quick test for transistors

[vinke]

hi guys,
im working on a board and i was wondering if there was a quick and simple way of testing transistors.
thks.

 

[JimB]

The first test, which is not 100% but is a good start:

Assuming you are using a DMM with a "diode" range,

1 Put the test meter positive lead to the base of the transistor and then measure to the emitter and then to the collector.

2 Put the test meter negative lead to the base of the transistor and then measure to the emitter and then to the collector.

With the leads one way you should measure about 0.7v, them the other way round you should measure an open circuit.

This simple test will usually find open or short circuit junctions in bipolar transistors (not FETS or MOSFETS), but is easily confused by the rest of the circuit still connected to the transistor.
Dont forget that the power to the circuit board must be turned off.

JimB

 

[vinke]

thks buddy.i would like to know if it also applies for darlingtons

 

[JimB]

Yes, should be no problem with darlingtons either.
What you should see with a darlington is a higher base-emitter voltage, about 1.2 to 1.4 volts.
This is due to there being two junctions in series.

JimB

 

[kchriste]

this simple test will usually find open or short circuit junctions in bipolar transistors

Don't forget to do a diode test from the collector to emitter. This can be shorted and the BE & CB tests will still give you a pass reading.

 

[Russlk]

I have never seen a collector-emitter short that was not shorted all the way around.

 

[stevez]

While possibly obvious to the professionals, mis-identification of leads can cause some confusion. I friend and fellow hobbyist was going in circles trying to debug a simple circuit using common 2N2222 transistors. Turns out that the leads were different between manufacturers. We dropped them into the transistor checker on my DMM - which identifies the leads - transistors worked fine once wired correctly.

 

[Russlk]

A 2N2222 is an NPN transistor in a TO-18 metal can, the tab indicates the emitter. You won't find one different unless it is mismarked by error.

 

[stevez]

It may be the result of memory lapse on exactly which transistor but it was a general purpose one - and we stumbled on the problem by putting the transistor in the tester. It identified the leads - indicating them as different than what he had been expecting (using a different mfr's datasheet). We pulled a datasheet for the device in hand - it matched the transistor. Once the leads were in the right place the circuit worked.

I do not recall if one might have been metal can - the other plastic. Again, maybe be one of those things that's obvious to the professionals but it was not obvious to my pal - or me initially.

 

[Russlk]

Plastic transistors are a problem, there are 3 leads and 9 ways to bring them out, and all 9 have been used.

 

[kchriste]

I have never seen a collector-emitter short that was not shorted all the way around.

I've encountered this over ten times now.

Plastic transistors are a problem, there are 3 leads and 9 ways to bring them out

The two most common configurations, when looking at the flat side of a TO-92 with the leads pointing down, are EBC and ECB.

 

[vinke]

With the leads one way you should measure about 0.7v, them the other way round you should measure an open circuit.


JimB

I knew about that but why for some transistors i dont get this base -emitter voltage drop.the meter i used can read very small voltages(mV) and the transistor is good as it operated a small amplifier circuit ive built.

 

[mvs sarma]

I knew about that but why for some transistors i dont get this base -emitter voltage drop.the meter i used can read very small voltages(mV) and the transistor is good as it operated a small amplifier circuit ive built.

Hi Vinke,
your DMM behavior also matter-- some don't rearad forward voltage of diodes or LEDs. how ever you can test them while the board is powered, you can measure the voltage across base and emitter-YOU GET AROUND .6 TO .7 VOLTS DC -OFCOURSE EXCEPT WHEN THE DEVICE IS WORKING AS AN OSCILLATOR.

the voltage drop across emitter resistor is another indicator after base emitter drop- that the device is functioning-

again not when collector emitter short as kchriste pointed out.

if this short is there, you can get direct behavior of potential divider formed by collector resistor and emitter resistor (almost).

 

[AllVol]

With most 2N2222's in T0-92 package, if you hold the transistor between thumb and forefinger (right-handed) with the flat side against your finger, the emitter will usually be the bottom-most pin.