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Old 24th June 2004, 04:38 AM   #1
azira is offline azira  United States
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Default Just had a power cap blow!

Interesting thing happened tonight. My buddy and I were glueing together a coffee table for his place and listening to some tunes on a GC I've had thrown together on my bench. Anyway, suddenly we hear a pop and a psheeewwwww like air coming out of a tire. Look up and one of my power caps (20,000uF powerlytic rated at 65 Volts) is spewing a 7ft cloud of smoke like one of those fogger bug killers you use in your living room. Strange thing is music still played as well as before but I ran over and shut off the power as quick as I could. About 10 minutes later after we finished the glueing, I went over to unplug the cap and toss it and noticed that it was very hot to touch and the bottom had rounded out like if you freeze a soda can.

Shame that I lost a cap, but I bought them from the Trading post here and I knew they had been on the shelf for a while. Still, been using it for about 3 months with no problems.

Anyway, does anyone know exactly what that smoke consists of? I'm trying to think of what's in the cap that would smoke like that. We opened some doors and ventilated just in case..
--
Danny
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Old 24th June 2004, 05:06 AM   #2
karma is offline karma  Canada
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electrolyte i have poped allot in the past and im still here. it just smells bad
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Old 24th June 2004, 05:45 AM   #3
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I had one go before and it made a nice fog but didn't smell, that I recall... different mix I guess.

What happens is it overheats and being the electrolyte is more or less water based, it boils and boiling water + confined space = what, children? A steam boiler...

Fortunately, most caps have those perforations on them so they blow in one direction, spewing not as much crud as say, the whole can exploding...(which used to be a problem in some old tube amps... )

Tim
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Old 24th June 2004, 05:56 AM   #4
karma is offline karma  Canada
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yes the old ones go off like a starter cap for explosives

nothing left but paper
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Old 24th June 2004, 06:01 AM   #5
azira is offline azira  United States
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Yeah.. it blew through a freeze plug in the top... Didn't smell though.
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Old 24th June 2004, 06:09 AM   #6
karma is offline karma  Canada
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not all do i only had one that did.
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Old 24th June 2004, 07:26 AM   #7
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Default WHAT BRAND OF CAPACITOR IS MOST RELIABLE

What brand of power supply capacitor is the most reliable? Has the longest rated lifetime?

Is is better to use a large number of small caps, or a few big caps for the power supply of an amplifier output stage?

Some designers use a large number of small modest size capacitors, like 48 at 8,200 uF each. Small capacitors like Panasonic are rated 10,000 hours MTTF. These can be soldered right on the PCB if dual sided 4 ounce copper is used and provide low resistance and inductance, plus a clean design.

Some designers use a few large size computer grade capacitors, like 6 at 68,000 uF each. These require bus bars and/or wires that add parasitics and make a true star ground complicated. Computer grade caps are typically rated > 20,000 hours MTTF, but some say that multiple on-off cycles can dramitically reduce this .

So, what is the best way to build a power supply for an amplifier output stage, and what capacitors are the most reliable?
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Old 24th June 2004, 07:33 AM   #8
azira is offline azira  United States
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I would suggest that if you were to parallel many smaller sized caps, then if one were to fail but not short, it would still act as a capacitor of smaller size. In addition, all the ESRs of the caps would be reduced because they are in parallel.
If the a cap fails and shorts, you're screwed anyway whether you have one or many..
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Old 24th June 2004, 07:36 AM   #9
Jennice is offline Jennice  Denmark
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Default just wondering...

Isn't it the high inrush current that eats away the life time? I mean... these peak currents (if you don't use limiting resistors) can be impressive, but way beyone the ripple current rating of a cap.

Any other suggestions?

Jennice
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Old 24th June 2004, 07:47 AM   #10
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Life time?

How the cap is designed, choice of electrolytics etc.

The temperature inside.

Other stress like to large currents.


I have seen RIFA caps with 250 000 hours life time!
10000 hours is only one year in 24/7 usage!


It's better to have many small caps but it costs more and take up more space.

Conclusion: Keep the caps cool and use them within their specifications and notice the mounting positiion, up right or horisontally, read the datasheet to determine this.

Which brand is the best? Mosts brands keep their promises but some brands are better. I have noticed that Rubycon is better than ERO Roederstein and Lelon is almost as good as Rubycon. RIFA is one if the best.

I'm also pleased with Elna, Jamicon, Sic-Safco, Frako and many more.
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