Electronic Fault Finding & Diagnosis
A circuit is a collection of componenets connected together to perform a perticular function. Failure of any of the components will drastically effect the expected operation or fail alltogether.
Faulty components in a circuit produces symptoms which could be used to pinpoint the failed component.Visual observation of the circuit
to find hints such as burnt, dis-colored (due to heat) or loose/dry joints,shorted due to residue/dust/moisture components in a printed circuit helps fast repair.Knowedge of the useage of the circuit is as important as the components used in the circuit and theory or operarion.
 
Stress induced in coponents due to continous temperature variations shorten life of materials used in components and results in eventual failure.Improper circuit design that operate components at or near there top end of performance will bring about stress to components, compounded by environmental stress(such as poor ventilation/moisture Etc.) hasten failure.Most circuit designers set there point of operation of a circuit in the safe middle grounds.Another way a circuit can effect and fail is fast hight speed trasient power pulses.These pulses that appear randomly due to proximity to high power /lightning, fast acting switch functions or borne by the mains supply will effect active components and results erratic operation or failure.
Circuit diagram of the circuit that is faulty when available repair /restore function time is reduced drastically.Besides diagram gives all input out put and component values.When diagram not available a sketch made saves time.
Ways in which passive components fail.
Resistors
A Resistor is failed if it is out of its given value of tolerance. Ex. if a resistor 100k at 10%+/- tolerance when measured out of the circuit
should have value between 90k -110k.any value above or below indicates the measure of failure of the resistor.Most resistors depending of type goes open or shorted.
Capacitors
Capacitors Ex.Leakage current of an electrolytic capacitor exceeds (given value is 10 micr Amp) to 20 micro Amp above the tolerance the capacitor has failed at least partially.Most capacitors depending of type and votage goes shorted(Ex.gets heated in a power supply or smokes or burst open) RF capacitors change value altering the resonance point.Tantalumn capacitors burst opnen and produce severe burns.
Inductors
Inductors gets shorted/open altering the frequency or burn depending on use.
Ways in which active components fail
 Ways in which active components fail.
Silicon diodes
If a P(germnium) or N(silicon) type diode is connected across a variable power supply that can adjust from minus to zero then to positive (Ex.-ninus3V-0 zero Volt +3V ) in series with a DC mA meter, and as the voltage is increased from minus or Reverse towards the zero value the mA meter indicate no current.From 0Voltage as the voltage is increased to about 100mV the DC mA meter in series start to show some effect and when the value reaches 200mV for germanium, 600mV for Silicon a sudden steep raise of current is shown.This point is known as Forward Bias without which the diode/rectifire remains Off and no conduction of current is shown in the DC mA meter.
In fact a few micro A will allways flows in the opposit direction of the diode circuit known as reverse current and reverse voltage.If this value is increased the diode rectifire breaks down and unless a limiting resistor is not placed in series will results a short circuit.Ref Fig 1.2. Typicaly when the diode is forward biased about 25 Ohm resistance at 1mA is shown.Refer to data for accurate values. How to test a diode.Use Diode function of Multimere.When marked end(K=Kathode) is connecte with + lead and -lead to Anode the diode goes to forward bias=ON,and the meter indicates PN junction on voltage typically 0.6V for Si diode and 0.3V for a Ge diode.When meter leads are reversed neglegibel amount of current is read.Shorted diodes will ring the multimeter and full voltage will be shown and open diode no voltage shown in any direction.
In circuit testing of diodes can be done the same manner.But one leg out and test will confirm any doubts.Diodes that are used to suppress trancient pluses (Ex.diode used accross the relay coil/diode used with series resistor across a switch contact )when fail get shorted.
Zenner voltage regulator Diodes
Zenner behaves much the same manner as a diode in forward bias mode.But when zenner is reverse biased(voltage going in negative direction) as the voltage reaches the Vz (this value is set during the manufacture.Ex.1.2VZat 100mWatt, 6.8VZ at 25Watt or more) the zenner opens fully brining the on resistance value of the PN junction to few Ohms. Ref Fig.1.3 and this quality of the zenner is used to make stabilise voltage across a load.Ref Fig 1.4 Rs is the current limiter resistor and RL is the load and the voltage remains constant set by the zenner diode operating V value. When the zenner gets shorted the power supply fuse opens or the limiting resistor gets heated(discolor) and burn.If not burn out put voltage will be lower than the required .Testing of zenner diodes is same as diodes.But zenner diode has wattage rating respect this value when replacing.
Transistors(BJT)
FET (IGFET)
MOSFET
- Basic fault diagnosis Circuits and test readings
- Single stage Transistor Amplifier
- basic principles
- Resistor faults...capacitor faults...transistor faults
- Components and common faults
- Operating principles of common active components
- Measuring instruments and testing methods
- Fault finding on electronic instruments and systems
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