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Electronic Component Testing Cheatsheet

When doing a repair job, it's nice to have a handy testing cheatsheet for a range of electronic components. So far I haven't found one, so I created by own here which covers everything from very basic resistors, to MOSFETs and gas glass surge arrestors using a multimeter and other tools. I include simple things like fuses and resistors to cater for absolute beginners.


The most simple of tests. Switch to resistance setting and put probe at both ends. Low resistance around 0Ω will indicate a good fuse. If you have a continuity setting, the multimeter will beep for you. If you have no beep, or you have OL (open line) the fuse is bad.


Set the multimeter to the ohms (Ω) reading and apply a probe to each end. Have a resistor chart handy to compare the colour code to the reading on your multimeter.

An OL reading means open line or broken connection. No current will pass, so is a bad resistor.

An almost 0Ω reading is a short. Of course, there are very low value resistors, for example BROWN BLACK SILVER GOLD, totalling 0.02Ω, so always check the resistor chart to be sure.


You can test for a short on a capacitor, on the PCB, using the resistance or continuity setting. Put a probe on both ends and if there is a very high ohms reading into the MΩ and finally to OL or open line then the cap is not shorted.

If there is a low resistance across the terminals, then you have a shorted capacitor which can allow damage to other components.

Beware of false positives. Parallel components can look like shorts so remove the cap and test it if unsure.

For capacitance tests, always best to test off the PCB if possible, though you can be lucky on the board. Use the capacitor test on your multimeter if you have one, and if you get the expected reading in nano-farads or micro-farads, then all well and good.

If you get an unexpected reading, remove from the board and try again after discharging it.

ESR tests can also be performed with an ESR meter on the board. If the values look pretty much around the values on the table printed on the meter, you are likely to be OK. You can dig into specific datasheets to check exact expected ESR values, but it's unlikely to be the cause of a fault if it's in the range.

You can also use a GM-328 to check both capacitance and ESR in one go. But the component needs to be off the board and plugged into the tester. Good option if you are still not sure.


Using the resistor setting, put the probes on either end of the inductor. There should be an initial reading that quickly drops to 0Ω. This means the inductor is OK.

If you get OL, you have a break in the inductor and it is bad.


Use the diode setting for this test.

Place the positive probe at the end without the line marker. Place the negative probe on the end with the line marker. There should be a reading somewhere between 0.2V and 0.7V which is the amount of forward voltage the diode needs to start passing current. So that reading indicates good in that direction.

Swap the leads and you should have a reading of 0L indicating no current can flow in that direction. Which is good.

You can also test for a short by using the resistance setting. If you get a very low reading around 0.01 etc in either direction, you have a shorted out diode.

Sometimes you can get false positives with parallel components, so remove the diode from the board to test if in doubt.

BJT Transistors

NPN transistors can be tested by first searching for the part number online to ensure it is an NPN. Locate the Base pin if it marked on the PCB. Otherwise find it by locating a datasheet for it.

Use the diode setting for this test.

Put the positive probe on the Base and the negative to the Collector and Emitter in turn. Both of those should show a forward Voltage of around 0.5V.

Switch the probes around with the negative on the Base and positive to either the Collector and Emitter pins. Both should show OL????

Finally, positive probe on the Emitter and negative probe on the Collector will should ???. Reverse should show ???.


Place you finger on all three pins for a moment to de-charge the MOSFET. MOSFETs have a little capacitance in them which is handy for the test.

Switch to diode setting.

Place the positive on the Source pin. Touch the Gate pin very briefly using the negative probe. You may see a few numbers display briefly in the multimeter output.

The MOSFET will contain a small charge.

Touch the Drain pin and you will get a reading of around ???V.

You can reset with you finger and repeat the test. If the above behaviour is not given, further investigation might be required, but the part should be suspected.

If you get no response from the MOSFET at all, add a little pressure to make better contact from probe to pin.

Reversing the test:

Positive probe to Source, and Negative probe to Gate should show ???

Positive probe to Source, and Negative probe to Drain should show ???

Gas Glass Surge Arrestors / Gas Glass Discharge Tubes

I include this because they look like a cross between a zener diode and the colour bands of a resistor. Little is written about them, but they can be located in some switch mode power supplies.

Switch to continuity mode.

Connect the probe at both ends.

If you get a continuity beep then the tube is good. Otherwise it is bad.


These are resistors that vary their value according to the temperature. The ohms value can go up or down when the temperature increases.

Switch to the resistance setting.

Connect your multimeter to both sides using alligator clips if you have them. You can put the thermistor into the fridge and close the door with the multimeter leads hanging out. Observe the increasing or decreasing resistance.

You can do the same by walking outside in the warm sun and observe the reverse effect.

Bridge Rectifiers

A bridge rectifier is really just a housing of four diodes with four leads:

+ ~ ~ -

Most demos just show the standard test. Here is the entire combination of lead test and approximate expected outputs:

Anything that does not line up with the results should be suspected.

Again, parallel components can cause false positives. Remove the part and test again.

Four Pole Relays

These relays have a pair of pins on either side. The side with two even pins are connected with a coil.

The other two pins are offset in position from each other and is the switching part.

If you measure with your resistance setting, you should receive a reading. This means the coil is fine.

If you measure the switch side of the relay with continuity, there should be no connection.

electronic_component_testing_cheatsheet.txt · Last modified: 2021/02/18 10:41 by sausage