Okay, so the other day I needed to figure out if a MOSFET in my project was fried or not. I’m no expert, but I’ve picked up a few tricks over time, and this is how I went about checking it.
Gathering My Tools
First things first, I grabbed my trusty multimeter. That’s pretty much the main tool you’ll need. I made sure it was set to the diode testing mode – you know, the one with the little diode symbol. Some multimeters also have a specific MOSFET testing mode, but the diode mode works just fine for a basic check.
Disconnecting the Power
Safety first! Before I touched anything, I made absolutely sure the power to the circuit was OFF and any big capacitors were discharged. Don’t want any nasty surprises.
Locating the MOSFET Pins
Next, I identified the MOSFET’s pins: Gate (G), Drain (D), and Source (S). I looked at my board to determine which was which pin on the MOSFET, and that’s easy enough to do.
Testing Between Drain and Source
- I put the multimeter’s black probe on the Source (S) pin and the red probe on the Drain (D) pin.
- Most of the time, I got an “OL” reading (open loop), which is what I expected. If you get a very low resistance reading, it could mean the MOSFET is shorted and likely bad.
- Then I switched the probes – red on Source, black on Drain. I should see a diode-like reading here (something around 0.4 to 0.7V, but it can vary). This is checking the MOSFET’s built-in body diode. If it’s way off, there might be a problem.
Testing the Gate
- I kept the black probe on the Source (S).
- Now, I quickly touched the red probe to the Gate (G) pin for just a second. This “charges” the gate.
- Then, I moved the red probe back to the Drain (D) pin. This time, I was looking for a low resistance reading – the MOSFET should be “on”.
- To “turn off” the MOSFET, I briefly touched the Gate and Source pins together to discharge the gate. Then, measuring between Drain and Source again should give me that “OL” (open loop) reading.
My Findings
I went through these steps a couple of times to be sure. In my case, the readings were behaving as expected – “OL” when off, low resistance when “on”, and the body diode reading was in the normal range. So, I concluded my MOSFET was probably okay. If I had seen a constant short (low resistance) between Drain and Source, or the gate voltage wasn’t turning the MOSFET on and off, I would have known I had a faulty component.
This isn’t a super in-depth test that covers every possible failure mode, but it’s a good, quick way to get a pretty good idea of whether your MOSFET is working or not. It has helped me troubleshoot many times!
