Why is the cap still so hot?

[cpemma]

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 )

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]

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

[I_Forgot]

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

I_F

[AndrewT]

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

[jean-paul]

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]

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]

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]

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

How hot is the cap really?

[cpemma]

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]

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.