14
\$\begingroup\$

We worked two years on and off on this project at my friend's place. Finally we finish, I take it home. There's a power supply unit but it can't be connected directly to mains, it expects some lower AC voltage. There's a bridge rectifier and a cap directly after the input. I had an 18V AC adapter and connected it. I knew that 18V was an overkill for the 10.5V DC we needed in the end, but the device doesn't draw so much so it wouldn't get too hot, I figured.

The device worked well. I had it turned on for a while and played a bit. Then I started to smell an odd scent. I move closer to the device, smell, and indeed, it's coming from the device. I want to turn it off, but was just half a sec too late and BANG. The big capacitor in the PSU was blown. It was rated 2200uF/16V. Stupid. I can be happy we put the device already in its case, otherwise it would've exploded in my face.

Anyway, what do I do now? Of course, I need to replace the capacitor itself. But I heard once that there's acid inside the capacitor. Can I safely touch the capacitor to remove it or are there safety issues? And what is the best way to remove the small fibers that are everywhere? How should I go about that, a safe way?

Then, do I need to worry about the circuit around it? Everything worked fine until it exploded, so I don't think other components suffered over-voltage or something like that - but could other components be damaged because of the bang? There's some LM317s, the bridge rectifier, some potentiometers, and some small capacitors. A bit further away, what I'm most worried about, the DDS and a crystal oscillator - you can see it on the picture, the DDS is on the green board next to the crystal oscillator.

And then there's the smoke and the smell - is it dangerous? This is my bedroom.

The PCB with the DDS (in the bottom right of the picture) seems to have gotten some damage on the bottom. The copper traces don't conduct anymore when measuring on the copper, there's some kind of foil on it. The parts themselves are still connected though, when measuring from part to part. Would this be a problem?

And finally, out of curiosity. What would have happened if there wouldn't have been a case around it? Now, the case of the capacitor blew against the case of the device, so the cap couldn't 'explode completely'. What would've been the results if it could explode completely?

enter image description here
enter image description here enter image description here

Here's the circuit diagram of the PSU. The white dot in the center is an alternative ground symbol.

enter image description here

\$\endgroup\$
  • 5
    \$\begingroup\$ You could post a video? \$\endgroup\$ – The Photon Dec 7 '14 at 22:11
  • 4
    \$\begingroup\$ I used to blow model trains up using capacitors... Ahhh, good times. Best I ever did was a 36V rated 10mF electrolytic connected backwards to 75V. That went with a bang believe you me. :D That one was too big to fit in a train, so I blew up the signal box with it. \$\endgroup\$ – Majenko Dec 7 '14 at 22:20
  • 1
    \$\begingroup\$ I don't think the electrolyte is particularly nasty stuff, but use rubber gloves getting it off, and wash those PCBs with flux cleaner to be on the safe side. One place I worked sent a not exactly glowing report to a PSU manufacturer, with a footnote that the brown stains on the cover were from their reservoir cap, which "failed" after the report was printed. \$\endgroup\$ – Brian Drummond Dec 7 '14 at 22:28
  • 1
    \$\begingroup\$ ..it happens. I recommend resistance measurements on the bridge rectifier, in case any of the diodes failed short-circuit (the cap may have short circuited briefly, stressing the bridge), but everything else should be OK. \$\endgroup\$ – Brian Drummond Dec 7 '14 at 23:26
  • 1
    \$\begingroup\$ There would be two big nasties with no case: 1) shrapnel, and 2) electrolyte steam. \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 7 '14 at 23:49
10
\$\begingroup\$

I don't think the electrolyte is particularly nasty stuff. Electrolytics have been in use for about 80 years now (and failing!) and I've not heard of any major health scares, so I'd suggest you're OK with basic precautions - use rubber gloves getting it off, and wash those PCBs with flux cleaner to be on the safe side.

One place I worked sent a not exactly glowing report to a PSU manufacturer, with a footnote that the brown stains on the cover were from their reservoir cap, which "failed" in similar fashion, right after the report was printed.

I recommend resistance measurements on the bridge rectifier, in case any of the diodes failed short-circuit (the cap may have short circuited briefly, stressing the bridge), but everything else should be OK. And as the bridge has passed your tests I wouldn't bother replacing it.

As these look like home etched boards, I suspect the difficulty making resistance measurements may simply be the original photo-resist;if pin to pin measurements are OK, that's what matters. Look at the PCB in a year or so : if the copper is bright green and corroded, I was wrong...

\$\endgroup\$
3
\$\begingroup\$

After you clean the PCB using Brian's instructions, replace the cap with a 25v rating (not 16v). Also, use a 12v AC source (not 18vAC). If the PCB does not works fine, then replace the bridge rectifier, also. Most likely the other parts were not damaged, but if they were, replace them as needed.

\$\endgroup\$
  • \$\begingroup\$ Could you give some explanation for this? Actually, 8.5V AC would be enough for me already and then 16V max for the cap would be fine. Where do you get your numbers? Also, wouldn't it be better to test the bridge rectifier with an ohm meter first? \$\endgroup\$ – Keelan Dec 11 '14 at 22:51
  • \$\begingroup\$ Depends what the intended AC input is; 12Vrms = 17V peak. Some headroom is a good idea as well. \$\endgroup\$ – pjc50 Dec 11 '14 at 23:33
  • \$\begingroup\$ Right, I was assuming the first. Still, Guill, would you mind providing some explanations for the statements in your post? \$\endgroup\$ – Keelan Dec 12 '14 at 8:23

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.