Electrolytic SMD caps getting too hot to touch

Thread Starter

daxliniere

Joined Aug 7, 2021
3
Hey gang,

I have tested two different brands of PCI-e riser boards with an audio DSP card containing 8x SHARC 21469 ICs. Both of the risers seem to work perfectly, but their capacitors get too hot to touch within about a minute.

The max current draw for that card is allegedly 8W, which seems to check out reading the Analog Devices datasheets. These are the kinds of cards that are used in cryptocurrency mining, so they normally have a whopping big GPU card plugged into them, not a little 8W DSP card. The risers have a 12V ATX connector (6-pin) for power input and send the data signals across a USB3 cable (not USB3 protocol, of course) to a dummy card that plugs into the motherboard's PCI-e slot. One of the riser cards I've tested has 4 SMD caps the other has 8 caps (4x2 in parallel).

My questions are:

1) Why would this be happening?
2) What could I do to mitigate it? I'm happy to change or add caps.


Thanks in advance to anyone who can shed some light on this strangeness.

All the best,
Dax.
 

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Papabravo

Joined Feb 24, 2006
17,307
Unvarnished speculation on my part would be that capacitors with higher than expected ESR were substituted for the ones that the designer of the board originally chose. There could be other reasons and I'm not there to look at the board so take that with a grain of salt.
 

dl324

Joined Mar 30, 2015
13,551
Welcome to AAC!

A schematic would be helpful.

"Hot to touch" is subjective. Most caps should be rated to operate at at least 85C. Most people would find that uncomfortable to touch.
 

Deleted member 115935

Joined Dec 31, 1969
0
Hey gang,

I have tested two different brands of PCI-e riser boards with an audio DSP card containing 8x SHARC 21469 ICs. Both of the risers seem to work perfectly, but their capacitors get too hot to touch within about a minute.

The max current draw for that card is allegedly 8W, which seems to check out reading the Analog Devices datasheets. These are the kinds of cards that are used in cryptocurrency mining, so they normally have a whopping big GPU card plugged into them, not a little 8W DSP card. The risers have a 12V ATX connector (6-pin) for power input and send the data signals across a USB3 cable (not USB3 protocol, of course) to a dummy card that plugs into the motherboard's PCI-e slot. One of the riser cards I've tested has 4 SMD caps the other has 8 caps (4x2 in parallel).

My questions are:

1) Why would this be happening?
2) What could I do to mitigate it? I'm happy to change or add caps.


Thanks in advance to anyone who can shed some light on this strangeness.

All the best,
Dax.
you have some inductors and what looks like a power supply chip there also
are they hot ? I would expect them to get hot, wonder if they are heating the capacitors.


As said by @dl324, the capacitors should not be hot, The heating method of capacitors is the ESR , which with a current charging / discharging the capacitor leads to I*2 R heating,
But
as you power is so low, this is very unlikely
 

schmitt trigger

Joined Jul 12, 2010
475
I believe PapaBravo’s theory is correct.
Once a successful product hits the market, product clones are not far behind.
For the clones to be successful, they are usually much lower in price. Which means among other things, substituting for lower performance components.
 

DickCappels

Joined Aug 21, 2008
7,989
Wait a minute! What do you mean by "too hot"? The maximum operating temperature is probably printed on the plastic jacket.
1628355664591.png1628355793409.png
The one on the left is rated at 85°C and the one on the right is rated at 105°C. If you touched either of them while running near its rating it would probably hurt quite a bit, but the capacitors would be just fine.

If you have a means of safely measuring the temperature of the capacitors you should be able to compare your measurement with the rating printed on the capacitor.

Usually we select these capacitors so they run well below their ratings because this extends service life, but I've come across capacitors operating at the high end of their range, and they were painful when touched.
 

Deleted member 115935

Joined Dec 31, 1969
0
This is basically a DC system,

with the capacitors for power filtering / keep up on what may be a switch mode supply

I wonder,

Just how much current would have to flow in and out of them to heat them to "to hot to touch"

and where that current flow would come from ?
 

Thread Starter

daxliniere

Joined Aug 7, 2021
3
Thank you everyone for your thoughtful replies.

I suspect that they might indeed be incorrectly spec'd caps. They're through-hole, so easy to put some low ESR caps in parallel.

How hot is "too hot"? Let's just say that if this was a voltage reg, I would be switching it off immediately and checking how much current I was drawing from it. Never before have I experienced

My understanding about the temperature ratings on caps is that is was more about being able to withstand the ambient operating environment, not that it itself would reach those temperatures. Either way, over time it would ruin the expensive DSP card that it's attached to, so I'm very keen to find the right solution before I put it in an enclosure. (You guessed it, a fan is not an option as it has to be silent.)

Thanks everyone!
Dax.
 
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