Society of Robots - Robot Forum

Electronics => Electronics => Topic started by: krich on October 30, 2007, 05:08:18 PM

Title: Capacitor voltage rating question
Post by: krich on October 30, 2007, 05:08:18 PM
I've got a number of capacitors laying around from a previous project.  The catch is that they are rated for 250V, for example.  I know the recommendation is to use a cap rated at about twice the voltage you are expecting to run through it, but is there any issue with using a capacitor that is rated at 20 or more times the voltage?   ???

These guys are left over from a monitor repair I did a while back, thus the larger voltage ratings.

Title: Re: Capacitor voltage rating question
Post by: frank26080115 on October 30, 2007, 05:44:13 PM
i've actually read that you need to be over the rating by at least %50, too high a rating will cause it not to work
i can't say for sure
Title: Re: Capacitor voltage rating question
Post by: krich on October 30, 2007, 06:36:48 PM
What about a 240V capacitor on a 5V circuit, assuming the proper number of uF's?
Title: Re: Capacitor voltage rating question
Post by: airman00 on October 30, 2007, 07:33:37 PM
Why not just make a simple circuit with a switch ,LED, and capacitor?

Should take like 5 minutes...  ;)
Title: Re: Capacitor voltage rating question
Post by: paulstreats on October 30, 2007, 07:38:52 PM
There should really be no problem in using a higher v rated capacitor, ive used a 25 kilovolt cap on a 24 volt circuit (although i did change it eventually once i got a suitable one), the load might alter slightly though
Title: Re: Capacitor voltage rating question
Post by: Admin on October 31, 2007, 07:26:09 AM
That 250V rating is actually for AC current (its a non-polarized cap, like ceramic, right?) . . . I don't remember what the conversion is, but I think its like 100V for DC.

For crude applications (like for motors) it won't matter.

The only real disadvantage to a voltage rating that high is that the capacitor is unnecessarily large.
Title: Re: Capacitor voltage rating question
Post by: krich on October 31, 2007, 09:59:38 AM
That makes sense.  I just got around to looking at some of the ratings and what you say about the size is definitely true.   A good example is this 350V 22uF capacitor.  Its a tenth the uF's as the one used in the $50 dollar robot, but its at least 5 to 6 times larger (wow).

Edit:  These are polarized caps.  I guess there's some high DC voltage flying around inside a monitor.