Page 120 - Testing Electronic Components
P. 120
Transistor Failure
Transistor can fail in a number of different ways. Transistors have
forward and reverse current and voltage ratings like diodes do. Exceeding
either rating can destroy a transistor. A bad transistor may short-circuit
from the base to the collector or from the base to the emitter.
Sometimes a transistor is damaged so badly that short circuits develop
between all three of the leads. A short-circuit often allows a large current
to flow, and causes the faulty transistor to heat up. The transistors also
can developed open circuit between base to collector or base to
emitter.
The first step in identifying a bad transistor is to check for signs of
overheating. A bad transistor may appear to be burnt or melted. 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 transistors heat sink. If the part has a large heat sink, you can
expect it to be too hot but not until the extend of burning the hand or
fingers. If the transistor has no heat sink, yet is very hot, you can suspect
a problem. DO NOT TOUCH A TRANSISTOR IF IT IS PART OF
THE CIRCUITRY THAT CARRIES 240VAC. Always switch off the
equipment before touching any components.
Transistor Replacement
If for some reason, you cant get the exact replacement, refer to one of the
transistor substitution guides, and try to identify a near replacement.
Beware, however a substitution guide will sometimes list a replacement
for your part, even though the two parts are not very similar. The
important parameters are:
• Voltage
• Ampere
• Wattage
The replacement part should have a voltage, ampere and wattage rating
equal to or higher than the original. The best is to get an exact part
number for transistor- its worth the trouble.
TRANSISTOR SPECIFICATIONS
Diodes are numbered 1N for example 1N4148, 1N4007, 1N5408 and etc.
Transistors are numbered 2N for example 2N3904, 2N3906 and etc. The
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