31
Subject to change without notice
giving an ellipse-shaped display. The position and opening width
of the ellipse will vary according to the impedance value (at 50Hz)
of the component under test.
A horizontal ellipse indicates a high impedance or a relatively
small capacitance or a relatively high inductance.
A vertical ellipse indicates a small impedance or a relatively
large capacitance or a relatively small inductance.
A sloping ellipse means that the component has a
considerable ohmic resistance in addition to its reactance.
The values of capacitance of normal or electrolytic capacitors
from 0.1µF to 1000µF can be displayed and approximate values
obtained. More precise measurement can be obtained in a
smaller range by comparing the capacitor under test with a
capacitor of known value. Inductive components (coils,
transformers) can also be tested. The determination of the
value of induct ance 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,
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 junction
changing from the conducting state to the non conducting
state. It should be noted that both the for ward and the reverse
characteristic are displayed simultaneously. This is a two-
terminal test, therefore testing of transistor amplification 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 reverse 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 for ward
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 (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 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. 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 components.
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
Component Texter
