Hot capacitors: Is that a problem?

Question

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.

Answer 1

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.]]

Answer 2

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.

Answer 3

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.