Testing diodes and transistors with a meter

A meter's diode-test mode will tell you whether a diode or an ordinary transistor is alive, which way round it is, and often what type it is — in under a minute and without a datasheet. It is the most useful bench skill on this page.

Diodes first

A diode conducts one way and blocks the other. In diode-test mode the meter pushes about a milliamp through and displays the forward voltage drop in volts.

1. 1

   Set the dial to the diode symbol, red lead in the volts socket

   Power off, and the part out of circuit or with one leg lifted.

2. 2

   Red on the anode, black on the cathode

   The cathode is the end with the printed band. You should read between 0.5 and 0.7 V for a normal silicon diode.

3. 3

   Swap the probes

   Now it should read OL — over limit, no conduction.

A transistor is two diodes back to back

This model is not how a transistor works — it explains nothing about gain — but as a test model it is exactly right, and it is what makes a meter useful here.

So an NPN reads like two diodes with their anodes joined at the base, and a PNP like two diodes with their cathodes joined there. Six measurements — every pair of legs, both ways round — identify the part completely.

Finding the base, and the type

1. 1

   Hunt for the leg that conducts to both others in the same direction

   Put the red probe on one leg and touch black to each of the other two in turn. Then move red to the next leg and repeat. One leg out of three will give you a reading of about 0.7 V to both of the others. That leg is the base.

2. 2

   Note which probe was on the base when it conducted

   If it conducted with red on the base, it is an NPN. If it needed black on the base, it is a PNP. That is the whole test.

3. 3

   Confirm collector and emitter are open to each other

   Between the two non-base legs you should read OL in both directions. A reading here means the transistor is shorted — a very common failure, and usually the reason a circuit blew a fuse.

Using hFE, if your meter has the socket

The little socket marked hFE measures current gain. Insert the transistor into the holes matching its type and pin-out and you get a number — typically 100 to 800 for a small-signal part.

Treat it as a pass or fail. The meter measures at one fixed current, gain varies enormously with current and temperature, and two transistors from the same bag can differ by three times. A reading in the hundreds means the transistor works. Comparing two readings tells you almost nothing worth having.

Parts this test does not work on

• Darlingtons (TIP122, BD679) contain two transistors in series, so the base-emitter reading is about 1.2 to 1.4 V — double. Many meters cannot supply enough voltage to turn one on at all and it reads open. That is not a fault.
• MOSFETs have an insulated gate that does not conduct in either direction, so the two-diode test does not apply. Gate to source reads OL both ways. You will find the body diode from source to drain, which reads like a normal diode one way — and that diode is present whether the part is good or blown.
• Transistors with a built-in resistor (digital transistors, marked DTC or DTA) read low resistance base to emitter, not a diode drop.
• Anything still soldered in circuit. Other parts sit in parallel with the junctions you are testing. Lift the base leg at minimum, and expect misleading readings if you do not.

Why a transistor failed

Replacing a shorted transistor without answering this just destroys the new one. Three causes cover most of it:

• No flyback diode across an inductive load. Switching off a relay coil or a motor produces a large reverse spike that punches straight through the transistor. Fit a 1N4007 across the coil, banded end to the positive side.
• No base resistor. Connecting a microcontroller pin directly to a base is a forward-biased diode across the pin. It takes out the transistor, the pin, or both. 1k is the usual value.
• Beyond its rating. Check the collector current and the power dissipation against the datasheet. A BC547 is a 100 mA part and will not drive a motor, however correct the circuit around it.