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Old 29th November 2007, 02:29 AM   #1
CBRworm is offline CBRworm  United States
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Default Capacitors get hot?

I am sure this is a stupid question, but nevertheless:

I have an amp that the original 470uf 80v caps were bulging. there were two of them, I replaced them both with new Panasonic 470uf 80v TS-HA 'large can' caps. They are similarly sized to the originals.

They sit just after a full wave bridge and I noticed that one of them gets warm.

The measured voltage is +80 at one cap and -80 at the other. The polarity is correct and the output is fairly smooth.

In retrospect I probably should have gotten 100v caps, but the last set of 80's lasted many years.

Is it normal for caps to get warm during operation? The rectifier is cooler than the caps.
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Old 29th November 2007, 02:41 AM   #2
spock is offline spock  Canada
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It's probably not a big deal for caps to be warm, but I always use components rated @ 1.5 x or more rated voltage/amperage.
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Old 29th November 2007, 02:44 AM   #3
Magura is offline Magura  Denmark
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Originally posted by spock
It's probably not a big deal for caps to be warm, but I always use components rated @ 1.5 x or more rated voltage/amperage.
It is actually a big deal for caps to be warm, as it decreases their expected lifespan considerably. As I recall, it's something like 50% per 10C.

Everything is do the impossible just takes a little while longer.
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Old 29th November 2007, 03:47 AM   #4
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is there any thing directly connected to it or in electrical contact with the tabs that gets warm? If no, you will probly wanna up the voltage rating. The leakage current at 80V is high enough to make it warm. If your meter says 80VDC, there is probly a ripple voltage component that exceeds the 80V rating. One other the amp running at high power when you are checking the cap temperature? If they only get warm then, then the ESR is high enough to be causing the temp to rise because of ripple current. Like magura said, lifespan goes down significantly with slight increases in temperature. Best to increase voltage rating anyway.
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Old 29th November 2007, 07:20 AM   #5
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If your original capacitors were bulging there is most likely a problem with the original power supply circuit.

Old electrolytic caps typically dry out but do not bulge.

Bulging is caused by overheating the electrolyte which causes gas vapors to rise out of the vents in the tops of the caps.

Another possibility is that 80V could be too close to their actual operating level and 100V caps should be used in their place.
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Old 29th November 2007, 10:38 AM   #6
Nordic is offline Nordic  South Africa
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Well the heat is a direct offspring of the series resistance of the cap and the amount of current passing through it, a la ohm's law...

2 solutions, find lower ESR caps..... or find caps with higher ripple current handleing...

A higher voltage cap, normally has lower series resistance, so that is starting to think in the right direction....
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Old 29th November 2007, 12:06 PM   #7
CBRworm is offline CBRworm  United States
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I think I will go ahead and get some higher voltage, low ESR caps.

The heat is mostly generated at idle. As for the power supply circuitry, it is so simple I can't imagine there is anything wrong. This particular piece is connected directly to the output side of a diode rectifier which is connected directly to the output of a transformer. This particular piece of the supply is low current capable of sourcing only 2 amps.
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Old 29th November 2007, 02:38 PM   #8
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Though rare, a leaky rectifier diode will do it. If they're easy, I'd change 'em out with the caps.
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Old 29th November 2007, 02:57 PM   #9
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How much is "warm". Do they selfheat or do you have a warm environment?
/Per-Anders (my first name) or P-A as my friends call me
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Old 29th November 2007, 03:09 PM   #10
AndrewT is offline AndrewT  Scotland
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I think it is a combination of voltage right up at the cap limit and lack of ripple current rating.

If the supply already shows 80Vdc across the caps, then when mains voltage rises it is going to be even worse.

You really do want your caps to run cold if possible.
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