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ELECTRONICS REPAIR ARTICLES PDF

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Electronic repair articles for the electronic repairer. Components Like A Professional! Learn How You Can Become A Professional In LCD Monitor Repair !. Become an electronics pro with this great book on testing electronic components. Jestine Yong's great book will get you into LCD monitor repair quickly. If you want to get started in LCD TV repair or electronics repair than this is a good book for you. r Adjustable High Voltage Power Supply (PDF, hvgenpdf). .. The purpose of these articles is not only to help you repair your CD or VCR.


Electronics Repair Articles Pdf

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Due to this investigative instinct, I decided to choose the electronics repair industry .. Some simple fault of electronic equipment can be repaired by using only. Electronics Repair Manual - Ebook download as PDF File .pdf), Text File .txt) or Your manual includes a detailed safety article which outlines the steps you. This article explains the difference between a cheap analogue meter, http:// ecogenenergy.info sends a pulse to the coil and counts the.

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. Devices required to work at an appreciable power level 1 W or more require heat-sinking. Furthermore, at power levels in excess of 20 W, discrete devices are generally preferred. Encapsulation The most popular form of encapsulation used for ICs is the dual-inline package.

The package itself may be fabricated from either plastic or ceramic material with the latter using a glass hermetic sealant. Common DIPs have 6, 8, 14, 16, 18,20,24 or 40 pIns.

No holes are required to accomodate the leads of such devices which are arranged on a 0. This form ofcasing requires special handling, and repair should not be attempted unless the correct re-working equipment is available.

IC Coding Once again we find a coding system which demonstrates how our world is becoming smaller. Conventions are almost interchangeable. This is made more complicated by users often reducing the coding ofa chip to a few digits e.

Efforts are being made to reduce this confusion, but it will take a long time. Meanwhile, the technician is once again forced to look through cross-reference manuals which are fortunately easy to find and constantly updated.

Logic gates operate using binary signals. 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.

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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.

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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.

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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.

However there is one major problem with measuring a faulty transformer or coil. It may only become faulty when power is applied. The voltage between the turns may be sparking or jumping a gap and creating a problem. A tester is not going to find this fault. Secondly, an inductance meter may produce a reading but you do not know if the reading is correct.

The circuit for a ring tester can be found here: You will only get one or two return pulses, whereas a good winding will return more pulses. One way to detect a faulty power transformer is to connect it to the supply and feel the temperature-rise when nothing is connected to the secondary. It should NOT get hot. Detecting shorted turns is not easy to diagnose as you really need another identical component to compare the results. Most transformers get very hot when a shorted turn has developed.

It may deliver a voltage but the heat generated and a smell from the transformer will indicate a fault. Many electrical appliances are fully insulated and only have two leads connected to the mains.

I am not suggesting you carry out the following tests, but they are described to show how an isolation transformer works. If you touch a soldering iron on the "live" active end of the heating element it will cause a short-circuit. As soon as you earth one lead of the output an isolation transformer, the other lead becomes "active. You can use any transformers providing the primary and secondary voltages are the same. The current capability of the secondary winding does not matter.

However if you want a supply that has almost the same voltage as your "Mains," you need two transformers with the same voltages. This handy isolation transformer will provide you with "Mains Voltage" but with a limited current.

In other words it will have a limited capability to supply "wattage. This has some advantages and some disadvantages. If you are working on a project, and a short-circuit occurs, the damage will be limited to 15 watts.

If you are using two transformers with different VA ratings, the lower rating will be the capability of the combination. If the secondaries are not equal, you will get a higher or lower "Mains Voltage. Colin Mitchell designs a lot of "LED lighting lamps" that are connected directly to the mains. He always works with an isolating transformer, just to be safe.

Working on exposed "mains" devices is extremely nerve-wracking and you have to very careful. You must be sure it is a mains transformer designed for operation on 50Hz or 60Hz. Switch-Mode transformers operate at frequencies 40kHz and higher and are not covered in this discussion. To be on the safe-side, connect the unknown transformer to the output of your isolating transformer. Since the transformer will take almost no current when not loaded, the output voltages it produces will be fairly accurate.

Measure the input AC voltage and output AC voltage. If the transformer has loaded your isolating transformer it will be faulty. VA stands for Volts-Amps and is similar to saying watts. Once you have the weight of the transformer and the output voltage, you can work out the current capability of the secondary. You can check the "quality" of the transformer, the regulation by fully loading the output and measuring the final voltage.

If the transformer has a number of secondaries, the VA rating must be divided between all the windings. A common opto-coupler is 4N The light is proportional to the signal, so the signal is transferred to the photo transistor to turn it on a proportional amount. The 4N35 opto-coupler schematic is shown below: You need to set-up the test-circuit shown above with a 1k resistor on the input and 1k5 on the output.

The opto-coupler should be removed from circuit to perform this test. A transistor is sometimes referred to as BJT Bi-polar Junction Transistor to distinguish it from other types of transistor such as Field Effect transistor, Programmable Unijunction Transistor and others. In the following diagram, two diodes are connected together and although the construction of a transistor is more complex, we see the transistor as two diodes when testing it.

All transistors have three leads. Base b , Collector c , and Emitter e. For an NPN transistor, the arrow on the emitter points away from the base. It is fortunate that the arrow on both symbols points in the direction of the flow of current Conventional Current and this makes it easy to describe testing methods using our simplified set of instructions. The symbols have been drawn exactly as they appear on a circuit diagram.

There is no difference between the two. The difference is the circuit. And the only other slight difference between transistors is the fact that some have inbuilt diodes and resistors to simplify the rest of the circuit. All transistors work the same way. The only difference is the amount of amplification they provide, the current and voltage they can withstand and the speed at which they work. For simple testing purposes, they are all the same.

When the base voltage is higher than the emitter, current flows though the collector-emitter leads. As the voltage is increased on the base, nothing happens until the voltage reaches 0.

At this point a very small current flows through the collector-emitter leads. As the voltage is increased, the current-flow increases. At about 0. That's how it works. If the voltage on the base is 0v, then instantly goes to 0. The transistor is said to be working in its two states: OFF then ON sometimes called: If the base is delivered 0.

Since a transistor is capable of amplifying a signal, it is said to be an active device. Components such as resistors, capacitors, inductors and diodes are not able to amplify and are therefore known as passive components. That's what this test will provide. It will measure resistance values normally used to test resistors - you can also test other components and Voltage and Current.

We use the resistance settings. It will be the top scale. The scale starts at zero on the right and the high values are on the left.

This is opposite to all the other scales. How to read: When the needle swings to "1" on the "x" setting, the value is ohms. Use this to work out all the other values on the scale. Resistance values get very close-together and very inaccurate at the high end of the scale.

For a PNP transistor, set the meter to "x10k" place the leads on the transistor and when you press hard on the two leads shown in the diagram below, the needle will swing almost full scale. Keep trying a transistor in all different combinations until you get one of the circuits below.

When you push on the two leads, the LED will get brighter. The leads of some transistors will need to be bent so the pins are in the same positions as shown in the diagrams. This helps you see how the transistor is being turned on. This heat must be conducted away from the transistor otherwise the rise may be high enough to damage the P-N junctions inside the device.

This draws heat away, allowing it to handle more current. Low-power signal transistors do not normally require heat sinking. Some transistors have a metal body or fin to connect to a larger heatsink. If the transistor is connected to a heatsink with a mica sheet mica washer , it can be damaged or cracked and create a short-circuit. See Testing Mica Washers.

Or a small piece of metal may be puncturing the mica. This is very important as mica is a very poor conductor of heat and the compound is needed to provide maximum thermal conduction.

They have forward and reverse voltage ratings and once these are exceeded, the transistor will ZENER or conduct and may fail. In some cases a high voltage will "puncture" the transistor and it will fail instantly. In fact it will fail much faster via a voltage-spike than a current overload.

It may fail with a "short" between any leads, with a collector-emitter short being the most common. However failures will also create shorts between all three leads. A shorted transistor will allow a large current to flow, and cause other components to heat up. Transistors can also develop an open circuit between base and collector, base and emitter or collector and emitter.

The first step in identifying a faulty transistor is to check for signs of overheating. It may appear to be burnt, melted or exploded. When the equipment is switched off, you can touch the transistor to see if it feels unusually hot. The amount of heat you feel should be proportional to the size of the transistor's heat sink. If the transistor has no heat sink, yet is very hot, you can suspect a problem.

Always switch off the equipment before touching any components. The important parameters are: Points to remember: The transistor will amplify analogue signals but when the signal is 0v then immediately goes to a voltage above 0.

It is tested like an ordinary transistor but the low value resistor between base and emitter will produce a low reading in both directions.

They are internally connected in cascade so the gain of the pair is very high. This allows a very small input signal to produce a large signal at the output. The second advantage of a Darlington Transistor is its high input impedance.

It puts very little load on the previous circuit. Darlington transistors are tested the same as an ordinary transistor and a multimeter will produce about the same deflection, even though you will be measuring across two junctions, and a base-emitter resistor is present.

These are all names given to a transistor when it is used in a particular circuit. ALL these transistors are the same for testing purposes.

We are not testing for gain, maximum voltage, speed of operation or any special feature. This allows them to pass very high currents without getting hot. All the leads will show a short circuit. This due to the fact that the Gate needs 2v - 5v to turn on the device and this voltage is not present on the probes of either meter set to any of the ohms ranges.

You need to build the following Test Circuit: Removing your finger will turn the LED off. It only conducts current between Anode and Cathode in one direction and it is mainly only used in DC circuits.

When it is used with AC, it will only conduct for a maximum of half the cycle. To understand how an SCR "latches" when the gate is provided with a small current, we can replace it with two transistors as shown in diagram B above. When the ON button is pressed, the BC transistor turns on.

This turns ON the BC and it takes over from the action of the switch. Measure the anode-to-cathode resistance in both the forward and reverse direction; a good SCR should measure near infinity in both directions.

Momentarily touch the gate lead to the anode while the probes are still touching both leads; this will provide a small positive turn-on voltage to the gate and the cathode-to-anode resistance reading will drop to a low value. Even after removing the gate voltage, the SCR will stay conducting. Disconnecting the meter leads from the anode or cathode will cause the SCR to revert to its non-conducting state.

When making the above test, the meter impedance acts as the SCR load.

The lamp should not illuminate. If it illuminates, the SCR is around the wrong way or it is faulty. Press Sw1 very briefly. The lamp or motor will turn ON and remain ON.

Release Sw 2 and press it again. The Lamp or motor will be OFF. This device will conduct current in both directions when a small current is constantly applied to the Gate. If the gate is given a small, brief, current during any instant of a cycle, it will remain triggered during the completion of the cycle until the current though the Main Terminals drops to zero. This means it will conduct both the positive and negative half-cycles of an AC waveform. If it is tuned on with a brief pulse half-way up the positive waveform, it will remain on until the wave rises and finally reaches zero.

TRIACs are particularly suited for AC power control applications such as motor speed control, light dimmers, temperature control and many others. The plastic eventually becomes carbonized and conducts current and can affect the operation of the appliance. You can see the difference between a mica sheet washer and plastic by looking where it extends from under the transistor. Replace all plastic insulators as they eventually fail.

These can consist of two wires inside a plastic holder or a glass tube or special resistive device. The purpose of a spark gap is to take any flash-over from inside the tube , to earth. This prevents damage to the rest of the circuit. However if the tube constantly flashes over, a carbon track builds up between the wires and effectively reduces the screen voltage. Removing the spark-gap will restore the voltage. These are not available as a spare component and it's best to get one from a discarded chassis.

The author has had a 3 metre cable reduce the signal to "snow" so be aware that this can occur. This can result in very loud bangs in the sound on digital reception. These two conductors are normally the Active and Neutral. Should the imbalance current reach 30mA the sensor will "trip" and remove the voltage and current from the line being monitored. Some detectors will trip at 15mA.

You cannot alter the sensitivity of the device however there are a number of faults in these devices that can be fixed. In some devices the contact pressure for the 10Amp or 15 Amp contacts is very weak and they arc and produce an open circuit.

The result is this: 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.

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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. In many applications. MT2 Gate. Triacs can be triggered by positive and negative voltages applied between G and MTI with positive and negative voltages present at MT2 respectively.

In order to simplify the design of triggering circuits. Like their half-wave counterparts. A typical diaG conducts heavily. Spikes can also be radiated from wiring and may cause an appreciable level of RF noise which may affect radio reception over some distance.

Once in the conducting state. Triacs thus provide full-wave control and offer superior performance in AC power control applications when compared with thyristors which only provide half-control.

MT1 Figure 1: Triac symbol and connections Page 1. The switching action generates spikes which are carried for some distance along the supply lines. Triacs have three terminals known as main terminal one MTl.

Faults can usually be detected by removing the device from the circuit and testing it with an ohmmeter. This causes the main fuses to blow. Damage in this area is usually easy to spot. Such filters can be subject to considerable loading. Thy" should be followed. Warning Triacs are often used in high-power applications and AC main supply lines.

When connecting or disconnecting such devices. Triac casings and terminals radiation. The filter capacitor normally fails in a short-circuitcondition.Fully recommended if you want to be a professional in modification of small switch mode power supplies repair. Electronic repair is a skill and once you had mastered it, no one can snatch your knowledge away. 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.

This device will conduct current in both directions when a small current is constantly applied to the Gate. A Load Resistor is generally connected across the output of a circuit and turns the energy it receives, into heat. It uses 3 ordinary diodes to increase the output voltage of a 3-terminal regulator by 2. The market is filled with loads of washing machine of various sizes manufactured by various brands.

Voltage "D" should be about v. This is because the meter is actually measuring a voltage across a component and calling it a "resistance.

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