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Old 29th October 2008, 11:04 PM   #21
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Gain,

with regards heating of capacitors you are correct provided you are referring to d.c. and the load is resistive. But as Andrew mentions, I also believe there has to be some heat generated when a.c. current flows through the capacitor, else there would be no reason to specify the ripple current limits.
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Old 29th October 2008, 11:26 PM   #22
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Quote:
Originally posted by gain
hence no power (heat) can be produced within a cap!
In an ideal, perfect capacitor, you would be right. But most caps, especially big electrolytics have significant resistance (ESR) and this does produce heat when large currents flow.

10A through 100milliohms = 10W
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Old 29th October 2008, 11:38 PM   #23
AndrewT is offline AndrewT  Scotland
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get yourself a carbon filament light bulb.
They last a long time.
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regards Andrew T.
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Old 30th October 2008, 01:32 AM   #24
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Quote:
Originally posted by AndrewT
Hi Gain,
capacitors do generate internal heat.
It comes from the ripple current and the ESR.
This heat is the reason for the limitation on ripple current and if ignored will destroy capacitors.

ok i will concede the debate to Andrew. if he says caps can produce heat then i believe he is correct. i have read enough of his posts to know that he knows his stuff. REALLY knows his stuff.

i do not understand though. i have gone over my calcs and can't see where i went wrong. could someone who is good with math (unlike me) please help point out the error of my analysis. i'm just naive i guess and don't get how a voltage and current wave that are 90 degrees out of phase can produce power.

thanks, especially to AndrewT.
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Old 30th October 2008, 01:49 AM   #25
gain is offline gain  United States
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Quote:
Originally posted by Nico Ras
Gain,

with regards heating of capacitors you are correct provided you are referring to d.c. and the load is resistive. But as Andrew mentions, I also believe there has to be some heat generated when a.c. current flows through the capacitor, else there would be no reason to specify the ripple current limits.

imho you are backwards in your statement. heating occurs when DC is applied at first due to the resistance present in the legs of the cap. this will, however, decay to (almost) zero as time goes on and exponential function asymptotically approaches V. so cap will heat initially with DC but once charged will stay cool.

with AC there will be no heating due to the 90 degree phase diffrence between the voltage and current waves. except for the heating in the legs of the cap due to resistance (not reactance!) of the i squared r law of current AC or DC through a resistor (in this case not a resistor but the leg of the cap)

fwiw, in all my years of dealing with electronics (almost 8 now) i have never once felt a cap that was warm. ever. even filter caps in PSU's that were playing metallica cranked to ten.
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Old 30th October 2008, 03:15 AM   #26
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Quote:
Originally posted by gain

i have never once felt a cap that was warm. ever. even filter caps in PSU's that were playing metallica cranked to ten.

You haven't put them in backwards, I take it.
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Old 30th October 2008, 03:18 AM   #27
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Quote:
fwiw, in all my years of dealing with electronics (almost 8 now) i have never once felt a cap that was warm. ever. even filter caps in PSU's that were playing metallica cranked to ten.
Maybe the ripple currents produced by a normal amplifier
running off a PSU just don't produce enough heat to be
noticable ,still..., a little heat is produced.
BUT , try hooking up a cap reversed biased (I did this by
accident..reversed rails ) ,the cap got so hot
I had to dip it in the sink!!

BTW ,the caps survived but giving it FULL AC at reverse
polarity could of boiled a cup of tea.

In cheap HT receivers where they use 63V/4700uf caps
(at +- 58V rails) for all FIVE channels I've felt warm caps
(with 6-10v ripple). The way we DIY'ERS use 40Kuf/100V
caps with our 70V rails one should enjoy 1-2V ripple
and 10K+ hours of listening pleasure (cold caps with overdriven
Metallica,as gain says )
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Old 30th October 2008, 03:36 AM   #28
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Quote:
Originally posted by MJL21193



You haven't put them in backwards, I take it.

...or overloaded the starter cap on a big induction motor.

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Old 30th October 2008, 04:59 AM   #29
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this thread has gotten off topic imo.

we do not care about whether or not caps make power when AC is passed through them. imho the answer to that question is obvious. i wait to be standed corrected.

i care about devices that OBVIOUSLY heat up. transistors, diodes, resistors, etc. whats you all's opinion?

will amp live longer if left on continuously or if turned off when not in use?
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Old 30th October 2008, 07:29 AM   #30
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gain,
Quote:
will amp live longer if left on continuously
Amp will live longer if turned off continuously.
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