6-
14
Important Note
Use of this instrument may alter the current gain (h
FE
or ß) of a bipolar transistor whenever the
emitter is tested. Either the base-emitter or collector-emitter test circuits satisfy this criterion.
While heating of the device due to the current produced by the instrument may cause a
temporary change in h
FE
(most noticeable in the low range), a permanent shift in h
FE
may occur
whenever the base-emitter junction is forced into reverse breakdown (~6-20 Volts). If the voltage
is above 6 Volts, then the magnitude of the shift depends on the duration of the test and the
resistance selected. Reducing the voltage to 5 Volts or less will avoid this problem.
Most bipolar transistor circuit designers take into account a wide variation in h
FE
as a normal
occurrence and design the related circuitry to function properly over the expected range of h
FE
.
The effects mentioned above are for the most part much smaller than the normal device variation
so that the use of this instrument will have no effect on the functionality of good devices and can
fulfill its intended purpose of a means to locate faulty components. However, some circuits may
depend on the h
FE
of the particular part in use e.g. instrumentation that is calibrated to certain h
FE
value, or precision differential amplifiers with matched transistors. In such instances, this
instrument should not be used as its use may cause the h
FE
to shift outside the limited range
where calibration can correct for any change.
Suggestions to minimize effects on bipolar transistors:
1. Use 5 Volts or less for testing the base-emitter or collector-emitter.
2. If using 6 Volts or greater, then keep the duration of the test as short
as possible.
3. Identify the base, emitter and collector pins of the device and then test
the collector-base junction to determine whether it is an NPN or PNP.
Since the emitter is not tested there will be no effect on h
FE
regardless of the selected
voltage. Then use the three terminal test described in the "Using The DC Source To Test
Transistor Operation" later in this section.
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