Atmega328 component tester killed with 400V cap - Page 1
Forum section: electronics / beginners

author: Xylitol King
author_rank: Newbie
timestamp: April 15, 2018, 08:51:55 am
content: Hey dudes,I acidentally killed one of those Ebay component tester things based on the Atmega328 microcontroller.I stupidly tried to test a 100 µF 400V capacitor from a SMPS without discharging it first. I connected probe 1 and 2 to the cap terminals, and when I put the 2 on there, there was a pretty big spark. After that, tester completely dead.I've looked inside and there's a 3V3 voltage regulator in there that might be shorted (it shows 3.4 mV [in circuit] between Ground/Adjust and Vin, which I don't know is normal or not).Any ideas on what might have been smoked? Is the tester done for?Here's a schematic that kinda shows how it's designed. It's not quite the same as mine, though, as there's no 3V3 regulator in there. It might only be used for the display, I'm not sure.Thanks, dudes.

author: Xylitol King
author_rank: Newbie
timestamp: April 15, 2018, 09:10:48 am
content: Further testing reveals VCC to be about 2.5V during operation. Diode drop between VCC and GND is about 24 mV (700 mV with reversed polarity).U2 (5V regulator) is a TSOT23-6 type, with two legs showing about 700 mV (those are the VCC/GND pins I suppose), and the other four basically shorted. Haven't been able to find a schematic with pinout for this component. It just says V05.

author: calmtron
author_rank: Regular Contributor
timestamp: April 15, 2018, 09:21:04 am
content: I would guess that the microcontroller took the brunt of the force and is goneski.

author: Twoflower
author_rank: Frequent Contributor
timestamp: April 15, 2018, 09:26:28 am
content: As the cap was connected direct to the atmega it is most likely fried to death too. The inputs are not rated for such excessive energies. At maximum I would spend a minute in silence to praise the god of the magic smoke.Also you've been lucky that the tester died and not you. Please be careful with such circuits. For example caps canrechargethemselves after discharge them. Mainly due to dielectric effects.

author: Xylitol King
author_rank: Newbie
timestamp: April 15, 2018, 09:46:57 am
content: Yeah, that's what I figured. I just wanted some confirmation. Guess I'll order up another tester then. Too bad, I invested so much time on this one, it died on me once before, but then I fixed it (turned out the be poor solder joints).At least the SMPS is fixed. The cap that killed the meter turned out to be bad as I suspected, because after replacing it, it works beautifully. Before it would run so hot that it stunk up the room with melting transformer smell.

author: Nusa
author_rank: Super Contributor
timestamp: April 15, 2018, 10:11:08 am
content: The Atmega is almost certainly toast, since the path from the inputs to the rest of the board is through it. If it's socketed, replacing it might be worth a shot, assuming you're also prepared to program it. Otherwise, the kits are so cheap, you just get another.

author: madires
author_rank: Super Contributor
timestamp: April 15, 2018, 10:30:23 am
content: Yep, the ATmega is toasted. And most likely also the display and a few parts more. The 3V3 regulator is for the graphics display.

author: Ian.M
author_rank: Super Contributor
timestamp: April 15, 2018, 11:05:01 am
content: After the third time I repaired the shop LCR meter, I stuck a strip of heavy copper sheet on foam tape next to the test sockets. *EVERY* cap got touched down there before it got plugged in. I didn't have to fix it again . . .However we did make the effort to discharge big caps with less excitement as if you made a notable bang, you owed the rest of the workshop coffee and donuts!

author: Xylitol King
author_rank: Newbie
timestamp: April 15, 2018, 11:25:27 am
content: I don't like discharging the caps directly over metal, it makes my eyes squirm.I usually use a big 33R resistor but I forgot all about it this time...That is a very good idea, though! I think I'll do the same on my next tester.