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Pop goes the capacitor!

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WKetel
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re: Pop goes the capacitor!
WKetel   11/25/2010 1:40:40 AM
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Capacitors do have quite a few specifications, and sometimes it is vital to consider all of them, or at least more than are usually considered. AC current and power dissipation were overlooked in this tale of woe. Leakage versus temperature is a good "gotcha" parameter, a bit more subtle than the "life versus temperature" rating. One very interesting rating is ESR,(effective series Resistance), which does cause problems with capacitors used on power buses, but can also cause problems in high-frequency capacitive impedance matching circuits. It winds up being very important to understand all of the parameters that are important when specifying capacitors, and also to make sure that purchasing department decisions do not substitute 100pF disc capacitors for the 100pF Silver Mica caps originally specified. The results from such a substitution, while not explosive, are impressive in the lack of correct circuit functionality.

David Ashton
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re: Pop goes the capacitor!
David Ashton   10/3/2010 11:29:36 PM
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I reckon connectors for power are one comonent that is difficult to overspec. I had a similar experience to yours, with a washing machine. 2 months after the warranty ran out it stopped spinning. I found a relay on the control board with dark black lines radiating from under it on the PC board. Trouble was, the whole PC board was encased in a clear resin type stuff. So with much cursing I forked out $180 for a new board. Then a week after I'd got it going, I happened to see a recall notice for that model. I did get a refund for my $180 but they wouldn't give me anything for my labour! In this case I think it was inadequate soldering on the relay pins - a common problem with automatic soldering processes.

ylshih
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re: Pop goes the capacitor!
ylshih   10/1/2010 5:29:10 AM
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In reference to your side comment about inadequate connector pins for the power lines, the number of marginally spec'd parts in even modern designs is discouraging. I've just had to repair a dishwasher control board (personal fix-it) where a relay controlling the heating element runs through a totally inadequate PCB trace and will burn out over a few years. The failure mode is well known and documented on appliance repair websites. The discouraging news is that this fault seems to have been in this product for at least 10 years and there seems to be no evidence that the manufacturer has ever issued an updated control board with a thicker/wider trace for this power run, a simple no-cost solution. They seem content to charge $200 for replacement control boards - I replaced the burned out run with heavy gauge copper braid and reinstalled the board.

WA7TUV
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re: Pop goes the capacitor!
WA7TUV   9/27/2010 8:15:33 PM
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I used to be a TV transmitter maintenance tech back in the days before remote control. I was once working on a 285 volt 1 amp power supply with tube regulators and had the supply upside down on the workbench. I leaned over to the right to grab my meter when the main electrolytic capacitor vented electrolyte steam that left colored speckles on our 12-foot ceiling! I had been looking down at the bottom of the capacitor just two seconds before. Timing is everything and I found a new respect for the bottom of those capacitors.

Duane Benson
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re: Pop goes the capacitor!
Duane Benson   9/23/2010 4:24:20 PM
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Electrolytic capacitors have always worried me when used close to their published specs. A good engineer will always give plenty of headroom, but when the cost constraints come in, that headroom is often one of the first targets. Some components in some designs seem to be able to deal with minimal headroom better than others, but I don't think that electrolytics are in that set. These days with RoHS I have even more concern. As an assembly house, we see a lot of these and while most RoHS components are in fact truly RoHS compatible, the metal can caps seem to be closest to the hairy edge. One big electrolytic with lots of space around it may reflow quite well. However, put a couple close together and their combined thermal mass often prevents the inside connections from soldering thoroughly. Slow the oven down a bit to fully solder it all and the cans are likely to bulge. If they are that close to maximum thermal tolerance, are they typically slightly damaged even if they don't show any external signs? Are they now pre-weakened and ready to pop if anything else is on the marginal edge?

camey
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re: Pop goes the capacitor!
camey   9/20/2010 7:47:42 PM
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In my early days of building computer systems a friend brought around an S-100 motherboard for me to test. After we had connected the +/-5 and +/-12 and powered it up I noticed the electrolytics down the side were arranged in random orientation. I commented that it was unusual given the otherwise symmetric nature of the backplane. My friend looked at me and asked "They go in a particular way?" Before any of us could hit of the off switch one exploded and we were left rather dazed standing in a cloud of mylar and dust. I've seen many other larger caps explode since but nothing quite as impressive as that little 100uF firecracker aka directional electrolytic.

W1PK
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re: Pop goes the capacitor!
W1PK   9/20/2010 1:28:28 PM
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That happened to my first ham transmitter. I bought a second-hand AT-1 that had been sitting on a shelf for some time. When I powered it up for the first time, there was a sizzle from inside, then all four B+ capacitors blew their seals and sprayed electrolyte all over the inside. I washed it out with a garden hose and left it in the sun to dry for a couple of days, before doing anything else. A guy at the local radio club gave a talk last year on how to bring up old tube gear with electrolytics inside. Use a variac, and bring up the voltage in small steps over a period of a day or so, to re-form the caps. Or just replace them. I've gotten away from designing power supplies with electrolytics, tantalum or otherwise, in the last couple of years, though, since high-frequency switching supply ICs have become easy to get. I'm using almost all ceramics, and not Z5U types, either.

Erickk
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re: Pop goes the capacitor!
Erickk   9/17/2010 10:55:12 PM
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About 25 years ago, I enocuntered an application note in the specification sheet for a multi-output switching power supply from a Japanese manufacturer. In factured english, the note said that the life of the product could be extended many times over by periodic replacement of all the filter capacitors. Despite this note, the power supplies were actually extremenly reliable, perhaps becuase we operated them at only about 75% of rated wattage. But it does point out that electrolytic capacitors have always been a known weak link in electronics.

David Ashton
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re: Pop goes the capacitor!
David Ashton   9/17/2010 10:23:07 PM
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A later line of terminals that were made by the same company that made mine above had exactly that problem. It was a sod to find the first couple of times but after that we'd just replace all the low side caps and the PSU was good for another couple of years. ASTEC PSUs too, from what I remember, they ought to have known better. It's great if they bulge, dead giveaway, but they don't always.

David Ashton
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re: Pop goes the capacitor!
David Ashton   9/17/2010 10:19:38 PM
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That would have been almost as good as working for Mythbusters!

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