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Capacitor ratings

Posted by nophead 
Capacitor ratings
June 14, 2007 08:17AM
This is in reply to Jonathan's question in the developer forum.

Capacitor temperature and voltage ratings are not just the maximum the capacitor can handle, they also define its lifetime. Usually they are rated for 2000 hours at max current and voltage. In most application you want them to last longer so you derate them according to the formulas here [www.illinoiscapacitor.com].

Lifetime doubles for each 10C drop in temperature and is a square law on voltage so you can get much longer life by using higher temperature and voltage specs.

The temperature is not just the amibient air temperature but includes a component due to heating caused by the ripple current and the internal resistance (e.s.r).

It is a good idea not to mount electrolytics close to heatsinks if you care about the life.

I wish the people that designed the electronics in my boiler had unserstood this hot smiley

Re: Capacitor ratings
June 14, 2007 11:01AM
Thanks. So, to get back to the practical question at hand: is C1 rated at 16V and 125C going to last any longer than one rated at 50V and 85C on the PowerComms board? Is it possible to determine that with any certainty? Are C2-C5 (50V 85C) appropriately rated too, and if not, what should their rating be for optimal cost/size/lifetime tradeoff?

I mean, we could go crazy and specify caps rated 100V and 125C, but they would be physically larger as well as several times the cost... probably not appropriate.

Using those forumlae, I plugged in what I think are the right numbers, and it appears the 16V 125C cap will last 200 years, and the 50V 85C cap will last a mere 20 years of run time. Somehow I doubt a Reprap Darwin v1.0 is going to be operated for that long in practice -- hopefully v2.0 Mendel will be out well before that :-) :-)


Re: Capacitor ratings
June 14, 2007 12:16PM
The other parameter to consider is the ripple current rating. That tends to be higher on larger caps, i.e. higher voltage ones. Sometimes a higher voltage part is specified to get a higher current rating.

It does look like both parts are somewhat overrated. I must admit that when I design something at home, i.e. a one off I just pick parts that are generously rated and don't bother with the sums, often I just use what is to hand. When I design for production then I do the calcutaions to get the cheapest / smallest parts. Also wide availability is a consideration.

If you can find parts that are significantly cheaper and still fit the PCB then it might be worth updating the BOM but I wouldn't bother changing the PCB at this stage. My guess is that all 16V 85C would do the job fine but I don't have the data to hand at the moment.

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