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Author Topic: testing a transitor  (Read 815 times)
KC2FZN
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« on: July 18, 2009, 11:15:47 AM »

Can a transitor be tested while still on the circuit board with the radio turned off? I have an analog and digital multimeter. If it can be tested this way, how do i go about doing it?
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OBSERVER11
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« Reply #1 on: July 18, 2009, 12:03:34 PM »

no.

You need to test the device, not the surrounding components too.
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K4DPK
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« Reply #2 on: July 18, 2009, 12:49:01 PM »

You might be able to just cut the base loose and then check the B-E and B-C juntions both forward and back.  That would tell you if it's open or shorted.

Phil C. Sr.
k4dpk
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K0BT
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« Reply #3 on: July 18, 2009, 01:04:51 PM »

The factual answer is "no" but the practical answer is "sometimes".  While I agree of course that the surrounding circuitry affects the readings, you can usually get a feel for whether or not the transistor is shorted or open.

If your meter has a semiconductor test position (usually indicated with a diode symbol), test the B-C and B-E junctions.  In one direction the meter will usually give a quick beep and will show a reading of about .500 to .600.  In the other direction the meter will probably remain silent.  If you see a solid and repeatable indication like this, the transistor is probably OK.  Check a few transistors out of circuit first to get a feel for how your meter behaves, then give it a try with a in-circuit component.  The problem with this approach is that you need to interpret the readings and experience helps there.

The best way to test the transistor in circuit is to check the voltages with the equipment turned on (assuming you have steady hands).
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N1DVJ
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« Reply #4 on: July 18, 2009, 02:52:17 PM »

The answer is 'yes', but a qualified yes.  

If you have a schematic, and you can take that into consideration when you do your test, and figure out correctly how the circuit components will adjust your test results, then go ahead.  Sometimes it's easy, sometimes it's not.

Even still, you can usually find failures easily enough in-circuit.  Or opens.  The real work is when you're trying to find 'inbetweens'

Mike
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AA4PB
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« Reply #5 on: July 18, 2009, 04:53:59 PM »

In addition you can only tell a limited amount with an ohmmeter even when the transistor is out of the circuit. You can tell if it is shorted or open but you can't tell if the gain is correct.
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K4DPK
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« Reply #6 on: July 18, 2009, 07:23:47 PM »

"If it can be tested this way, how do i go about doing it?"

OK, put your DVM in "diode check"

Clip the base of the transistor loose from the circuit board.  Leave enough of the lead sticking up so you can solder it back together if the transistor is OK.

For NPN transistors, put the positive lead of the DVM on the base.  You should see .400 to .700 when you touch the negative probe to the emitter ; same with the collector.  

Reverse the probes by putting the negative DVM probe on the base and touch first the emitter, then the collector.  You should see an open circuit or infinite resistance.

Do the same thing for PNP transistors, except reverse the probes.

If you don't get this sort of indication, clip one more of the transistor leads loose from the board and re-test it.  If you don't get the correct indication then, the device is bad.

The exceptions to this would be on diode-protected mosfets and darlington transistors.

Phil C. Sr.
k4dpk
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G4AON
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« Reply #7 on: July 19, 2009, 03:35:40 AM »

When I worked in the mobile radio service industry, we all tended to have home made bipolar transistor testers and tested transistors out of circuit with them, if only to confirm the suspect one was really faulty and the replacement looked OK.

The one I made was from July 1976 "Ham Radio" magazine and was the Shirt Pocket Transistor Tester. I still have it and use it occasionally. I don't know how good or bad those transistor test facilities built into many multi-meters are, the home made one as above is spot on. I no longer have the circuit and can't locate it on-line.

The bottom line is that if you determine the device is suspect by measuring the in circuit voltages, you will remove it anyway... So you might as well accept the fact it's coming out and you can test it out of circuit.

73 Dave
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K1BXI
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« Reply #8 on: July 19, 2009, 06:29:59 AM »

I used one of these back in the 70's:

 http://www.tequipment.net/BK520B.html

Also check this:

http://www.elecfree.com/In_Circuit_Transistor_Checker.php


John
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KE3WD
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Posts: 5689




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« Reply #9 on: July 19, 2009, 09:41:08 AM »

>>"If it can be tested this way, how do i go about doing it?"

OK, put your DVM in "diode check"

Clip the base of the transistor loose from the circuit board. Leave enough of the lead sticking up so you can solder it back together if the transistor is OK.

For NPN transistors, put the positive lead of the DVM on the base. You should see .400 to .700 when you touch the negative probe to the emitter ; same with the collector.

Reverse the probes by putting the negative DVM probe on the base and touch first the emitter, then the collector. You should see an open circuit or infinite resistance.

Do the same thing for PNP transistors, except reverse the probes.

If you don't get this sort of indication, clip one more of the transistor leads loose from the board and re-test it. If you don't get the correct indication then, the device is bad.

The exceptions to this would be on diode-protected mosfets and darlington transistors.

Phil C. Sr.
k4dpk <<

Listen to Dr. Phil, This is how to do it.  

Another method, with power on for the Unit Under Test, is to use the DC voltmeter scale.  

Any silicon bipolar transistor circuit should read between 0.6 to 0.7VDC (~600-~700mVDC) from Base to Emitter.  

If you don't read that Base-Emitter volt drop, turn the unit off and check the transistor using the Diode Scale, in circuit.  If replacing a suspect transistor doesn't bring that portion of the circuit back to operation, suspect the surrounding components, reistors out of spec or perhaps a leaky capacitor nearby.  

In most circuits that are dead or drawing too much current (fuse blowers, surrounding parts overheating, etc.) you can simply "walk" the transistors using that Diode Scale method and leave them IN circuit while you do so.  

KE3WD
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AC5UP
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« Reply #10 on: July 19, 2009, 03:11:50 PM »

K4DPK wrote:

" The exceptions to this would be on diode-protected mosfets and darlington transistors. "
____________________________________

It has been a while since I've done this, but the first time I tried the diode check trick on a MOSFET it was a real learning experience... SOP for me on a bipolar device is to check B to E, B to C and C to E in both directions. Should see infinity when reversed and both ways for C to E, normal voltage drop in the forward direction with B to C showing a slightly lower number than B to E.

(everybody follow all that?)

With a MOSFET, when I touched the gate with the right polarity, source to drain turned on AND STAYED ON until I discharged the device by tying all three points together. Thought fer'shure I fried the dude first time I did that.

One other notable exception to testing in-circuit is pass transistors in a regulated linear power supply. They're in parallel so if any one of them is shorted they'll all test shorted.

Best bet is to pull the transistor(s) judged most likely to have lost their smoke as determined by voltage and signal pass-through checks. If you have a scope you can go fairly fast up through a string of circuits until you get zilch.

Back up one step and anything between those points is a prime suspect.
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K4DPK
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« Reply #11 on: July 19, 2009, 03:34:03 PM »

Had to go back and re-read what I wrote...

I got ahead of my typing.  It should have read:  diode protected transistors, mosfets and darlingtons.

Thanks for spotting that.  Sorry.

Phil C. Sr.
k4dpk
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