Test A Transistor

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

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How a transistor works

For a standard NPN transistor, you need to apply a voltage of about 0.7V between the base and the emitter to get the current flowing from base to emitter. When you apply 0.7V from base to emitter you will turn the transistor ON and allow a greater current to flow from collector to emitter.

Explanation of how transistors work

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:

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

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.

The schematic symbols for these transistor looks like this: Image result for npn symbol

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

NPN Transistor

PNP Transistor

N (-ve)

P (+ve)

N (-ve)

P (+ve)

N (-ve)

P (+ve)

Emitter

Base

Collector

Emitter

Base

Collector

Alternatively

Buy a DCA55 and forget testing with a multimeter.

failedvt308

One thought on “Test A Transistor

  1. The suggested method works well, but can give misleading results depending on the meter used……

    1/ Using an analogue meter e.g. AVO 8
    The polarity of the test leads is reversed i.e. the positive lead for volts/amps becomes is negative on the ohms range.

    On the highest Ohms range (x100) the open circuit voltage is 15v, which is enough to breakdown the E-B junction of most transistors. It is better to use the (x1) range as the open circuit voltage is only 1.5v.

    2/ Using a Digital meter
    The full scale voltage on the ohms ranges is less than 500mV for most DMMs (I’ve measured 100-300mV on mine), which is not enough to turn on a silicon transistor.
    Better to use the ‘Diode’ test which will measure forward voltages up to 2v or so.
    Of course a lot of the cheaper models have a dedicated ‘Transistor Test’ function.

    Jim

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