Re: Capacitor Wattage Capability

From: John Bachman (johnREMOVE_at_anatek.REMOVEmv.REMOVEcom)
Date: 03/29/05 

Date: Tue, 29 Mar 2005 15:15:11 -0500

On Tue, 29 Mar 2005 12:59:05 -0330, "Terry" <tsanford@nf.sympatico.ca>
wrote:

>
>"John Bachman" <johnREMOVE@anatek.REMOVEmv.REMOVEcom> wrote in message
>news:g3ii41hn8c7ij4sbbrr6rmhihug25722lu@4ax.com...
>> On 29 Mar 2005 02:51:37 -0800, "Glynn R." <kittyslave75080@yahoo.com>
>> wrote:
>>
>>>Hey-- I've kinda wondered this too! In a circuit, I might choose a 16v
>>>in a circuit I expect to have 5 volts running through. I have been told
>>>I could put a 63v there and it works just the same. BUT, would it last
>>>better? Would the extra expense do anything at all for reliability?
>>
>> Reliability is generally improved by "derating" but only up to a
>> limit. Derating means operating the capacitor at a voltage less than
>> its rating, ie, a 25 volt capacitor operated at 15 volts is derated
>> 15/25 = 60%. In other words, it is being operated at 60% of its rated
>> voltage.
>>
>> Capacitor voltage ratings can be derated 25% - 100% depending on the
>> type but going beyond that is a waste of money and space except in the
>> most demanding applications.
>>
>> John
>>
>Are there any other considerations; for electrolytic caps?
>What I'm getting at is that an electrolytic capacitor rated at say 16
>microfarads at say 450 volts DC, if operated at a very low voltage, lets
>assume, say 25 volts for sake of argument, may not have the stated capacity
>of 16 mfd. because of the manner of the electrochemical action of the
>capacitor.
>Or am I off track with this idea? Note this ? is only in reference to
>electrolytic caps.

No, the capacitance is not a function of applied voltage.

>
>I have operated 2 mfd. 'paper' caps rated for 50v DC at 350v DC (with
>ripple) knowing that they were tested to 2000v static DC when manufactured!
>Also have operated similar 0.5 mfd. cap at 115v RMS (60 HZ.) in a fan
>circuit with only one failure in 30+ years.

That is not the result I would expect.

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