Alright, let me tell you how I usually go about testing a MOSFET when I suspect one might be bad. It’s something I end up doing fairly often when fixing stuff.
Getting Started
First thing, I grab the suspect MOSFET and my multimeter. Gotta have a multimeter that has a diode test function – most decent ones do. It’s usually marked with a little diode symbol, kinda like an arrow hitting a wall. I switch the meter to that setting.
Next, I need to know which pin is which. You got the Gate (G), the Drain (D), and the Source (S). If I’m lucky, it’s a common package like a TO-220 and I remember the pinout (usually G-D-S looking from the front). If not, or if it’s some weird surface mount thing, I absolutely have to look up the datasheet for that specific part number. No guessing here, you’ll just confuse yourself.
The Actual Testing Steps
Okay, let’s assume it’s an N-channel MOSFET, which is pretty common. Here’s my routine:
- Initial check for shorts: Before doing anything fancy, I just quickly probe between all pairs of pins (G-D, G-S, D-S) in both directions. I shouldn’t find a dead short (like 0 ohms or a constant beep) between any of them right off the bat. If I do, it’s probably fried right there.
- Testing the Body Diode: Most MOSFETs have an internal body diode between the Drain and Source. For an N-channel, it runs from Source to Drain. So, I put the black probe (negative) on the Drain and the red probe (positive) on the Source. My meter should show a voltage drop, typically somewhere between 0.4V and 0.9V, just like a regular diode. Then I reverse the probes (red on Drain, black on Source). This time, it should read open – usually “OL” or “1” on the display. If this diode test fails, the MOSFET’s bad.
- Turning the MOSFET ‘On’ (Charging the Gate): This is the key part. The gate needs a voltage (relative to the source) to turn the MOSFET on. With the meter still in diode mode, I put the black probe on the Source pin. Then, I briefly touch the red probe to the Gate pin. Just a quick touch is enough. The meter’s voltage is usually enough to charge the gate capacitance.
- Checking if it Turned On: Okay, keep the black probe on the Source. Now, move the red probe over to the Drain pin. If the MOSFET is working and the gate got charged, the meter should now show a very low reading, maybe close to zero volts or it might even beep if it has a continuity function tied to the diode setting. This means the Drain-Source path is now conducting – the switch is ‘on’.
- Turning the MOSFET ‘Off’ (Discharging the Gate): To turn it off, we need to discharge the gate. The easiest way I do this is to just briefly short the Gate and Source pins together. Sometimes just touching them both with my finger works, or I might use a screwdriver tip. You just need to equalize the charge.
- Checking if it Turned Off: After discharging the gate, I test between Drain and Source again. Black probe still on the Source, red probe on the Drain. The meter should go back to reading open (“OL” or “1”), just like it did during the reversed body diode test. The switch is now ‘off’.
Wrapping Up
So, if the MOSFET passed all these steps – the body diode checked out okay, it turned on (low reading between D-S) when I charged the gate, and it turned off (high/OL reading between D-S) when I discharged the gate – then I’d say it’s probably a good component.
If it failed any of those steps, like being shorted initially, the body diode reading weird, not turning on, or not turning off, then into the bin it goes. It’s not a foolproof test for every possible failure mode, especially for high-frequency issues, but for basic go/no-go checks on the bench, this process works pretty well for me most of the time.
