So, I had this project on my workbench, a little amplifier circuit that suddenly went quiet. My first suspect? One of those MOSFET things. You know, those little black chips with three legs that control the power. Figured I needed to check if it was fried before I started ripping other stuff apart.
Getting Ready
First, I needed my multimeter. Dug it out of the toolbox, dusted it off. Set it to the diode test mode – the one that makes a beep sound when there’s a connection. That’s usually the best way I find to check these semiconductor bits.
Then, the tricky part. The MOSFET was soldered onto the circuit board. Sometimes you can test them in-circuit, but it can give funny readings. I decided to play it safe and desoldered the little guy. Took a bit of careful work with the soldering iron and a solder sucker, but I got it out clean without damaging the board too much.
Once it was out, I needed to know which leg was which. Usually, they are Gate (G), Drain (D), and Source (S). Had to squint at the tiny markings on the MOSFET and look up its datasheet online just to be sure. Got the pinout figured out. Okay, ready to test.
Doing the Actual Check
Alright, here’s how I did it with my multimeter:
- Step 1: Check if it’s shorted. I put the black probe (-) on the Drain (D) pin and the red probe (+) on the Source (S) pin. My meter showed an open circuit reading (like ‘OL’ or just a ‘1’). Good start. If it beeped or showed a low number here, it’d be shorted and definitely bad.
- Step 2: Turn it ‘on’. This is the neat part about MOSFETs. You gotta charge the Gate. I kept the black probe on the Drain, and then touched the red probe to the Gate (G) pin for just a second. Didn’t even need to hold it there long.
- Step 3: Check if it turned on. Now, keeping the black probe still on the Drain, I moved the red probe back to the Source (S) pin. Success! The multimeter beeped and showed a very low resistance reading. This meant the MOSFET switched on, allowing current to flow between Drain and Source, just like it should when the Gate is triggered.
- Step 4: Turn it ‘off’. To discharge the Gate and turn the MOSFET off, I simply touched all three pins (G, D, S) together with my finger for a moment. You could use a small metal tool too, just shorting the Gate to the Source works fine.
- Step 5: Check if it turned off. Finally, I went back to the first test position: black probe on Drain, red probe on Source. The meter went back to showing an open circuit (‘OL’ or ‘1’), just like in Step 1. It turned off correctly.
Final Thoughts
Did one last check between the Gate and the other two pins (Gate-Drain, Gate-Source). Both showed open circuit, which is what you want. No shorts there.
Turns out, this particular MOSFET was actually okay. It passed all the steps. So, my amplifier problem wasn’t this component after all. A bit frustrating, as I had to put it back in, but at least I know the MOSFET is good. It’s a handy little routine to check them this way. Doesn’t take long once you remember the sequence.