Re: Any SS foks here : 15003 readings

On Mar 23, 10:52 am, jpopel...@xxxxxxxx wrote:
On Mar 23, 8:04 am, "Peter Wieck" <p...@xxxxxxx> wrote:
(snip)> You will need a VOM capable of forward-biasing that
particular transistor, most are, some are not. You will get near-
infinite resistance in one direction low resistance in another.

They are identical transistors, so the readings should be very close
to each other. Near-identical if they are even vaguely matched. I have
not looked up that particular transistor, but forward-bias resistance
could be anything from 30-or-so to 250 ohms or more.

You will have to measure them out-of-circuit unless you have a tester
capable of in-circuit use. Most VOMs will not do that. I keep a Heath
tester that does it well-enough in-circuit to determine if the
transistor is good or bad, and I can actually match them _OUT_ of
circuit.

(snip)

The concept of an in circuit tester is pretty simple. It is an ohm
meter that applies less voltage to make its measurement, than it takes
to forward bias a silicon diode (somewhere between .1 and .4 volts,
open circuit). This allows you to check if a silicon diode junction
is short circuited (no normal diode drop needed to produce a low
resistance value) or to measure the values of resistors connected to
transistors and diodes, while keeping all the junction voltages low
enough that nothing gets forward biased enough to conduct and distort
the ohm readings.

Diode check meters (usually show a diode symbol on the selector) apply
a limited current through the probes, while displaying the open
circuit voltage (often with an implied decimal point). So if you
connect the leads across a diode or base to collector junction, etc.
you may get a reading of 50 to 70 (or 500 to 700, depending on where
the implied decimal point is), indicating .5 to .7 volts forward bias
drop. A shorted junction will read very close to zero and an open
circuit usually blanks the display, instead of showing the open
circuit voltage. Testing diode junctions in circuit will include the
effect of any other current paths on the voltage drop across a
junction, so it is not generally an in circuit test.

Yabbut...

Those two transistors (cited in the OP) are very nearly in parallel.
So any results are questionable if in-circuit. A simple diode tester
may not be enough to tell one from the other. Those resistors are only
5 ohms.

Peter Wieck
Wyncote, PA

.

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