How To Test A Transistor Using A DMM Or Analogue Multimeter.

Created OnAugust 28, 2019

bycrustytv

How a transistor works

A transistor acts as a switch, main current flows from collector to emitter and the signal current flows from base to emitter. The signal current at the base is what controls the switch. So a current flowing from the base to emitter is what opens the switch allowing current to flow from the collector to emitter.

NPN

To have an NPN transistor turned on to allow current to flow between the collector and emitter, you must have at least .7v applied between the base and emitter, otherwise it’s off.

PNP

Transistors are different, instead of the minimum .7V on the base, they require a minimum difference of .7V between the collector emitter voltage and the base emitter voltage. So a difference of less than .7V the transistor is off, a difference of more than .7V and the transistor is on.

The four transistor operation modes are

Saturation – The transistor acts like a short circuit. Current freely flows from collector to emitter.
Cut-off – The transistor acts like an open circuit. No current flows from collector to emitter.
Active – The current from collector to emitter is proportional to the current flowing into the base.
Reverse-Active – Like active mode, the current is proportional to the base current, but it flows in reverse. Current flows from emitter to collector (not, exactly, the purpose transistors were designed for).

When a transistor fails

One of two things usually happens:

A junction (or junctions) go short circuit (its resistance becomes very low or zero).
A junction (or junctions) go open circuit (its resistance becomes very high or infinity).

Image result for npn symbol

The above could also include junctions that may become leaky (slightly low resistance), though this is rare. Invariably this condition is usually followed soon by a complete short circuit.

A transistor has three pins: Base (b), collector (c) and emitter (e). And it comes in two versions: NPN and PNP.

DMM Or Analogue Multimeter

Set your meter to ohms on the highest range or if you’re using an auto-ranging meter just set it to ohms. With this method we will be testing only NPN or PNP you cannot test FETs this way.

You will need to ensure you have the datasheet for your transistor so you know the exact layout of the pins.

NPN

First we need to measure resistance between emitter and base, so negative probe to the emitter, positive to the base. For a good transistor you should get an ohms reading, if not your transistor will not work
Now swap the probes around, positive to the emitter, negative to the base. For a NPN you should not get a reading, if it does you transistor will not work.
Assuming test 1 and two passed correctly then we now need to measure the resistance between collector and Base. So negative probe to the collector and positive to the base. For a good transistor you should get an ohms reading, if not your transistor will not work
Now swap the probes around, positive to the collector, negative to the base. For a NPN you should not get a reading, if it does you transistor will not work.
Finally test that there is no reading betweem Collector and Emitter if there is your transistor has failed or you got the pin-outs wrong

PNP

First we need to measure resistance between emitter and base, so positive probe to the emitter, negative to the base. For a good transistor you should get an ohms reading, if not your transistor will not work
Now swap the probes around, negative to the emitter, positive to the base. For an PNP you should not get a reading, if it does you transistor will not work.
Assuming test 1 and two passed correctly then we now need to measure the resistance between collector and Base. So positive probe to the collector and negative to the base. For a good transistor you should get an ohms reading, if not your transistor will not work
Now swap the probes around, negative to the collector, positive to the base. For a PNP you should not get a reading, if it does you transistor will not work.
Finally test that there is no reading between Collector and Emitter if there is your transistor has failed or you got the pin-outs wrong.

If you have a transistor with unknown pin-out, the above method can be used to identify which type it is, NPN or PNP