Hameg HM 203-7 Manual page 19

Testing Semiconductors
Most semiconductor devices, such as diodes, Z-diodes,
transistors, 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 semiconductor
testing is the voltage dependent knee caused by the junc¬
tion changing from the conducting state to the non conduct¬
ing 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 amplifica¬
tion is not possible, but testing of a single junction is easily
and quickly possible. Since the
CT
test voltage applied is
only very low (max. 8.5Vrms), all sections of most semicon¬
ductors can be tested without damage. However, checking
the breakdown or reverse voltage of high voltage semicon¬
ductors is not possible. More important is testing compo¬
nents 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. Z-
diodes always show their forward knee and, up to approx.
10V, their Z-breakdown, forms a second knee in the oppo¬
site direction. A Z-breakdown voltage of more than 12V can
not be displayed.
Type:
Terminals:
Connections:
Normal Diode High Voltage Diode
Cathode-Anode Cathode-Anode
(CT-GD) (CT-GD)
Z-Diodel2V
Cathode-Anode
(CT-GD)
The polarity of an unknown diode can be identified by com¬
parison with a known diode.
Testing Transistors
Three different tests can be made to transistors: base-emit¬
ter, base-collector and emitter-collector. The resulting test
patterns are shown below.
The basic equivalent circuit of a transistor is a Z-diode
between base and emitter and a normal diode with reverse
polarity between base and collector in series connection.
There are three different test patterns:
P-N-PTransistor:
Terminals:
Connections:
b-e
(CT-GD)
b-c
(CT-GD) (CT-GD)
N-P-N Transistor
l| If
-W-^—
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 cir¬
cuit condition.
These transistor test patterns are valid in most cases, but
there are exceptions to the rule (e.g. Darlington, FETs). With
the
CT,
the distinction between a P-N-P to a N-P-N transis¬
tor is discernible. In case of doubt, comparison with a
known type is helpful. It should be noted that the same
socket connection
(CT
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.
Pay attention to the usual caution with single MOS-
components relating to static charge or frictional elec¬
tricity!
In-Circuit Tests
Caution! During in-circuit tests make sure the circuit is
dead. No power from mains/line or battery and no sig¬
nal inputs are permitted. Remove all ground connec¬
tions including Safety Earth (pull out power plug from
outlet). Remove all measuring cables including probes
between oscilloscope and circuit under test. Otherwise
both CT test leads are not isolated against the circuit
under test.
In-circuit tests are possible in many cases. However, they
are not so well-defined. This is caused by a shunt connec¬
tion of real or complex impedances - especially if they are
of relatively low impedance at 50 or 60 Hz - to the compo¬
nent under test, often results differ greatly when compared
with single components. In case of doubt, one component
terminal may be unsoldered. This terminal should then be
connected to the insulated
CT
socket avoiding hum distor¬
tion of the test pattern.
Another way is a test pattern comparison to an identical cir¬
cuit 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.
Terminals:
Connections:
b-e
(CT-GD)
b-c
(CT-GD) (CT-GD)
The test patterns on page Ml6 show some typical displays
for in-circuit tests.
Subject to change without notice
Ml5 203-7