Re: Wondering...Diode test on transistors/BRs/predrivers

Ok, I'm gonna try to explain a bit about this test.
The diode test on our DMMs is meant as a ballpark check of the forward
voltage drop across a single P/N junction - diodes.
Testing with a DMM on the ohms scale without a diode test range, can
be unpredictable, as the applied voltage and forward current may not
be sufficient to give a meaningful reading.
For diode test, they use a small applied voltage typically less than 2
volts, and a current limiting resistor internally, to limit to a
couple of mA.
On a good diode, you should see a nominal forward voltage drop of .3
to .7 volts, and nothing when reverse biased.
These are ball park values and are dependant upon the type of material
used to make the PN junction, such as silicon, germanium, point
contact, etc.
A standard transistor has three legs and has either NPN or PNP
junctions, but can be tested as if it is two diodes in series using
those three legs.
The diode test can be used to check these transistors with fairly good
reliability, but the testing more accurately defines a transistor is
bad, and cannot not certify the transistor is good, as too many other
variable come into play depending on the component and circuit it is
used in.
TIP102s are Darlington transistors, and have two transistors stacked
on one substrate in one case. The diode test is much less reliable on
this type of transistor as you have several PN junctions to deal with
at once, and some Darlingtons have internal biasing resistors to
complicate things too.
Again a zero reading is a good indicator of a shorted transistor, but
any other readings are nebulous and should not be relied upon to
certify a Darlington transistor as defacto good.
In all cases, the best methode is to test the component out of circuit
to remove influences from other circuitry.
For example, a good coil diode will test as shorted, unless you remove
it from the coil first.
So again, testing in-circuit is only going to give a general
impression of a components condition. For in circuit testing, an A-B
comparison with a working circuit board is a good practice.

Scott


On Sep 16, 10:28 pm, Paul Drabik <pdra...@xxxxxxxxxxx> wrote:
True!  When testing in-circuit, I to have seen various values outside
the usual .4-.6 range.

Just to clarify my situation, I am replacing several TIP102
transistors on a System11a board (from F14) along with the predrivers.
The customer had about 3 coils melt down on him, all on the special
coil area (J118/J119) ... so I will test to see if the upstream chip
is needed or not.

Anyway, I installed 3 Tip102's and one of the reads .38 where it
usually would be .4 to .6 which may be due to overheating on my part.
The other two are fine.

So that is why I am wondering what the possible consequences might
be.  Shorter life of the component or would it be stressing the board
elsewhere?
Remounting a System11 board is such a pain without the tear drop
shaped screw holes to hang it on.

BTW - Can I use 2N5401 in place of the 2N4401 pre-drivers?   (just
like I am using Tip102 instead of Tip122 which clay's direction
mention ... Clay's directions do not say if the predrivers can be
substituted.)

.

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