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-   -   Why is the cap still so hot? (http://www.diyaudio.com/forums/everything-else/88200-cap-hot.html)

cpemma 12th October 2006 11:32 PM

Why is the cap still so hot?
 
My router/modem runs from a 9V 9VA AC wart, and has its own bridge rectifier and 1000uF 16V 150°C smoothing cap; this feeds a couple of regulators, 3.3V and 5V.

The original no-name cap showed swelling after some months in service so I replaced it with a low-ESR 1000uF 25V I had in stock; runs fine, but too hot to touch for more than a second. OK, it's 105°C rated and the ripple spec is 1.7A so maybe I should ignore it, but I had the thought to feed the bridge with regulated and smoothed 12V DC from another adaptor (9V RMS = 12V peak, near enough), thinking the cap temperature would show a big drop; it didn't, still too hot to touch. I've dropped the supply to a regulated 9V DC, feeding the bridge input, the router runs fine but the cap's still hot. (Regulators are a lot cooler though :D )

What's going on? Do the bridge diodes in each supply rail between the caps prevent the adaptor smoothing and router smoothing from "sharing" the heat produced, or am I misunderstanding why a smoothing cap gets hot?

pinkmouse 12th October 2006 11:49 PM

Silly question. It's not connected the wrong way round is it?

I_Forgot 12th October 2006 11:49 PM

Maybe the cap gets hot because it is so close to the transformer inside a little wall wart with no ventilation.

I_F

AndrewT 12th October 2006 11:59 PM

Hi,
could the bridge be shorted?
Feeding AC directly into the cap?
What voltage is on the cap?

jean-paul 13th October 2006 12:01 AM

Quote:

Originally posted by pinkmouse
Silly question. It's not connected the wrong way round is it?
I just repaired a Rega radio which had reversed caps ( from factory ). Simple but effective error.

cpemma 13th October 2006 12:24 AM

No red herrings - nothing wrong with the cap wiring, the hot cap is in the router, not the wart; it's its own heat, not picked up from adjacent components, the warts get warm (as they usually do when on 24/7) but they're sealed units so I can't say what temperature the wart caps are running at.

At 12V DC input I've used both SMPS and 50Hz linear supplies, no apparent difference.

Obviously to rate a cap at 105°C it's expected to run hot; I'm just surprised that post-smoothing a regulated voltage still gives such a rise.

SY 13th October 2006 12:32 AM

No, the 105C rating is for caps placed in hot environments- it shouldn't generate much heat of its own unless there are enormous ripple currents or oscillations. Or if the cap is leaky, which might be the issue here.

peranders 13th October 2006 09:44 AM

If the power supply is of the type SMPS caps get warm or even hot, nothing peculiar.

How hot is the cap really?

cpemma 14th October 2006 04:58 PM

Quote:

Originally posted by SY
No, the 105C rating is for caps placed in hot environments- it shouldn't generate much heat of its own unless there are enormous ripple currents or oscillations.
Surely not the only reason for using 105°C caps? I can't think of too many environments that hot. My motherboard certainly isn't and it's crowded with them.

Maybe the high-frequency on/off loading of a wholly digital circuit?

I guesstimate "idle" temp as around 60°C (140°F), several degrees hotter during prolonged downloading. Not alarming, but routers are on 24/7 and cap life goes down with temp head. Plus routers seem to be the one PC peripheral that often fails, judging by any "What's a good router?" thread elsewhere.

SY 14th October 2006 05:34 PM

Quote:

I can't think of too many environments that hot.
No, but in an elevated temperature situation (for example, tube amps or some of the solid state class A space heaters, where 80 degrees is not out of the question), they'll last longer than the lower temp rated caps.


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