Exploding bypass caps - Page 1
Forum section: electronics / beginners

author: rexxar
author_rank: Frequent Contributor
timestamp: August 10, 2013, 04:47:24 am
content: I just built up a bench power supply from a kit (thisone), and when I was testing it, I shorted the output, which caused one of the bypass caps (C3 in the attached schematic) to blow up. Okay, so much for "short circuit protection". I replaced it with one I had on hand of the same value, and carried on. I then tried pulling 1A at the rated 30V, and the other bypass cap (C2) exploded as well.Judging by the pcb footprints, a much larger cap was designed in here, but was changed out for something smaller. Same thing happened with one of the main filter caps. I wrote to customer support, but I don't really expect anything useful, so I'll ask my question here.Is there another value cap that's supposed to be here, or just one rated at higher current? The ones they gave me are puny little 16v 10uf caps.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 10, 2013, 05:05:46 am
content: Did you put tantalum capacitors in backwards?

author: rexxar
author_rank: Frequent Contributor
timestamp: August 10, 2013, 05:10:49 am
content: Quote from: ejeffrey on August 10, 2013, 05:05:46 amNo, they're electrolytics, and I put them in the right way round both times.

author: c4757p
author_rank: Super Contributor
timestamp: August 10, 2013, 05:16:15 am
content: Got a DSO? Stick it across a capacitor, then make it blow up and see what the capacitor sees.On a hunch, stick diodes antiparallel to the outputs and try again.

author: TerminalJack505
author_rank: Super Contributor
timestamp: August 10, 2013, 05:22:19 am
content: They didn't spare any expenses there, did they? 16V caps for a 15V rail. I'd put some 25V or 35V caps in instead.10uF should be fine. You usually don't want to put more capacitance on a power supply's output than is necessary--which is basically enough to keep the regulator happy.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 10, 2013, 05:59:32 am
content: I wouldn't use 16 V caps in a 15 volt supply, but it should still work, even if the longevity is compromised. Could be the caps are fakes. In principle the output could be oscillating, and the ripple current is blowing up the output caps, but I don't see any reason that would happen in this circuit.

author: rexxar
author_rank: Frequent Contributor
timestamp: August 10, 2013, 06:44:05 am
content: Quote from: TerminalJack505 on August 10, 2013, 05:22:19 amWell, I had some 25V caps in my junk bin, but when I put them in, I accidentally put the one for the positive rail in backwards, and the magic smoke got released from something.Now the positive rail is dead, and the negative seems to be stuck at about -25V regardless of what I turn the pot to.Did I somehow kill both the regulators? Nothing inside looks burnt, so I don't know where the smoke came from, but I definitely smell burnt silicon.

author: megajocke
author_rank: Regular Contributor
timestamp: August 10, 2013, 07:00:53 am
content: If you short from negative to positive output, the strongest regulator will "win" and reverse the polarity of the other output, for example driving +15V into the -15V output which would blow up C3. Diodes to prevent reverse-biasing of those capacitors would be well advised.The designer seems to have only considered shorts from either or both polarities to the output common terminal.Your new problem could also be caused by an open potentiometer or some bad connection, for example.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: August 10, 2013, 07:30:27 am
content: You probably blew or weakened the LM317 on the positive supply rail, when you shorted the positive rail. Both regulators are missing a protection diode across R1 and R2, respectively. These are needed when C1 and C4 are used. Check the datasheet for the LM317.The open ends of VR1 and VR2 is a bad design decision. If the wiper arm goes open circuit, then the output voltage will soar to the full, unregulated input voltage across C5/C6, probably killing C2/C3 and perhaps even the load. The wiper arm should always be connected to one end of the potentiometer. This way the circuit will - worst case - see the full resistor value of the potentiometer, if there is a bad connection at the wiper.A preferred solution would be to connect the pots in series with R1 and R2, with a fixed resistor located where the potentiometers are now. This requires calculating new resistor and potentiometer values, to ensure the regulator pin current stays within spec, yet would result in the output going down/toward common if there is a contact problem in either potentiometer.

author: cube1us
author_rank: Contributor
timestamp: August 10, 2013, 03:44:37 pm
content: Interesting. I built a dual output power supply from a Radio Shack "curkit" back in the day. I have shorted its output lots of times, had it fight with batteries, etc. etc. and it just works. It also has built-in switchable current limiting, and dual meters. (No +5 fixed supply, though). The thing looks kind of awful, but just works.

author: rexxar
author_rank: Frequent Contributor
timestamp: August 11, 2013, 09:46:54 pm
content: They're sending me a replacement kit. To beef up the circuit, I should replace the 16v caps with 25V, and if I'm reading megajocke correctly, I should put a diode in series with the caps? Does it matter what type, or just anything I find in my junk bin?

author: c4757p
author_rank: Super Contributor
timestamp: August 11, 2013, 09:59:06 pm
content: Just standard 1N400x. Not in series - across each output in antiparallel, cathode to positive.

author: rexxar
author_rank: Frequent Contributor
timestamp: August 12, 2013, 04:13:34 am
content: Quote from: c4757p on August 11, 2013, 09:59:06 pmWhy would we want to let a reverse voltage pass through, rather than just block it?

author: David_AVD
author_rank: Super Contributor
timestamp: August 12, 2013, 04:26:16 am
content: Quote from: rexxar on August 12, 2013, 04:13:34 amYou don't want to add voltage drop in the output lines. Even if you did, the Vf of the diodes will vary (with temp and load) and lead to less accurate outputs. You could sense the voltage after the outputs, but then you've just come full circle and defeated the protection for the feedback.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: August 12, 2013, 07:35:58 am
content: Quote from: rexxar on August 12, 2013, 04:13:34 amWhen you disconnect an inductive load, there will be a potentially very powerful back-EMF voltage spike. This is how the inductor attempts to keep the current flowing at its previous value. A diode in series with the output will do nothing to help in this situation.With a 'catch diode' connected in reverse across the output, you will still get a small spark at the contact point, but the reverse voltage spike within the PSU is kept to safe levels.

author: rexxar
author_rank: Frequent Contributor
timestamp: August 15, 2013, 05:10:34 am
content: Okay, so replacement came in the mail, and it's not blown up yet.Interestingly, in this one they gave me25V caps instead of 16. I'd already bought 36V replacements for the ones in question, so I used those, and stuck a 1n4007 in parallel with them, all good so far. I also changed R1 and R2 to 1/2W resistors because the 1/4W ones looked a bit singed from overheatingAnyway, I wanted to look at the output on my scope, and I noticed that there's still voltage on the output for a long time after I turn the supply off. Since the power switch is on the primary side of the transformer, it has to be charge stored in the caps.My guess is that discharge resistors across C5 and C6 (and C7) would be in order, but I don't know what value to put in. Second, what's the purpose of these caps? Is it just filtering?Also, the output of this power supply is really clean, but I guess that's to be expected of a linear supply?

author: mariush
author_rank: Super Contributor
timestamp: August 15, 2013, 06:24:57 am
content: A 1 kohm or something a bit higher would probably be a reasonable discharge resistor.As for why that capacitor is required, have a look at the LM317 datasheet.

author: MrAureliusR
author_rank: Unknown
timestamp: August 15, 2013, 02:36:12 pm
content: Quote from: rexxar on August 15, 2013, 05:10:34 amDid you make sure to put the diodes inantiparallelas c4757p mentioned?

author: rexxar
author_rank: Frequent Contributor
timestamp: August 15, 2013, 05:42:35 pm
content: Quote from: MrAureliusR on August 15, 2013, 02:36:12 pmYes, and 1k resistors across the caps seem to be working just fine to discharge them.Thanks for the help, all!