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First I want to say that I have no experience with capacitors heating up, so this one may be obvious.

I've recently replaced a set of aluminium electrolytic capacitors on an ADSL modem. The original ones vented. They were rated at 105 °C, so I got some 105 °C (of same voltage rating) capacitors as a replacement. Unfortunately, I couldn't find capacitors of the same size, so the modem is now outside of its plastic case.

Anyway, I noticed that the capacitors are getting hot. This is first time I noticed something like that. All other capacitors I've touched were always cool, even when used on a warm PCB. So I'm getting 45.5 °C on the cap of the capacitors. The outside temperature is 27.8 °C. The temperature of the PCB itself (measured from an exposed, unpopulated, solder pad) is 35.7 °C.

I do understand that the capacitors should be able to take the temperature without any problems, but it still seems a bit too high to me.

So is it normal for capacitors to heat up this much? I was unable to determine exact properties of the original capacitors (and Google isn't familiar with inscriptions on them), so I got some "low-ESR" capacitors just in case.

UPDATE: The capacitors are rated at 25 V (working at 12 V) and have capacitance of \$470 \mu F\$. The modem was bought in 2008. and was working more or less continuously (with no more than two months downtime for that period) since then until a month ago.

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not an answer, but just a link to paper about capacitors wattage. Large can caps can have thermal power up to several watt. digikey.com/Web%20Export/Supplier%20Content/CDE_338/PDF/… –  user924 Jul 8 '11 at 13:58
    
After running this paper through Excel one can get an interesting number. Typical wattage for very large caps is between 5..35 mW per square cm of device surface. The smaller cap/the better. Very small caps (like in question) then could have about 100 mW power typically. –  user924 Jul 8 '11 at 14:08
    
@Rocket Surgeon Interesting information! The replacement capacitors seem to have \$\sqrt{2}\$ time bigger surface area than original capacitors, if my calculations are correct. –  AndrejaKo Jul 8 '11 at 14:42
    
If extrapolate data to zero sized cap, the number is 40mW per square cm. Where 1 cm2 is probably the smallest lytic cap possible. So as long as design is below 40mW per any cap, then the design is thermally safe. I'd start worrying may be after exceeding 100mW for caps sized 470uF. –  user924 Jul 8 '11 at 14:48

3 Answers 3

up vote 11 down vote accepted

I'll assume that these are power supply capacitors.

Short answer:

  • 45C is tolerable.
  • Cooler would be better.
  • Taking steps to minimise temperature will improve lifetime, especially in a continuously on application.
  • All similarly specified capacitors are not created equal. Brand may matter.

Long! answer:

That's hotter than you'd expect in a simple power supply circuit, but should be tolerable - but see below re operating lifetime that you can expect. "Rule of thumb" is that capacitor life halves for every 10 degrees C rise in temperature. If your capacitors are ~ 45C externally assume that the core is at say 55C. That's (105-55) = 50C lower than rated so lifetime will be about 2^5 = 32 times longer than nominal rating. Most capacitors (especially 105C rated ones) have a 2000 hour or better rating so you could expect a lifetime of about 2000 x 32 = 64000 hours or about 8 years of continuous operation. Even if core temperature was 65C that would give 4 years continuous. If the modem is run 24/7, as it may well be, then capacitor failure in the say 2 to 10 year timescale is not unexpected. What lifetime did you get from the original capacitors? And was the modem operated continuously?

Capacitors are also rated for "ripple current" and exceeding the ripple current rating will increase internal heating and reduce lifetime. This is an additive effect with temperature. eg If two capacitors are operating at 50C then the one with a larger ripple current will have a shorter lifetime. Formulae are available to allow ripple current lifetime derating calculations (not to hand at present, I can provide if useful).

Ripple current ratings can vary widely between capacitor model and manufacturer. Using a known reputable brand of capacitor is recommended in demanding applications as specification sheets for unknown brands are often suspect, often having been copied from those of other manufacturers. [[This claim is based on my having personally tracked down the source of a significant number of data sheets of capacitors and other products when the claims did not seem to match reality. Internet searching on an unusual phrase will often allow the source to be located.]]

Operating your modem without its case is liable to reduce capacitor operating temperature and increase lifetime. Anything else you can sensibly do to reduce ambient temperature will also help. If you measure a 45C cap temperature in a 20C ambient room, if you then operate the modem in a 30C enclosure the cap temperature will probably be 55C or higher.

Fan cooling may make sense. But just replacing the caps when they fail or buying a new modem may be preferable. Heatsinks for capacitors are not unknown but are not common. Anything you can do to sensibly improve airflow will help. eg if it has no case then orientation may not matter much, so orienting it to improve air flow may be possible.

A datasheet or manufacturer's information should tell you

  • Rated operating temperature.
  • Lifetime at rated temperature.
  • Ripple current.
  • ESR (less commonly)

If they don't tell you the first three, buy another brand. ESR is important but is reasonably well correlated with the other parameters. You can buy capacitors with 3000 hour or 5000 hour or even longer lifetimes at rated temperature, but cost is liable to be higher to much higher. You can buy capacitors with higher than 105C temperature ratings but they are usually much less common and probably expensive.

There are many well known & reputable brands. Panasonic make a wide range of grades, generally seem to "know their stuff" and often are not much dearer than little known or unknown brands. They are certainly not the only brand to consider but are a good starting place.

Distributors like Digikey (www.digikey.com) stock a vast range of brands and models. Digikey have an excellent parametric search engine that allows you to selectively subset based on many different parameters. Even if you buy elsewhere their product search engine is a useful tool. Also see www.findchips.com [[No association with Panasonic or Digikey apart from being a satisfied user and customer/ database user.]]

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Where can I find a list of capacitor manufacturers of good reputation? –  AndrejaKo Jul 8 '11 at 9:23
    
@AndrejaKo - No such list, but Panasonic has good specs, and we've never had problems with them. –  stevenvh Jul 8 '11 at 14:52

45C itself is not an issue.

They might be heating up due to some high-frequency stuff going on - try to solder some 1-10uF ceramic in parallel to it - maybe it will improve things a little if working frequency is very high (though this is unlikely).

Also, if original were polymer ones, your low-ESR might not be good enough (also, unlikely for ADSL modem).

And finally, doublecheck polarity & rated voltage of the capacitor and compare it to actual voltage.

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The polarity is fine and the are at 11.8 V, which is much lower than the 25 V (my other option are 35 V non low-ESR) they are rated at. As for high frequencies, my multimeter is showing 0.625 kHz on them, but it's been known to show all kinds of things on frequency range. I'll see if ceramics help. –  AndrejaKo Jul 8 '11 at 1:05
1  
@Andreja - Use your oscilloscope (hopefully with an FFT option) not your multimeter to determine the frequency content of a signal. Of course, if you've got a spectrum analyzer, use that. –  Kevin Vermeer Jul 8 '11 at 1:14
    
@Kevin Vermeer I don't have a real scope and "probe" for my sound card based one has fallen apart yesterday :(. –  AndrejaKo Jul 8 '11 at 1:19
3  
any HF stuff of interest here will be far above what a multimeter or soundcard scope will work for. Just stick a ceramic across the electrolytic and see what happens. –  Connor Wolf Jul 8 '11 at 3:58
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AndrejaKo: Get Rigol DS1052 form China for 350$, and hack it to 100Mhz, and you will have unbeatable performance/$. I personally own one, and it's just perfect. –  BarsMonster Jul 8 '11 at 10:55

Aluminium electrolytic capacitors have a short life and are the weak spot in electronic products, and their lifetime is affected by temperature. Russell already mentioned the double-life-per-10-degrees rule: for every 10°C you can stay below the specified temperature lifetime will double. So the cooler you can keep them, the better. The old ones may have suffered from a poor design, where there's to much power dissipation in a small enclosure.

45°C is not hot at all, especially not when the environment is nearly 28°C (nice temperatures you're having in Belgrade). That's less than 18°C difference, a value that can easily be expected from any electrolytic capacitor which has to do some work (read: sees ripple). The cooler caps probably have a rather flat DC voltage over them.

If you're worried about lifetime (modem will probably be on continuously?) you can look out for a capacitor specified for 125°C, this should give you 4 times longer lifetime, or a long-life type. 5000 hours is not uncommon, and I've seen values up to 10000 hours, look for "Lighting Ballast Applications". (Want real long life? How about 100 years expected operating life?). Unfortunately parts which are both long-life and high temperature are rather uncommon.

You don't mention capacitance or voltage, but tantalum in lieu of aluminium will also increase reliability.

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+1 for 100 years expected life! :-) –  Federico Russo Jul 8 '11 at 8:18
    
This is the first time I've ever seen "electrolytics" abbreviated to "elcos". Is that a common abbreviation that I've just missed? –  Kevin Vermeer Jul 8 '11 at 16:07
    
@Kevin - No, that's just me. Force of habit. I'll try to avoid it in future. We use it in Dutch as abbreviation of "ELectrolytische COndensator", which is way too long. But admit that you understand what I mean! ;-) Is there an English abbreviation? –  stevenvh Jul 8 '11 at 16:13
    
@Kevin Vermeer♦ I've seen it written on capacitors themselves few times, so I guess it's common in some circles. Also the entire first page for elco capacitor in Google was about electrolytic capacitors for me. –  AndrejaKo Jul 18 '11 at 16:17
    
@Andreja - I got a link for Elco, Pennsylvania, but you're right, it does seem more popular than I initially surmised. –  Kevin Vermeer Jul 18 '11 at 19:20

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