20
Subject to change without notice
hf transistors etc. should not be tested). The limitation to 10
V
p
with bipolar transistors will suffi ce mostly as usual defects
will show up.
The best method to verify whether a component is defective is
the comparison to a good one. If the lettering of a component
is not legible at least it is possible to see whether it is a npn or
pnp transistor or which end of a diode is the cathode.
Please note that reversing the test leads will also invert the
picture, i.e. turn it 180 degrees.
STOP
Before testing a component, please check its data
to be sure that it is not overloaded by the test volta-
ge of ± 10 V.
In most cases, e.g. with service and repair, it will be suffi cient
to receive a good/bad result (open, short). With MOS compo-
nents the usual precautions are to be observed, but note, that
except for a possible short MOSFETs and JFETs can not be
suffi ciently tested. Indications to be expected depend strongly
on the kind of FET:
– With depletion type MOSFETs and all JFETs the channel
will conduct if prior to testing the gate was connected to the
source. The Rdson will be shown. As this can be very low it
may look like a plain short although the part is good!
– With enhancement type MOSFETs an open will be seen in
all directions, as the threshold voltage G – S is not available.
With power MOSFETs the antiparallel diode S – D can be
seen.
Tests of components within circuits are possible in many cases
but less indicative because other components may be in parallel.
But also here the comparison with a good circuit might help. As
both circuits must be deenergized it is only necessary to switch
the test leads back and forth between both in order to localize
a defective spot. Sometimes like with stereo amplifi ers, push-
pull circuits, bridge circuits there is a comparison circuit right
on the same board. In cases of doubt one component lead can
be unsoldered, the other one should then be connected to the
ground lead. This is labelled with a ground symbol. The pictures
show some practical examples:
C o m p o n e n t T e s t e r
