Subject to change without notice
30
Component Tester
transformers) can also be tested. The determination of the
value of inductance needs some experience, because inductors
have usually a higher ohmic series resistance. However, the
impedance value (at 50Hz) of an inductor in the range from 20
Ohm to 4.7k
Ω
can easily be obtained or compared.
Testing Semiconductors
Most semiconductor devices, such as diodes, Z-diodes, tran-
sistors, FETs can be tested. The test pattern displays vary
according to the component type as shown in the figures
below. The main characteristic displayed during semiconduc-
tor testing is the voltage dependent knee caused by the
junction changing from the conducting state to the non
conducting state. It should be noted that both the forward and
the reverse characteristic are displayed simultaneously. This
is a two-terminal test, therefore testing of transistor amplifi-
cation is not possible, but testing of a single junction is easily
and quickly possible. Since the test voltage applied is only very
low, all sections of most semiconductors can be tested
without damage. However, checking the breakdown or re-
verse voltage of high voltage semiconductors is not possible.
More important is testing components for open or short-
circuit, which from experience is most frequently needed.
Testing Diodes
Diodes normally show at least their knee in the forward
characteristic. This is not valid for some high voltage diode
types, because they contain a series connection of several
diodes. Possibly only a small portion of the knee is visible.
Zener diodes always show their forward knee and, depending
on the test voltage, their zener breakdown forms a second
knee in the opposite direction. If the breakdown voltage is
higher than the positive or negative voltage peak of the test
voltage, it can not be displayed.
The polarity of an unknown diode can be identified by compari-
son with a known diode.
Testing Transistors
Three different tests can be made to transistors: base-
emitter, base-collector and emitter-collector. The resulting
test patterns are shown below.
The basic equivalent circuit of a transistor is a Z-diode be-
tween base and emitter and a normal diode with reverse
polarity between base and collector in series connection.
There are three different test patterns.
For a transistor the figures b-e and b-c are important. The figure
e-c can vary; but a vertical line only shows short circuit condition.
These transistor test patterns are valid in most cases, but there
are exceptions to the rule (e.g. Darlington, FETs).
With the COMPONENT TESTER, the distinction between a P-
N-P to an N-P-N transistor is discernible. In case of doubt,
comparison with a known type is helpful. It should be noted
that the same socket connection (COMP. TESTER or ground)
for the same terminal is then absolutely necessary.
A connection inversion effects a rotation of the test pattern by
180 degrees round about the center point of the scope
graticule.
In-Circuit Tests
Caution!
During in-circuit tests make sure the circuit is dead. No
power from mains/line or battery and no signal inputs
are permitted.
Remove all ground connections including Safety Earth
(pull out power plug from outlet). Remove all measur-
ing cables including probes between oscilloscope and
circuit under test. Otherwise both COMPONENT TESTER
leads are not isolated against the circuit under test.
In-circuit tests are possible in many cases. However, they are
not well defined. This is caused by a shunt connection of real
or complex impedances - especially if they are of relatively low
impedance at 50Hz - to the component under test, often
results differ greatly when compared with single compo-
nents.
In case of doubt, one component terminal may be unsoldered.
This terminal should then not be connected to the ground
socket avoiding hum distortion of the test pattern.
Another way is a test pattern comparison to an identical circuit
which is known to be operational (likewise without power and
any external connections). Using the test prods, identical test
points in each circuit can be checked, and a defect can be
determined quickly and easily.
Possibly the device itself under test contains a reference
circuit (e.g. a second stereo channel, push-pull amplifier,
symmetrical bridge circuit), which is not defective.
