Re: capacitor value

From: John Popelish (jpopelish_at_rica.net)
Date: 02/25/05 

Date: Fri, 25 Feb 2005 17:14:04 -0500

aman wrote:
>
> I use a simple rule that if I use capacitor in main supply filter
> circuit of say
> 24V DC i use it at atleast twice the rated voltage(48V).
>
> I use the same rule for the bypass capacitors. Does this make sense ?

You are being very conservative, unless you need a capacitor to
approach either its ripple current rating or its case temperature
rating. I usually choose electrolytic caps rated one or two ranges
above actual. E.g.. For 5 volts, 6.3 volt rated. For 15 volts,
either 16 or 25 volt rated.

I have sometimes gone more conservative than this if I needed lowest
possible internal resistance, or got too close to the ripple current
rating. If I am expecting the cap to spend long periods of time near
its maximum rated temperature, I may go as far as double the actual
voltage, especially for very low voltages.

As to bypass capacitors, I often use ceramic surface mount caps that
are rated for 2 or more times the actual voltage, because the large
sizes are sometimes cheaper (you have to check) or because I want a
bit lower ESR. I never use Z5U or Y5V types, because they lose a lot
of their capacitance as you approach their rated voltage, and as they
get hot. I like X5R and X7R.
  

-- 
John Popelish

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