Rant: Found out the hard way about using component tester on active circuits

I was tracing a bad capacitor in my Fuzz Face clone last night and decided to test components while the board was still powered up. My $30 transistor tester instantly fried, showing random numbers and now it won't even turn on. Has anyone else wrecked a multimeter or component tester by forgetting to disconnect power first?

2 comments

2 Comments

emmam327d ago

Read somewhere that some cheap component testers don't have reverse polarity protection or any input clamping diodes. That means any voltage over like 5 volts just nukes the microcontroller directly. Your fuzz face probably has a 9v battery so that would explain the instant death. I've seen people mod those cheap testers with a zener diode and a resistor to protect them just for this reason. Might be worth looking up if you can repair yours. If the microcontroller is fried though it's basically a paperweight now.

charles_price7d ago

Probably worth mentioning that some of those cheap testers run their whole system at 3.3 volts internally even if you power them with 9 volts. There's usually a little voltage regulator on the board that drops it down. So if you feed 9 volts into a test pin that goes straight to the microcontroller without that regulator in between, yeah you're toast. I blew up a tester myself testing a transistor from an old car radio that had some weird voltage on it. Learned that lesson the hard way. You can sometimes save the microcontroller if the input goes through a resistor first but most of these cheap boards skip that too. Definitely worth opening yours up to see if the chip is socketed or soldered because if it's socketed you might be able to swap in a new one.