Alright folks, let me tell you about my weekend adventure: testing MOSFETs! I was digging through my parts bin, trying to figure out if these MOSFETs I salvaged from an old power supply were any good. Turns out, it’s not as scary as it sounds!
First things first, I grabbed my multimeter. A multimeter is your best friend when it comes to electronics testing. Make sure it’s set to diode test mode – that little diode symbol is what we’re after.
Now, MOSFETs have three legs: Gate (G), Drain (D), and Source (S). The trick is knowing which leg is which! Most of the time, you can find a datasheet online for the specific MOSFET model. I googled the part number on mine – luckily, I found a datasheet pretty quick. If you can’t find a datasheet, you can sometimes figure it out by looking at how it’s connected on the board it came from.
Okay, datasheet in hand, I started testing. Here’s the basic idea: MOSFETs are like electronically controlled switches. We’re going to use the diode test to see if the internal diodes are working, and also to “turn on” the MOSFET to see if current flows between the Drain and Source.
For an N-Channel MOSFET (which is what I was testing):
- Source-Drain Diode Test: Put the red probe on the Source and the black probe on the Drain. You should see a voltage drop, typically around 0.4-0.7V. If you see something drastically different (like 0V or a very high voltage), the MOSFET is probably fried. Switch the probes – you should see an open circuit (no reading).
- Gate “Turn On” Test: This is the fun part. Touch the red probe to the Gate, then touch it briefly to the Drain. This should charge the Gate capacitance and “turn on” the MOSFET. Now, put the red probe on the Drain and the black probe on the Source. You should see a very low resistance (close to 0V). The MOSFET is conducting!
- Gate “Turn Off” Test: To turn it off, touch the black probe to the Gate and then briefly to the Source. This discharges the Gate. Now, put the red probe back on the Drain and the black probe on the Source. You should see an open circuit again (no reading). The MOSFET is off.
I repeated these steps a few times on each MOSFET to make sure the results were consistent. If the MOSFET failed any of these tests, it went straight into the “dead parts” pile.
It’s important to note that this is a very basic test. It doesn’t tell you everything about the MOSFET’s performance, like its switching speed or on-resistance. But it’s a good way to quickly weed out the obviously bad ones.
For P-Channel MOSFETs, the polarity is reversed. You’ll put the black probe on the Source for the Source-Drain diode test and use the black probe to “turn on” the Gate.
I gotta say, it felt pretty satisfying to sort through all those MOSFETs and figure out which ones were still good. Now I can finally use them in my next project! Hope this helps you folks out there trying to test your own MOSFETs. Happy tinkering!