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Or, place your finger over the I meaning that you want to calculate the current. Despite the fact that the label is European. For example: When an exact or a near equivalent is unavailable. It will then protect a power supply from delivering a high current to a circuit that has failed. A typical characteristic for a 5.
DO NOT count on them to protect you or the equipment. A television set. When replacing components. For example. It is generally of low value. Work on nonconductive sUlface. The purpose of the breaker or fuse is to reduce the danger of fire. The greatest danger in these areas is to the equipment itself. When clipping wires. The danger to you begins at the wall outlet and could continue well past the power supply. Unless the device is powered by batteries. Always use the right toolfor the job.
NEVER on a metal table. Be sure the work area is sufficiently lighted. Some components get very hot during operation. The work area in general should be clean and well organized.
One is that you may not be able to get the fire under control. If you have to get out of the area fast. This must be of the right type. Don't rely on the gauge. The first step is to be sure that there is a safe and quick exit from the work area.
Fire Hopefully you will never have to deal with this problem. Page 3. This is yet another reason to keep the working area clean and uncluttered. Having a fIre extinguisher around won't do much good if you don't know how to use it. If this happens you won't be able to let go ofwhat is causing the shock. With just a fraction of an amp more. The unit should also be serviced on a regular basis some suggest once per year as a minimum.
Merely keeping that other hand back isn't enough. Placing it in your pocket will force you to think 'about whatyou are doing. You might be tempted to reach forward with it. The lesson.
It won't! As stated above. That fuse or breaker allows 15 or more amps to flow almost indefinitely. At least one fire extinguisher should be immediately at hand. If you are careful. This is hundreds of times what it takes to kill a person. The other is that some electrical fires can release poisonous gases in the air. It is not overly cautious to work on an insulated surface. Even ifit shows "good" on the dial.
Even if the fire is out. Why would you suddenly have to flee? There are two main reasons. For electrical fires you'll need the dry powder type Type C. Even then. Page 4. Do this ONLY if it can be done safely and without risk of shock to yourself. The Shock Victim In the event of an electrical shock the first thing to do is to disconnect the power supply. As supplies are used from it.
The ABC of first aid in these circumstances is: Figure 1: Turn the person onto their back and tilt the head. You should know the basicsand know them well enough to apply them calmly under an emergency situation. This may mean standing on insulating material if available and pushing the live conductor with an insulator.
The fire alarm. Many hospitals and medical clinics offer free classes in CPR. If the person is not breathing. The working area should contain a complete first aid kit. Make sure that the kit is always fully stocked. You might even consider taking classes. First Aid It is a good idea to have a quality first aid manual.
Experts suggest that once per month is not too frequent. Now check for a pulse as an indicator that the heart is beating Figure 4. When natural breathing has started. Repeat the process. Remove your mouth and see ifthe chest sinks. If it is not. Breathing Figure 2: Pinch the nose. Pinch the victim's nose with your fingers and close it.
Circulation The mostreliable way for an amateur to check the pulse ofan unconscious person is as follows: See Figure 1. Watch the chestto see ifitrises Figure 2. That is. The right arm should be down and beside the body. This has the effect of lifting the tongue forward so that it does not block the airway. The left knee should be bent.
If the heart is beating. If the chestdoes not go up and down. Figure 4: Check the pulse for sign of heartbeat. The victim must be on his or her back and on a firm surface. Place two fingertips on the voice box Adam's apple and slide them around the neck to either side. Relax the pressure and repeat the process at a rate of just over one per second.
If the chest does not rise and fall. Figure 6. Kneel beside the victim and find thepoint where the ribs join atthe bottom ofthe breastbone. Don't Page 6. A pulse should be felt in either of the two carotid arteries that run up the sides of the neck Figure 4.
Place the heel of one hand on the breastbone about two finger-widths above this point. Put your other hand on top of the first and get into position over the victim with your arms straight and your shoulders directly above the breastbone. Keeping your arms straight. Continue mouth-tomouth until natural breathing starts. Complete 15 of these actions and then go back to the head. Stop the chest compressions as soon as a pulse is detected. If necessary. Figure 6: Pushing on the spot shown causes pressure on the heart.
With breathing restored. Page 7. Try to simulate the smooth. Continue with 15 chest compressions followed by two mouth-to-mouth cycles. PVC Quantities. They can amplify it. Semi-conductors are smaller. Even here tubes are disappearing. In an ideal theoretical transformer. Even with a transformer. Active components can do a variety of things to the current.
Page 1. You may encounter tubes in old equipment or in equipment that handles large amounts of power such as large transmitters. Vacuum tubes are active components. In reality there is a net loss. This voltage is known as the foward voltage drop. In order to obtain conduction. Very little current flows in the reverse direction.
The direction in which current flows is referred to as the forward direction. The amount of reverse current is negligible in most silicon devices. When a diode is conducting. The maximum reverse voltage that a diode can tolerate is usually specified in terms of its reverse repetitive maximum voltage. The direction of current flow is from anode to cathode when the diode is conducting as shown in Figure 1.
Forward biased conducting diode Diodes exhibit a low resistance to current flow in one direction and a high resistance in the other. The N-type connection is the cathode. Page 2. Thus germanium diodes are used primarily for signal detection purposes whereas silicon devices are used for rectification and for general purpose applications. Diodes are often divided into signal and rectifier types according to their principal field of application.
Signal diodes require consistent forward characteristics with low forward voltage drop. J Figure 2: Reverse biased non-conducting diode Typical values of forward current and forward voltage for commonly available silicon and germanium diodes are given below: The forward resistance of a conducting silicon diode is also generally much lower than that of a comparable germanium type.
Rectifier diodes need to be able to cope with high values of reverse voltage and large values of forward current. Typical forward and reverse characteristics for comparable germanium and silicon diodes are shown in Figure 3. Bridge rectifier Page Rectifier diodes are often available in the form of a bridge see Figure 4 which proves full-wave rectification.
VF -6 uA Reverse current. Typical characteristics for comparable silicon and germanium diodes Consistency of characteristics is of secondary importance in such applications Cross references for diodes that fulfill the same specifications are easily located and are sometimes printed on the package.
At times. In the European labeling: First Letter Material germanium silicon gallium arsenide. Despite the fact that the label is European.
This is the tolerance rating. Complete cross reference guides are also available should you wish to purchase one. J Dia. Virtually every parts supplier can cross reference. Do not worry about this too much.
And the same is available as Case If those are contained. Diode casings Page 5. These contain data about all the different semiconductors. JA Cia. To make matters more confusing.
A DO-7 case. This same casing can be DOAA. D Components Active Components Diodes technical data for the devices are often given. Those with computers can get all this information on disk. If you can't find it in your area. The software is available through a Motorola sales office. As is the case with conventional silicon diodes. Zener diode casings are generally plastic or glass and appear identical to conventional silicon diodes.
Zener diodes are available in various families according to their general characteristics. A typical characteristic for a 5. Note that the forward characteristic has exactly the same shape as that of a conventional silicon diode conducting rapidly at mY. The reverse characteristic has a much greater slope such that the current rises very rapidly beyond the zener voltage. Zener diode symbol and typical casing Page 8. Zener diode characteristics Page 9. This is the integrated circuit IC. Many ICs are standard and have common pinouts.
Devices could be made much smaller and lighter. How the IC is structured inside. Another problem is that some ICs are proprietary. The radio that once filled a large cabinet in the corner of a room. It allows devices to be even smaller. This may help you to locate which IC has gone bad-or the failure could be caused by a cascading effect with one chip or component elsewhere causing others to malfunction.
The combination of the two often puts the technician in a position in which the "fix" is to replace an entire circuit board. They may contain the equivalent of as few as 10 or as many as This means that even if you locate which Ieis causing the trouble.
Then came the technology that made it possible to build hundreds or even thousands of transistors. In electronics the IC is a mixed blessing. It also means that repair at the component level is quickly disappearing.
Integrated circuits are complex circuits fabricated on a tiny slice of silicon. The following terminology is often used: ICs do have some shortcomings when high currents or high voltages are involved. Scale of Integration A relative measure of the number ofindividual semiconductor devices within an intergrated circuit is popularly used to describe the scale of integration achieved within digital devices. Typical examples of linear integrated circuits include the vast majority of consumer ICs used in radio.
ICs are commonly used in almost every branch of electronics. Digital integrated circuits are designed for use in conjunction with digital signals i.
Typical examples of digital ICs are logic gates. In high-power audio applications it is. Devices required to work at an appreciable power level 1 W or more require heat-sinking. Not only are they the most cost-effective method of realizing many practical circuit configurations.
The immensely popular timer. Some integated circuits combine both digital and analog technology.. Efforts are being made to reduce this confusion. The package itself may be fabricated from either plastic or ceramic material with the latter using a glass hermetic sealant. Encapsulation The most popular form of encapsulation used for ICs is the dual-inline package. Common DIPs have 6. No holes are required to accomodate the leads of such devices which are arranged on a 0.
This is made more complicated by users often reducing the coding ofa chip to a few digits e. TO-3 and TO casings are also found the latter being commonly used with voltage regulators. This form ofcasing requires special handling. Conventions are almost interchangeable. IC Coding Once again we find a coding system which demonstrates how our world is becoming smaller. Each device within the family is coded with the prefix 74 and variants within the family are identified by letters which follow the 74 prefix as follows: Such signals have two states.
In conventional positive logic. TTL logic gates and related digital devices are found in the popular series of ICs. The logic family to which adevice belongs is largely determined by its operational characteristics such as power consumption.
Logic gates operate using binary signals. In practice. The logical function of a gate is specified in tenns of a truth table that relates the logical state of its output to every possible combination of input. A gate with three inputs can have eight possible input combinations. Its logical function is described by the logical condition which relates its output to the input s. Buffers are nonnally used to provide extra current drive at the output but can also be used to regulate the logic levels present at an interface.
OR and NOR. Inverters are used to complement the logical state. Variants within the family are identified by suffix letters as follows: Suffix none Meaning A B.
The symbol and truth table for a buffer is shown in Figure 1. Buffers Buffers do not affect the logical state of a digital signal. BE UB. Inverters also provide extra current drive and.
Inverters Page 2. Inverters and buffers each have one input. Since a digital signal can have two states 0 or 1 a gate with two inputs can have. Several functions are commonly encountered including AND. Symbol and truth table for an inverter OR Gates OR gates produce a logic 1 output whenever anyone or more input is at logic 1. Any other input combination results in a logic 0 output. Symbol and truth table for a buffer c ffij y o 1 0 1 The symbol and truth table for an inverter is shown in Figure 2.
Symbol and truth table for an AND gate Page 3. The OR gate produces a logic 0 output only when all of its inputs are simultaneously at logic O. The symbol and truth table for an AND gate is shown in Figure 3. The symbol and truth table for an OR gate is shown in Figure 4. The symbol and truth table is shown in Figure 7.
Any other combination produces a logic 1 output. Exclusive-OR gates produce a logic 0 whenever both inputs have the same logical state. The symbol and truth table for a NOR gate is shown in Figure 6. Monostables Basic Function A logic device which has only one stable output state is known as a monostable.
The output of such a device is initially at logic 0 low until an appropriate level Page 4. Any other combination produces a logic 0 output. A NAND gate. A NOR gate. The device then awaits the arrival of the next trigger. Control inputs AI. Upon receipt of a valid trigger pulse. Control Inputs PageS. The chip has complementary outputs Q and Q and requires only two timing components one resistor and one capacitor.
This level change can be from 0 to 1 positive edge trigger or negative edge trigger depending on the particular monostable device or configuration. Example The most common example of a TTL monostable device is the A2 and B are used to determine the trigger mode and may be connected in anyone of the following three ways: Symbol and truth table for an Exclusive-OR gate change occurs at its trigger input.
This device can be triggered by either positive or negative edges depending upon the configuration employed. The monostable then triggers on a negative edge to B. A2 and B connected to logic 1.
Monostable arrangement based on the The monostable then triggers on a negative edge applied to A2. The monostable then triggers on a negative edge applied to AI. The only requirement is that.. A is an ideal device to perform this function. Page 6. Circuit Arrangements. This simply means that once a timing period has been started. It is be triggered by a very short duration pulse and continues its timing period. The minimum recommended value of external capacitor is 10 pF. In normal use.
Usage Monostable devices are often used for stretching pulses of very short duration. In most practical circuit arrangements see Figure 8 the values of the external timing resistor should normally lie in the range of 1.
Once set. This device has two inputs.. R-5 Bistables The simplest form ofbistable is the R-S bistable. R-S a Ootype K a Clea. J-K Figure 9: Bistable symbols Various forms of bistables are found see Figure 9.
These arrangements are.. In either case. A logic 1 applied to the SET input causes the Q outputto become orremain at logic 1.
The larger the noise margin the better the ability to perform in a noisy environment. J-K bistables have two clocked inputs J and K. Operation is thus said to be synchronous.
J-K bistables are the most sophisticated and flexible bistables. Additional subsidiary inputs which are invariably active low are provided which can be used to directly set or reset the bistable. D-type bistables are used both as latches a simple form of memory and as binary dividers.
CMOS logic levels are relative to the supply voltage used while the logic levels associated with TTL devices tend to be absolute as shown in the following table. Noise margin is defined as the difference between the minimum values of high state output and high state input voltage. VDDIS the posnive s! Jpply associated with CMOS devices. That is.. Noise Margin The noise margin of a logic device is a measure of its ability to reject noise. The data input logic 0 or 1 is clocked into the bistable such that the output state only changes when the clock changes state.
Page 8. In particular. As with R-S bistables. Vlli MIN is the minimum value of high state logic 1 input voltage. These outputs can be placed in a high impedance state Le. Bus Compatible Devices Microprocessor bus compatible digital integrated circuits invaria.. L Components Active.
The fan-out of a logic gate is a measure of its ability to drive further inputs. Power consumption for CMOS devices tends to be proportional to switching speeds. For this reason it is essential to ensure that a replacement device has the same or improved fan-out.
The absolute maximum supply voltage for TIL devices is nominally 7 V. This explains why CMOS-based equipment sometimes fails to peIform to specification when the supply voltage is. In most cases. While modem CMOS devices are fitted with input static protection diodes. At speeds in excess of several MHz. Page In order to maintain peIformance at high switching speeds. If the supply voltage ever exceeds this value. A small circle is often used to denote an active low enable or chip select input on the device symbol.
With TIL devices. Early CMOS devices were easily damaged by stray static charges and required careful handling. Such an input may be active high the output of the gate is valid when the enable or chip select input is taken to logic 1 or active low the output of the gate is valid when the enable or chip select is taken to logic 0.
A typical red LEO provides a reasonable amount of light output with a forward current of as little as lOrnA. Used in conjunction with photo-sensitive components. With the rectangular types. LEOs operate from significantly smaller voltages and currents and are much more reliable. LED symbol and round casings Page 1. LEOs are available in a variety of colors including red. Most commonly they are used as general purpose indicators. Another common format is the 2mmx 5mmrectangular structure.
Compared with conventional filament lamps and neon indicators. Light emitting diodes are available in various formats. Round LEOs are available in the 3 mm and 5 mm diameter plastic packages see Figure 1. In order to limit the forward current to an appropriate value. To maintain an equal light output when several LEDs are used together. The value of the. VF is usually about 2 V. Section 3. Basic LED circuit Page As a rule ofthumb. Reverse voltage in excess ofabout 5 V will destroy the junction.
AC -OpAmp Operational Amplifiers Basic Function Operational amplifiers op-amps are general purpose integrated circuits with a wide variety of applications.
The following terminology is applied to operational amplifiers.. As amplifiers they possess near-ideal characteristics virtually infinite voltage gain and input resistance together with low output resistance and wide bandwidth.
In this case: Closed-loop Voltage Gain This is the ratio of output voltage to input voltage when negative feedback is applied. Operational amplifiers can be considered to offer a "block of gain" The closed-loop voltage gain is normally very much less than the open-loop voltage gain.
The open-loop voltage gain is sometimes expressed in decibels dB rather than as a ratio. In linear voltage amplifying applications. Vour and VIN are the output and input voltages repectively under open-loop conditions. The open-loop voltage gain is given by: Open-loop Voltage This is the ratio of outputvoltage to input voltage measured without feedback Gain applied.
V our and V IN are output and and input voltages respectively under closed-loop conditions. Offset may be minimized by applying large amounts of negative feedback or by using the offset null facility provided by certain types of op-amps Figure 1. Input Offset Voltage The input offset voltage is the voltage which when applied at the input provides an output voltage ofexactly zero.
VIN is the input voltage and lIN is the input current. Output Figure 1: Operational amplifier symbol Page 2. Note that due to imperfect balance and very high internal gain. The input resistance of operational amplifiers is very much dependent on the semiconductor technology employed.
Note that the input of an operational amplifier is normally assumed to be purely resistive. CMRR is a measure of an operational amplifier's ability to reject signals e. Active Components Operational Arnplifiers Slew-rate. Operational amplifier with feedback Page 3. The maximum output voltage swing is dependent on the positive and negative supply voltages. The bandwidth of an operational amplifier is the range offrequencies over which the device is able to provide its rated gain.
OpAmp Components. Bandwidth is closely related to slewrate in that the greater the slew-rate. Maximum Output Voltage Swing This is the maximum range of output voltages that the device can produce: The formula is: The Certain characteristics. PET devices. Op-amps are available in standard bipolar.
The output resistance of a low-power operational amplifier is usually in the region of 20 n to n while that for a high-power device may be as low as 2 Q. Figure 3: Offset-null facility -VE supply Page 4.
Operational amplifiers are packaged singly. Eachjunction within the transistor. The current flowing in the emitter circuit is often or more times greater than that flowing in the base. In either case the electrodes are labeled collector. Two diodes back-to-back. The base region is. Silicon transistors are superior when compared with germanium transistors in the vast majority of applications particularly at high temperatures and thus germanium devices are very rarely encountered in modern equipment.
The direction of current flow is from emitter to collector in the case of a PNP transistor. When base current is applied to the transistor. IB is the base current and Ie is the collector current. The base current sets up a corresponding standing current quiescent current in the collector circuit.
This bias is usually applied to the base by means ofa single resistor or a potential divider. When a signal is applied. In transistor circuits designed to provide linear amplification e. AF amplifiers or preamplifiers a static bias curent is applied to the transistor in order to obtain satisfactory operation. Thus the baseemitter voltage drop for a silicon transistor is in the region of 0.
Bipolar transistor connections Page 3. Ie is the change in collectorcurrentwhich results from a corresponding change in the base current. Ie is the collector current. T Components Active Components Transistors associated with a forward biased diode of the same material. When small. In the case of an NPN device. The small-signal AC current gain is then given by: The large-signal DC current gain in common emitter mode is thus given by: The current gain offered by a transistor is a measure of its effectiveness as an amplifying device.
In this mode. The most commonly quoted parameter is that which relates to common emitter mode. The emitteris effectively common to both the input and output circuits. VCEO max. Bipolar transistors potentials. PT max. Maximum value of collector-emitter voltage with the base terminal left open-circuit.
Parameter Meaning Maximum value of collector current. Maximum power dissipation. Maximum value of collector-base voltage with the base terminal left opert-circuit. Transition frequency Le. Apart from hfe. VCBO max. Such transistors are often used in the output stages of amplifiers where the high value of hfe can be used to achieve a very high power gain.
MaS or silicon on sapphire SOS technology. Once again. The effective resistance between the source and drain is thus determined by the voltage present at the gate. The effective width of the channel is controlled by a charge placed on the third gate electrode. The ends of the channel in which conduction takes place form electrodes known as the source and drain. The European system again uses more letters. These guides are available at parts suppliers.
On the other hand. Note that FET devices offer very much higher input impedances at the source than bipolar transistors at the base.
Field effect transistors are available in two basic forms. The gate-source junction of a junction gate field effect transistor JFET is effectively a reverse-biased P-N junction. JFET devices offer source input impedance of around Mil compared with the Among these are: Linear Switching Power High-frequency Low-frequ ency Low noise High voltage such as precision voltage or current amplification.
As with diodes. IGFETs use either metal on silicon. In either case a standing quiescent value of drain current results. Typical values of gate-source bias voltage are in the region of IGFETs may be designed for either depletion mode or enhancement mode operation. The method of applying the bias voltage differs according to the mode of operation depletion or enhancement.
Linear FETcircuits require the application ofa bias voltage between the gate and source. In the latter case. T Components Active Components Transisto rs combine low drain-source resistance in the on-state with very high drain-source resistance in the off-state. In the former case. VGS max. Other parameters normally quoted by manufacturers include: Parameter lomax. Symbols and connections for various types of FET FET Parameters The gain offered by a field effect transistor is normally expressed in terms of its forward transfer conductance gfs or Yfs in common source mode.
Maximum value of drain-source voltage. Typical value of output fall-time in response to a perfect rectangular pulse input.
ROS on max. Maximum value of resistance between drain and source when the transistor is in the conducting state on. Vos max. Maximum value of drain power dissipation.
Meaning Maximum value of drain current. Typical value of output rise-time in response to a perfect rectangular pulse input. The units of forward transfer conductance are Siemen S. Maximum value of gate-source voltage. Ti Timers Basic Function Integrated circuit timers are used in a wide variety of pulse generating applications in almost every branch of electronics.
The device operates on supply volages between The comprises two operational amplifiers used as comparators together with an RS bistable element. In addition. This versatile Ie is based on a neat hybrid arrangement of analog and digital circuitry see Figure A single transistor switch TRl discharges the external timing capacitor. The generic timer is the device. The standard timer is housed in an 8-pin DIP with the pin connections shown in Figure 2.
In monostable modes. These variants employ the same pin connections as their bipolar counterparts. Low-power s. The uses CMOS technology and thus demands on the power supply are minimal. The pin connections for the are shown in Figure 3.
In astable mode. The is a dual device in a pin DIP comprised of two identical independent timers. Accuracy is determined primarily by the external timing components.
Timers are used in either astable mode to generate a continuous pulse train or in monostable mode to generate a single pulse of accurately defined length. Monostable pulse generator based on a timer Low-power timers e. CMOS The output waveform has a period determined by C. An astable pulse generator based on a timer is shown in Figure 4. L ILr 1 n T Figure 4: CMOS devices can.. In many applications Rl and R2. The square wave produced by this arrangement has a frequency which is: Components Timers the mark to space ratio of the output waveform will be given by: The monostable timing period is initiated by a falling edge i..
The duration of the monostable pulse output is: Ti Components Activ. The output pulse has the following characteristics: Period for which the output is high: The amplitude of the pulse is approximately equal to the supply voltage. The device is triggered into the conducting on state by means of the application of a current pulse of sufficient amplitude to this terminal.
In the off state nonconducting the thyristor has negligible leakage current while in the on state conducting. A Figure 1: Thyristor symbol and terminal connections Page 1. Like their conventional silicon diode counterparts.. AC -Thy Thyristors Basic Function Thyristors are silicon controlled rectifiers with three terminals that can be used for switching and controlling ACpower.
In DC applications this necessitates the interruption or disconnection of the supply before the device can be reset into its non-conducting state. Control is applied by means of a gate terminal see Figure 1.
Thy Components Acti". Thyristors can switch very rapidly from a conducting state to a nonconducting state.. The device can then be triggered on the next half-cycle having the correctpolarity to permitconduction.
C Gate. Where the device is used with an alternating supply e. Once switched into the conducting state. This results in very little power loss within the thyristor even when appreciable power levels are being controlled. An ohmmeter on a low-resistance range connected between the anode and cathode with the probe connected to the anode indicates low-resistance when the thyristor is triggered on.
Thyristors may fail in one ofseveral ways. Thy Components Active Components Thyristors Figure 2 shows the details of the casing and connections commonly used with thyristors. Warning Thyristors are often used in high-voltage and AC mains power control applications.
Thyristors can be checked for correct triggerging by applying a 9 V battery connected in series with a switch and a n resistor between the gate and cathode. When connecting or disconnecting such devices it is essential to ensure that the equipment is switched off and that the AC mains are completely disconnected. Note that once triggered. Great efforts put it to find the list of articles which is very useful to know, Definitely will share the same to other forums.
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Thanks for sharing such a nice blog. And that's what we will do in this eBook: There are two types: You really need both types to cover the number of tests needed for designing and repair-work. We will discuss how they work, how to use them and some of the differences between them.
The cost is determined by the number of ranges and also the extra features such as diode tester, buzzer continuity , transistor tester, high DC current and others. Since most multimeters are reliable and accurate, buy one with the greatest number of ranges at the lowest cost. This article explains the difference between a cheap analogue meter, an expensive analogue meter and a digital meter.
You will then be able to work out which two meters you should buy. Some Digital Multimeter DMMs are auto ranging; they automatically select the correct range of voltage, resistance, or current when doing a test. Before making any measurement you need to know what you are checking. If you are measuring resistance, select the Ohms range x1, x10, x, x1k, x10k. If you are measuring current, select the appropriate current range DCmA 0.
Every multimeter is different however the photo below shows a low cost meter with the basic ranges. The most important point to remember is this: You must select a voltage or current range that is bigger or HIGHER than the maximum expected value, so the needle does not swing across the scale and hit the "end stop. A "-1" indicates the leads should be reversed for a "positive reading. The red lead fits into the red socket for Voltage and Resistance.
The following two photos show the test leads fitted to a digital meter. The probes and plugs have "guards" surrounding the probe tips and also the plugs so you can measure high voltages without getting near the voltage-source. Analogue meters have an "Ohms Adjustment" to allow for the change in voltage of the battery inside the meter as it gets old.
If the pointer does not reach full scale, the batteries need replacing. Digital multimeters do not need "zero adjustment. Before taking a reading, you should select the highest range and if the needle does not move up scale to the right , you can select another range. Always switch to the highest range before probing a circuit and keep your fingers away from the component being tested. If the meter is Digital, select the highest range or use the auto-ranging feature, by selecting "V.
DC means Direct Current and the voltage is coming from a battery or supply where the voltage is steady and not changing and AC means Alternating Current where the voltage is coming from a voltage that is rising and falling. You can measure the voltage at different points in a circuit by connecting the black probe to chassis.
This is the 0v reference and is commonly called "Chassis" or "Earth" or "Ground" or "0v. Sometimes there are "test points" on a circuit and these are wires or loops designed to hold the tip of the red probe or a red probe fitted with a mini clip.
It may be the voltage across a transistor, resistor, capacitor, diode or coil. In most cases this voltage will be less than the supply voltage.
Here's a simple case. The circuit below consists of two 1M resistors in series. Here how it works: Every meter has a sensitivity. If this multimeter is used to test the following circuit, the reading will be inaccurate. See how easy it is to get a totally inaccurate reading.
This introduces two new terms: In other words it has a very HIGH input impedance. Most Digital Multimeters have a fixed input resistance impedance of 10M - no matter what scale is selected. That's the reason for choosing a DMM for high impedance circuits. You can measure "across" a component, or between any point in a circuit and either the positive rail or earth rail 0v rail.
In the following circuit, the 5 most important voltage-measurements are shown. Voltage "A" is across the electret microphone. It should be between 20mV and mV. Voltage "B" should be about 0. Voltage "C" should be about half-rail voltage.
This allows the transistor to amplify both the positive and negative parts of the waveform. Voltage "D" should be about v. Voltage "E" should be the battery voltage of 12v. Measuring the current of a circuit will tell you a lot of things. If you know the normal current, a high or low current can let you know if the circuit is overloaded or not fully operational.
Current is always measured when the circuit is working i. The easiest way to measure current is to remove the fuse and take a reading across the fuse-holder. Or remove one lead of the battery or turn the project off, and measure across the switch.
If this is not possible, you will need to remove one end of a component and measure with the two probes in the "opening. You have to get an "opening" so that a current reading can be taken.
This voltage may be too high for the circuit being supplied and the result will be damage. Measuring current through a resistor Measuring the current of a globe. Do not measure the "current a battery will deliver" by placing the probes across the terminals. It will deliver a very high current and damage the meter instantly. There are special battery testing instruments for this purpose.
When measuring across an "opening" or "cut," place the red probe on the wire that supplies the voltage and current and the black probe on the other wire. If you are using a Digital Meter, a negative sign "-" will appear on the screen to indicate the probes are around the wrong way. No damage will be caused. It just indicates the probes are connected incorrectly. If any voltage is present, the value of resistance will be incorrect. In most cases you cannot measure a component while it is in-circuit.
This is because the meter is actually measuring a voltage across a component and calling it a "resistance. If any other voltage is present, the meter will produce a false reading. If you are measuring the resistance of a component while still "in circuit," with the power off the reading will be lower than the true reading. Measuring resistance.
Measuring resistance of a heater via the leads. Measuring the resistance of a piece of resistance-wire. Measuring the resistance of a resistor. Do not measure the "Resistance of a Battery". Do not measure the "resistance of a battery. It is measured by creating a current-flow and measuring the voltage across the battery.
Do not try to measure the resistance of any voltage or any "supply. If the bar is thinner, the resistance is higher.
If the bar is longer, the resistance is higher. If the material of the bar is changed, the resistance is higher. It's a bit like standing on a hose. The flow reduces. When current flow is reduced, the output voltage is also reduced and that why the water does not spray up so high.
Resistors are simple devices but they produce many different effects in a circuit. A resistor of nearly pure carbon may be 1 ohm, but when non-conducting "impurities" are added, the same-size resistor may be ohms, 1, ohms or 1 million ohms. Circuits use values of less than 1 ohm to more than 22 million ohms.
The letter "E" is also sometimes used and both mean "Ohms. The size determines the wattage of the resistor - how much heat it can dissipate without getting too hot. Every resistor is identified by colour bands on the body, but when the resistor is a surface-mount device, numbers are used and sometimes letters. If 3rd band is gold, Divide by 10 If 3rd band is silver, Divide by to get 0. The first 3 bands produce the resistance and the fourth band is the "tolerance" band.
These two bands provide the digits in the answer. But it's easy to follow. This represents a ZERO in the answer. This is different to 4-band resistors where black represents the word OHMS! The following list covers 10 ohms 10R to 1M. Surface Mount Resistors. All the SM resistors in the above photos conform to a 3-digit or 4-digit code. But there are a number of codes, and the 4-digit code caters for high tolerance resistors, so it's getting very complicated.
Here is a basic 3-digit SM resistor:. A k SM resistor. Three Digit Examples. Four Digit Examples. If you want to create a "Special Value," simply connect two resistors and read the value with a Digital Meter.
Keep changing the values until you get the required value. We are not going into series or Parallel formulae. You can easily find a value with a multimeter. You simply ADD the values. This can be done with any to two values as shown. Three equal-value resistors in series is three times the value. Three equal-value resistors in parallel is equal to one-third the value.
If you want a particular value and it is not available, here is a chart. Use 2 resistors in series or parallel as shown: There are other ways to combine 2 resistors in parallel or series to get a particular value. The examples above are just one way. The surrounding components can affect the reading and make it lower. You can take the reading of a resistor "in-circuit" in one direction then the other, as the surrounding components may have diodes and this will alter the reading.
You can also test a resistor by feeling its temperature-rise. Resistors are just "resistors" and they can be in AC circuits or DC circuits. It is a low-value resistor and has a voltage-drop across it but this is not intentional. The voltage-drop is to create a "heating-effect" to burn out the resistor.
In all the other types of resistor, the voltage-drop is intentional. A Ballast resistor is a normal resistor and can be called a Power resistor, Dropper resistor, Supply resistor or Feed resistor. It is designed to reduce the voltage from one source and deliver a lower voltage. It is a form of: A Load Resistor is generally connected across the output of a circuit and turns the energy it receives, into heat. It is made with many resistors of the same value, all in one package. One end of each resistor is connected all the other resistors and this is the common pin, identified as pin 1 and has a dot on the package.
These packages are very reliable but to make sure all the resistors are as stated, you need to locate pin 1. All values will be identical when referenced to this pin. Some resistor networks have a "4S" printed on the component.
The 4S indicates the package contains 4 independent resistors that are not wired together inside. The housing has eight leads as shown in the second image. Independent resistors have an even number of pins and measuring between each pair will produce identical values. R esistance between any pair will indicate leakage and may be a fault.
When cold, it has a very low resistance and a large current flows when the monitor or TV is switched on. This current heats up the Posistor and the resistance increases.
This causes the current to decrease and any magnetism in the shadow mask is removed. The posistor can one or two elements and it is kept warm so the resistance remains high. Many Posistors have a second element inside the case that connects directly to the supply to keep the Positive Temperature Coefficient resistor high so that the current through the degaussing coil falls to almost zero. This constant heat eventually destroys the package. The heavy current that flows when a set is turned ON also causes the posistor to crack and break and this results in poor purity on the screen - as the shadow mask gradually becomes magnetic..
Posistors have different resistance values from different manufacturers and must be replaced with an identical type. They can be checked for very low resistance when cold but any loose pieces inside the case will indicate a damaged component. The resistance of a "burnt" resistor can sometimes be determined by scraping away the outer coating - if the resistor has a spiral of resistance-material.
You may be able to find a spot where the spiral has been damaged. Clean the "spot" burnt section of the spiral very carefully and make sure you can get a good contact with the spiral and the tip of your probe. Measure from one lead of the resistor to the end of the damaged spiral.
Then measure from the other lead to the other end of the spiral. Add the two values and you have an approximate value for the resistor. You can add a small amount for the damaged section. This process works very well for damaged wire-wound resistors. They can be pulled apart and each section of the resistance-wire nichrome wire measured and added to get the full resistance.
There is another way to determine the value of a damaged resistor. Get a set of resistors of the same wattage as the damaged component and start with a high value. It's handy to know if the resistor is in the range: Start with a very high value and turn the circuit ON.
You can perform voltage tests and if you know the expected output voltage, decrease the resistance until this voltage is obtained. If you do not know the expected voltage, keep reducing the value of resistance until the circuit works as designed.
This is the best advice in a situation where you do not know the value of a resistor. There is a third way to determine the value and this requires measuring the voltage drop across the resistor and the current-flow.
By multiplying the two you will get a wattage and this must be less than the wattage of the resistor being replaced. A Rheostat is a variable resistor using only one end and the middle connected to a circuit. The resistance between the two outside pins is the value marked on the component and the centre leg will change from nearly zero to the full resistance as the shaft is rotated. Cleaning with spray fixes the bad focus but if the pot is leaking to chassis from inside the pot due to the high voltage on the terminals simply remove it from the chassis and leave it floating this will restore the high voltage to the picture tube or you can use one from an old chassis.
We have already covered placing resistors and capacitors in parallel and series: Two 1k 0. Zener diodes can be connected in series to get a higher voltage. Two 12v zener diodes in series produces a 24v zener. You can use the resistance scale "x1" or "x10" to detect low values of resistance. Set the pointer to "0" right end of the scale by touching the probes together and adjusting the "zero ohms" control. When taking a reading, you will have to decide if a low value of resistance is a short-circuit or an "operating value.
The "resistance of a circuit" may be very low as the electrolytics in the circuit are uncharged. This may not indicate a true "short-circuit. Leads and wires and cords have a small resistance and depending on the length of the lead, this small resistance may be affecting a circuit. Remember this: When a circuit takes 1 amp, and the resistance of the leads is 1 ohm, the voltage drop across the leads will be 1v.
That's why a 12v battery supplying a circuit with these leads will have 11v at the circuit. Turn off the equipment before making any continuity tests. The presence of even a small voltage from an electrolytic can give a false reading. You can determine the resistance of a lead very accurately by taking the example above and applying it to your circuit. If the battery is By making the lead shorter or using thicker wire, the resistance will be less and the voltage on the project will increase.
When taking readings in a circuit that has a number of diodes built-into IC's Integrated Circuits and transistors, some Continuity Testers will beep and give a false reading. The following circuit has the advantage of providing a beep when a short-circuit is detected but does not detect the small voltage drop across a diode.
This is ideal when testing logic circuits as it is quick and you can listen for the beep while concentrating on the probe. Using a multimeter is much slower.
You can build the circuit on Matrix Board and add it to your Test Equipment. You will need lots of "Test Equipment" and they can be built from circuits in this eBook. Turn off all power to the equipment before testing for shorts and continuity. All fuses, leads and wires should have a low, very low or zero resistance.
This proves they are working. If the inside of the glass tube of the fuse is totally blackened, the fuse has been damaged very quickly. This indicates a very high current has passed through the fuse. Depending on the rating of the fuse, current rating you will be able to look for components that can pass a high current when damaged - such as high power transistors, FETs, coils, electrolytics.
This is done by measuring them on a low OHMs range in one direction then reverse the leads to see if the resistance is low in the other direction.
A reading can be very low at the start because electrolytics need time to charge-up and if the reading gradually increases, the power rail does not have a short. An overload can occur when the supply voltage rises to nearly full voltage, so you sometimes have to fit a fuse and see how long it takes to "blow.
They are all current ratings as a fuse does not have a voltage rating. Some fuses are designed for cars as they fit into the special fuse holders. Some fuses are fast-blow and some are slow-blow. A "normal" fuse consists of a length of thin wire. Or it may be a loop of wire that is thin near the middle of the fuse. This is the section that will "burn-out. For instance, a 1amp fuse will remain intact when up to 1. When a circuit is turned on, it may take amps for a very short period of time and a normal 1 amp fuse will get very hot and the wire will stretch but not "burn-out.
If the current increases to 2amps, the fuse will still remain intact. It needs about 3 amp to heat up the wire to red-hot and burn out. A slow-blow fuse uses a slightly thicker piece of wire and the fuse is made of two pieces of wire joined in the middle with a dob of low-temperature solder.
Sometimes one of the pieces of wire is a spring and when the current rises to 2. Thus the fuse is not gradually being damaged and it will remain in a perfect state for a long period of time. A fuse does not protect electronic equipment from failing. It will then protect a power supply from delivering a high current to a circuit that has failed.
If a slow-blow fuse has melted the solder, it could be due to a slight overload, slight weakening of the fuse over a period of time or the current-rating may be too low. You can try another fuse to see what happens. You can replace a fast-acting fuse normal fuse with a slow blow if the fast-acting fuse has been replaced a few times due to deterioration when the equipment is turned on.
But you cannot replace a slow-blow fuse with a fast acting fuse as it will be damaged slightly each time the equipment is turned on and eventually fail.
The wire may be wrapped around a core made of iron or ferrite. It is labeled "L" on a circuit board. You can test this component for continuity between the ends of the winding and also make sure there is no continuity between the winding and the core. The winding can be less than one ohm, or greater than ohms, however a coil of wire is also called an INDUCTOR and it might look like a very simple component, but it can operate in a very complex way.
The way it works is a discussion for another eBook. This causes the fuse to "blow. You can then compare the inductance with a known good component. An inductor with a shorted turn will have a very low or zero inductance, however you may not be able to detect the fault when it is not working in a circuit as the fault may be created by a high voltage generated between two of the turns. A TV or monitor screen is the best piece of Test Equipment as it has identified the fault. It is pointless trying to test the windings further as you will not be able to test them under full operating conditions.
The solution is to measure a larger inductor and note the reading. This way you can measure very small inductors. However these components can become intermittent due to dirt or pitting of the surface of the contacts due to arcing as the switch is opened. It is best to test these items when the operating voltage and current is present as they quite often fail due to the arcing.
A switch can work 49 times then fail on each 50th operation. The same with a relay. If the contacts do not touch each other with a large amount of force and with a large amount of the metal touching, the current flowing through the contacts will create HEAT and this will damage the metal and sometimes reduce the pressure holding the contact together. This causes more arcing and eventually the switch heats up and starts to burn. Switches are the biggest causes of fire in electrical equipment and households.
A relay also has a set of contacts that can cause problems. There are many different types of relays and basically they can be put into two groups. The contacts allow a current to flow and this current can damage the contacts. Connect 5v or 12v to the coil or 24v and listen for the "click" of the points closing. Measure the resistance across the points to see if they are closing. You really need to put a load on the points to see if they are clean and can carry a current.
The coil will work in either direction. The two pins that energise the relay the two input pins must be connected to 5v or 12v around the correct way as the voltage is driving a LED with series resistor. The LED illuminates and activates a light-sensitive device. That's because they don't give a reading on a multimeter and their value can range from 1p to ,u. A faulty capacitor may be "open" when measured with a multimeter, and a good capacitor will also be "open.
Both are correct and you have to combine them to get a full picture. But it works in another way. Suppose you have a strong magnet on one side of a door and a piece of metal on the other.
By sliding the magnet up and down the door, the metal rises and falls. The metal can be connected to a pump and you can pump water by sliding the magnet up and down. A capacitor works in exactly the same way. If you raise a voltage on one lead of a capacitor, the other lead will rise to the same voltage. This needs more explaining - we are keeping the discussion simple. It works just like the magnetic field of the magnet through a door. The next concept is this: Capacitors are equivalent to a tiny rechargeable battery.
They store energy when the supply-voltage is present and release it when the supply drops. These two concepts can be used in many ways and that's why capacitors perform tasks such as filtering, time-delays, passing a signal from one stage to another and create many different effects in a circuit.
The easiest way to understand capacitor values is to start with a value of 1u. This is one microfarad and is one-millionth of a Farad. A 1 microfarad capacitor is about 1cm long and the diagram shows a 1u electrolytic. Smaller capacitors are ceramic and they look like the following. This is a n ceramic: To read the value on a capacitor you need to know a few facts. Capacitors from 1p to n are non-polar and can be inserted into a circuit around either way.
They must be fitted so the positive lead goes to the supply voltage and the negative lead goes to ground or earth. There are many different sizes, shapes and types of capacitor. They are all the same. They consist of two plates with an insulating material between. The two plates can be stacked in layers or rolled together. The important factor is the insulating material. It must be very thin to keep things small. If a capacitor sees a voltage higher than its rating, the voltage will "jump through" the insulating material or around it.
If this happens, a carbon deposit is left behind and the capacitor becomes "leaky" or very low resistance, as carbon is conductive. This is especially true for surface-mount capacitors. All capacitors are marked with a value and the basic unit is: For testing and repair work, they are all the same. Simply replace with exactly the same type and value.
A tantalum is smaller for the same rating as an electrolytic and has a better ability at delivering a current. They are available up to about 1,u, at about 50v but their cost is much higher than an electrolytic. Electrolytics are available in 1u, 2u2 3u3 4u7 10u, 22u, 47u, u, u, u, u, 1,u, 2,u, 3,u, 4,u, 10,u and higher.
The "voltage" or "working voltage" can be: There is also another important factor that is rarely covered in text books. This is the amount of current that can enter and leave an electrolytic.
This current heats up the electrolytic and that is why some electrolytics are much larger than others, even though the capacitance and voltage-ratings are the same. If you replace an electrolytic with a "miniature" version, it will heat up and have a very short life.
This is especially important in power supplies where current energy is constantly entering and exiting the electrolytic as its main purpose is to provide a smooth output from a set of diodes that delivers "pulsing DC. It sometimes has the letters "NP" on the component. Sometimes the leads are not identified. This is an electrolytic that does not have a positive and negative lead but two leads and either lead can be connected to the positive or negative of the circuit.
A non-polar electrolytic can be created from two ordinary electrolytics by connecting the negative leads together and the two positive leads become the new leads. For example: In the circuit below, the non-polar capacitor is replaced with two electrolytics. If you do not have the exact value, two or more connected in parallel or series can produce the value you need.
Capacitors connected in series will produce one with a higher voltage rating. Capacitors connected in parallel will produce a larger-value capacitance. Here are examples of two equal capacitors connected in series or parallel and the results they produce: This specifies the maximum voltage that can be applied across the capacitor without puncturing the dielectric.
Voltage ratings for "poly," mica and ceramic capacitors are typically 50v to VDC. Ceramic capacitors with ratings of 1kv to 5kv are also available. Electrolytic capacitors are commonly available in 6v, 10v 16v, 25v, 50v, v, v, and v ratings. CAUTION If a capacitor has a voltage rating of 63v, do not put it in a v circuit as the insulation called the dielectric will be punctured and the capacitor will "short-circuit.
High voltage electrolytic caps can pose a safety hazard. These capacitors are in power supplies and some have a resistor across them, called a bleed resistor, to discharge the cap after power is switched off. If a bleed resistor is not present the cap can retain a charge after the equipment is unplugged.
How to discharge a capacitor Do not use a screwdriver to short between the terminals as this will damage the capacitor internally and the screwdriver.
Use a 1k 3watt or 5watt resistor on jumper leads and keep them connected for a few seconds to fully discharge the electro. Test it with a voltmeter to make sure all the energy has been removed. Before testing any capacitors, especially electrolytics, you should look to see if any are damaged, overheated or leaking. Swelling at the top of an electrolytic indicates heating and pressure inside the case and will result in drying out of the electrolyte.
Any hot or warm electrolytic indicates leakage and ceramic capacitors with portions missing indicates something has gone wrong. A short-circuit within the capacitor 2. Capacitor values above 1u. You can test capacitors in-circuit for short-circuits. Use the x1 ohms range. To test a capacitor for leakage, you need to remove it or at least one lead must be removed.
Use the x10k range on an analogue or digital multimeter. For values above 1u you can determine if the capacitor is charging by using an analogue meter. The needle will initially move across the scale to indicate the cap is charging, then go to "no deflection. You can reverse the probes to see if the needle moves in the opposite direction.
This indicates it has been charged. Values below 1u will not respond to charging and the needle will not deflect. This does not work with a digital meter as the resistance range does not output any current and the electrolytic does not charge. Rather than spending money on a capacitance meter, it is cheaper to replace any suspect capacitor or electrolytic. Capacitors can produce very unusual faults and no piece of test equipment is going to detect the problem.
In most cases, it is a simple matter to solder another capacitor across the suspect component and view or listen to the result. This saves all the worry of removing the component and testing it with equipment that cannot possibly give you an accurate reading when the full voltage and current is not present.
You are fooling yourself. If the Test Equipment says the component is ok, you will look somewhere else and waste a lot of time. Here is a simple circuit that can be added to your meter to read capacitor values from 10p to 10u.
A capacitor may be slightly important in a circuit or it might be extremely critical. A capacitor just doesn't have a "value of capacitance. This is due to the way it is constructed. Some capacitors are simply plates of metal film while others are wound in a coil. Some capacitors are large while others are small. They all react differently when the voltage fluctuates.
Not only this, but some capacitors are very stable and all these features go into the decision for the type of capacitor to use. You can completely destroy the operation of a circuit by selecting the wrong type of capacitor.
No capacitor is perfect and when it gets charged or discharged, it appears to have a small value of resistance in series with the value of capacitance.
This effectively makes the capacitor slightly slower to charge and discharge. We cannot go into the theory on selecting a capacitor as it would be larger than this eBook so the only solution is to replace a capacitor with an identical type.
However if you get more than one repair with identical faults, you should ask other technicians if the original capacitor comes from a faulty batch. The author has fixed TV's and fax machines where the capacitors have been inferior and alternate types have solved the problem. Some capacitor are suitable for high frequencies, others for low frequencies. Open circuit in both directions. Low resistance in both directions. Breakdown under load. It will not allow any current to flow.
Thus the needle will not move. This position represents the voltage drop across the junction of the diode and is NOT a resistance value. If you change the resistance range, the needle will move to a slightly different position due to the resistances inside the meter. This indicates the diode is not faulty. The needle will swing to a slightly different position for a "normal diode" compared to a Schottky diode.
This is due to the different junction voltage drops. However we are only testing the diode at very low voltage and it may break-down when fitted to a circuit due to a higher voltage being present or due to a high current flowing. The best thing to do with a "suspect" diode is to replace it. This is because a diode has a number of characteristics that cannot be tested with simple equipment. Some diodes have a fast recovery for use in high frequency circuits.
They conduct very quickly and turn off very quickly so the waveform is processed accurately and efficiently.
If the diode is replaced with an ordinary diode, it will heat up as does not have the high-speed characteristic. Other diodes have a low drop across them and if an ordinary is used, it will heat up. Most diodes fail by going: This can be detected by a low resistance x1 or x10 Ohms range in both directions.
To locate this fault, place an identical diode across the diode being tested. A leaky diode can be detected by a low reading in one direction and a slight reading the other direction. However this type of fault can only be detected when the circuit is working. The output of the circuit will be low and sometimes the diode heats up more than normal. A diode can go open under full load conditions and perform intermittently. Diodes come in pairs in surface-mount packages and 4 diodes can be found in a bridge.
They are also available in pairs that look like a 3-leaded transistor. The line on the end of the body of a diode indicates the cathode and you cannot say "this is the positive lead. The cathode is defined as the electrode or lead through which an electric current flows out of a device.
The following diagrams show different types of diodes: Suppose you touch both wires. You will get a shock. The neutral is connected to an earth wire or rod driven into the ground or connected to a water pipe at the point where the electricity enters the premises and you do not get a shock from the NEUTRAL. You never get a v shock.
It is a v shock. In other words, if you touch the two wires at a particular instant, you would get a POSITIVE v shock and at another instant you would get a negative v shock. This is shown in the diagram below.
We now transfer this concept to the output of a transformer. The diagram shows an AC waveform on the output of the secondary. The bottom lead is called "zero volts. The diode only conducts when the voltage is "above zero" actually when it is 0. This is shown on the output of the Power Diode. Only the positive peaks or the positive parts of the waveform appear on the output and this is called "pulsing DC. We have used it to describe how the diode works.
The electrolytics charge during the peaks and deliver energy when the diode is not delivering current. This is how the output becomes a steady DC voltage.
The signal that it squelches is a voltage that is in the opposite direction to the "supply voltage" and is produced by the collapsing of a magnetic field. Whenever a magnetic filed collapses, it produces a voltage in the winding that is opposite to the supply voltage and can be much higher.
This is the principle of a flyback circuit or EHT circuit. The high voltage comes from the transformer. The diode is placed so that the signal passes through it and less than 0.
A damper diode can be placed across the coil of a relay, incorporated into a transistor or FET or placed across a winding of a flyback transformer to protect the driving transistor or FET. It does not have to be a high-voltage diode as the high voltage in the circuit is being absorbed by the diode.
When reading in the LOW direction, the needle will swing nearly full scale and the reading is not a resistance-value but a reflection of the characteristic voltage drop across the junction of the diode. As we mentioned before, a resistance reading is really a voltage reading and the meter is measuring the voltage of the battery minus the voltage-drop across the diode.
Since Silicon, Germanium and Schottky Diodes have slightly different characteristic voltage drops across the junction, you will get a slightly different reading on the scale. This does not represent one diode being better than the other or capable of handling a higher current or any other feature. The quickest, easiest and cheapest way to find, fix and solve a problem caused by a faulty diode is to replace it.
There is no piece of test equipment capable of testing a diode fully, and the circuit you are working on is actually the best piece of test equipment as it is identifying the fault UNDER LOAD. Using this, a silicon diode should read a voltage drop between 0.
For a germanium diode, the reading will be lower, around 0. The LED does not emit light when it is revered-biased. The light produced by a LED can be visible, such as red, green, yellow or white. They are used in remote controls and to see if they are working, you need to point a digital camera at the LED and view the picture on the camera screen. An LED needs about 2v - 3. The simplest way to deliver the exact voltage is to have a supply that is higher than needed and include a voltage-dropping resistor.
The value of the resistor must be selected so the current is between 2mA and 25mA. The life expectancy of a LED is about , hours.
LEDs rarely fail but they are very sensitive to heat and they must be soldered and de-soldered quickly. They are one of the most heat-sensitive components.
Light emitting diodes cannot be tested with most multimeters because the characteristic voltage across them is higher than the voltage of the battery in the meter. However a simple tester can be made by joining 3 cells together with a R resistor and 2 alligator clips: Connect the clips to a LED and it will illuminate in only one direction.
The colour of the LED will determine the voltage across it. You can measure this voltage if you want to match two or more LEDs for identical operation. Red LEDs are generally 1. Orange LEDs are about 2. The illumination produced by a LED is determined by the quality of the crystal. It is the crystal that produces the colour and you need to replace a LED with the same quality to achieve the same illumination. Never connect a LED across a battery such as 6v or 9v , as it will be instantly damaged.
You must have a resistor in series with the LED to limit the current. For instance a 1N is a v zener diode as this is its reverse breakdown voltage. And a zener diode can be used as an ordinary diode in a circuit with a voltage that is below the zener value. For instance, 20v zener diodes can be used in a 12v power supply as the voltage never reaches 20v, and the zener characteristic is never reached. Most diodes have a reverse breakdown voltage above v, while most zeners are below 70v.
A 24v zener can be created by using two 12v zeners in series and a normal diode has a characteristic voltage of 0.
This can be used to increase the voltage of a zener diode by 0. It uses 3 ordinary diodes to increase the output voltage of a 3-terminal regulator by 2. To tests a zener diode you need a power supply about 10v higher than the zener of the diode. Connect the zener across the supply with a 1k to 4k7 resistor and measure the voltage across the diode. If it measures less than 1v, reverse the zener. If the reading is high or low in both directions, the zener is damaged.
Here is a zener diode tester. The circuit will test up to 56v zeners. This clever design uses 4 diodes in a bridge to produce a fixed voltage power supply capable of supplying 35mA. If we put 2 zener diodes in a bridge with two ordinary power diodes, the bridge will break-down at the voltage of the zener.
This is what we have done. If we use 18v zeners, the output will be 17v4. When the incoming voltage is positive at the top, the left zener provides 18v limit and the other zener produces a drop of 0.
This allows the right zener to pass current just like a normal diode. The output is 17v4. The same with the other half-cycle. You cannot use this type of bridge in a normal power supply as the zener diode will "short" when the input voltage reaches the zener value.
The concept only works in the circuit above. Providing the input voltage is 4v above the output voltage, the regulator will deliver a fixed output voltage with almost no ripple. In most cases, a voltage regulator gets quite hot and for this reason it has a high failure-rate. If a regulator is not getting hot or warm it has either failed or the circuit is not operating.
A regulator can only decrease the voltage. It cannot increase the current. This means the current being supplied to a circuit must also be available from the circuit supplying the regulator. All regulators have different pin-outs, so you need to find the input pin and output pin and make sure the voltage-difference is at least 4v. Some regulators will work with a difference as low as 1v, so you need to read the specifications for the type you are servicing.
You need to test a voltage regulator with the power "ON". With the power turned off or the regulator removed from the circuit, you can test it with a multimeter set to resistance to see if it is ok. If any resistance readings are very low or zero ohms, the regulator is damaged. This includes chokes, coils, inductors, yokes, power transformers, EHT transformers flyback transformers , switch mode transformers, isolation transformers, IF transformers, baluns, and any device that has turns of wire around a former.
All these devices can go faulty. The coating on the wire is called insulation or "enamel" and this can crack or become overheated or damaged due to vibration or movement. When two turns touch each other, a very interesting thing happens. The winding becomes two separate windings. We will take the case of a single winding such as a coil. This is shown in the first diagram above and the winding is wound across a former and back again, making two layers.
Winding B C becomes a separate winding as shown in the second diagram. This short-circuit causes the transformer to get very hot. However when a transformer or coil is measured with an inductance meter, an oscillating voltage or spike is delivered into the core as magnetic flux, then the magnetic flux collapses and passes the energy into the winding to produce a waveform.
The inductance meter reads this and produces a value of inductance in Henry milliHenry or microHenry. This is done with the transformer removed from the circuit and this can be a very difficult thing to do, as most transformers have a number of connections. If the coil or transformer has a shorted turn, the energy from the magnetic flux will pass into the turns that are shorted and produce a current.
Almost no voltage will be detected from winding. The reading from the inductance meter will be low or very low and you have to work out if it is correct.