Hameg HM504-2, Manual

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Subject to change without notice
34
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 comparison
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 between 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 (
CT or ground) for the same terminal is then absolutely
necessary. A connection inversion effects a rotation of the test
pattern by 180 degrees about the centre point of the scope
graticule.
Pay attention to the usual caution with single MOS
components relating to static discharge or frictional
electricity!
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 measuring 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. Complex displays may be caused by a shunt
connection of real or complex impedance, especially if they
are of relatively low impedance at 50Hz, to the component
under test, often results differ greatly when compared with
single components. In case of doubt, one component terminal
should 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 under test itself may
contain a reference circuit (e.g. a second stereo channel, push-
pull amplifier, symmetrical bridge circuit), which is not defective
and can therefore be used for comparison.
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