20
Subject to change without notice
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.
In most cases, e.g. with service and repair, it will be sufficient
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
sufficiently 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 circuit 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.
Both circuits must be de energized and it is only necessary to
switch the test leads back and forth between them in order
to localize a defective spot. Sometimes, such as with stereo
amplifiers, pushpull circuits, bridge circuits, there is a com-
parison 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
Test Equipment Depot - 800.517.8431 - 99 Washington Street Melrose, MA 02176
FAX 781.665.0780 - TestEquipmentDepot.com
