Purpose of small cap across PSU rails?

porkchop61 · 24th October 2005, 05:29 PM

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 · 24th October 2005, 05:53 PM

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 · 24th October 2005, 06:19 PM

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 · 25th October 2005, 12:59 AM

Here's a URL to a good article on filter/bypass caps.
http://www.seattlerobotics.org/encod...97/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 · 25th October 2005, 05:03 AM

good one james.

great piece of info

porkchop61 · 25th October 2005, 11:16 AM

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 · 26th October 2005, 04:38 AM

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

24th October 2005, 05:29 PM	#1
porkchop61 diyAudio Member Join Date: Aug 2004 Location: Mansfield, Connecticut	Purpose of small cap across PSU rails? 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

24th October 2005, 05:53 PM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	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. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

25th October 2005, 12:59 AM	#4
james_b diyAudio Member Join Date: Jun 2004 Location: Herndon, Virginia	article on caps Here's a URL to a good article on filter/bypass caps. http://www.seattlerobotics.org/encod...97/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.

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24th October 2005, 06:19 PM	#3
sagarverma diyAudio Member Join Date: Feb 2005 Location: NEW DELHI	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

25th October 2005, 05:03 AM	#5
sagarverma diyAudio Member Join Date: Feb 2005 Location: NEW DELHI	good one james. great piece of info

25th October 2005, 11:16 AM	#6
porkchop61 diyAudio Member Join Date: Aug 2004 Location: Mansfield, Connecticut	Thanks for the reply everyone! I think I'll add one to the B+ right before it goes into the first tube. Glenn

26th October 2005, 04:38 AM	#7
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: Jakarta	Yes, good advice! I think, also, that a small cap is also useful across the output of an SS bridge, to help suppress hash.

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