Purpose of small cap across PSU rails?

[porkchop61]

Hi all-
Can someone tell me why some PSU's have a small (0.1uF or so) non-electrolytic capacitor across the rails? Is this for RF noise?
Does the position on the PSU rail matter?
Sorry for all the questions.
Glenn

 

[SY]

It is used to bypass higher frequencies where the usual electrolytic becomes ineffective. The best position is as close to the circuit being bypassed as possible.

 

[sagarverma]

what i do is.solder the leads of small caps to board such that long leads of cap is still left.i connect the circuit wires to those leads so that max noise bipassing is achieved.its really a very imp thing and must never be missed

 

[james_b]

article on caps

Here's a URL to a good article on filter/bypass caps.
http://www.seattlerobotics.org/encoder/jun97/basics.html

This is a short summary:
Another great place for a bypass cap is on power connectors. Anytime you have a power line heading off to another board or long wire, I would recommend putting in a bypass cap. Any long length of wire is going to act like a little antenna. It will pick up electrical noise from any magnetic field. I always put a bypass cap on both ends of such lengths of wire.

Bypass capacitors help filter the electrical noise out of your circuits. They do this by removing the alternating currents caused by ripple voltage. Most digital circuits have at least a couple of bypass capacitors. A good rule of thumb is to add one bypass capacitor for every integrated circuit on your board. A good default value for a bypass cap is 0.1uF.

 

[sagarverma]

good one james.

great piece of info

 

[porkchop61]

Thanks for the reply everyone!
I think I'll add one to the B+ right before it goes into the first tube.
Glenn

 

[ray_moth]

Yes, good advice! I think, also, that a small cap is also useful across the output of an SS bridge, to help suppress hash.