Capacitor across A/C inputs in power supply?

Cloth Ears · 3rd June 2004, 09:22 AM

Hi,

Very dumb question, but I don't want to get burned...

I have been looking at a number of schematics of power supplies on the Internet (mainly pointed to by various threads from this site) and quite a number have a 'mains' capacitor across the positive/neutral A/C input or the switch. Are all these methods valid? Or should I just 'leave it out' as so many of the articles on power supplies suggest?

1) is from Anthony Eric Holton of http://www.aussieamplifiers.com/psu1.htm from the N-Channel Amplifier PSU page
2) is from Dejan V. Veselinovic of http://zero-distortion.com in his "Designing your own Power Supply" article
3) is from Nelson Pass of http://www.passdiy.com from the zenlite.pdf article

I tried having a search through this site but have found nothing what the various 'pre-transformer' circuits do (although I know they, generally, are for noise reduction or slow power up).

Ouroboros · 3rd June 2004, 09:40 AM

These are special Class-Y or Class-X2 ac rated capacitors. They are there to act as a simple filter to stop high-frequency rubbish getting back onto the mains from the equipment. The only thing that will happen if you don't fit them is that you might generate a bit more RFI.

Cloth Ears · 3rd June 2004, 09:50 AM

Thanks Ouroboros,

Should I be connecting them between the positive and neutral A/C, or bypassing the off/on switch? From your post, it sounds like the first option only.

Ouroboros · 3rd June 2004, 10:14 AM

The cap across the on/off switch will be to stop switch arcing at the moment of opening the switch.

Also these days, most countries require double-pole on/off switches to meet safety requirements. (Or at least that's the case here in Europe). In that case both poles will need a cap across them.

To be honest, I doubt if you'll ever need a capacitor across the switch, as proper 230V ac rated switches shouldn't suffer too much from arcing.

jfd_212 · 3rd June 2004, 01:53 PM

Hi,

I don't normally use it in my design. I don't find that it effect the sound quality. If youre really serious about reducing line noise then I suggest that you build a proper line conditioner.

Normally, its for AC line noise and radiation countermeasure.

cheers.

JFD_212

Cloth Ears · 4th June 2004, 12:58 AM

Thanx for your responses, now I know what it's for. For $4, I think it's worth putting in even if I have a line conditioner.

dantwomey · 4th June 2004, 02:08 AM

Have a look at Jon Risch's AC line filter for some interesting thoughts.

Regards,
Dan

jaycee · 4th June 2004, 04:08 AM

A good source for AC line conditioner parts is a PC power supply - nearly all of these have one X2 rated cap, two Y2 rated caps and some even have a line choke.

I have a dead PC PSU that has a 2 stage AC filter which I'm thinking of using in my integrated amp. I'm not sure if it'll handle the current though.

LBHajdu · 4th June 2004, 01:36 PM

If putting a cap across the switch extends the life of the switch is seems like a good thing to do. However, putting a cap across the switch would mean that a little current, is always flowing through it when the switch is open. So the amp always consumes some power even then it’s off. Does anyone know what this draw is, or how to calculate it?

3rd June 2004, 09:50 AM	#3
Cloth Ears diyAudio Member Join Date: Dec 2001 Location: Melbourne, Australia	Thanks Ouroboros, Should I be connecting them between the positive and neutral A/C, or bypassing the off/on switch? From your post, it sounds like the first option only. __________________ Jont. "It is impossible to build a fool proof system; because fools are so ingenious."

4th June 2004, 12:58 AM	#6
Cloth Ears diyAudio Member Join Date: Dec 2001 Location: Melbourne, Australia	Thanx for your responses, now I know what it's for. For $4, I think it's worth putting in even if I have a line conditioner. __________________ Jont. "It is impossible to build a fool proof system; because fools are so ingenious."

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Power supply Capacitor, bad design?	gain-wire	Solid State	6	8th December 2008 04:36 PM
Power supply capacitor grenaded	valterdaw	Car Audio	6	18th July 2006 04:44 PM
Sunfire Power Supply Capacitor	ainami	Solid State	19	12th September 2004 01:43 PM
power supply capacitor questions	roofingboom	Solid State	5	26th May 2004 04:36 PM
Car Audio Power Supply Capacitor	Dustin Haug	Swap Meet	0	15th May 2003 02:15 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

3rd June 2004, 09:40 AM	#2
Ouroboros diyAudio Member Join Date: Dec 2003 Location: Nottingham UK	These are special Class-Y or Class-X2 ac rated capacitors. They are there to act as a simple filter to stop high-frequency rubbish getting back onto the mains from the equipment. The only thing that will happen if you don't fit them is that you might generate a bit more RFI.

3rd June 2004, 10:14 AM	#4
Ouroboros diyAudio Member Join Date: Dec 2003 Location: Nottingham UK	The cap across the on/off switch will be to stop switch arcing at the moment of opening the switch. Also these days, most countries require double-pole on/off switches to meet safety requirements. (Or at least that's the case here in Europe). In that case both poles will need a cap across them. To be honest, I doubt if you'll ever need a capacitor across the switch, as proper 230V ac rated switches shouldn't suffer too much from arcing.

3rd June 2004, 01:53 PM	#5
jfd_212 diyAudio Member Join Date: Jun 2004 Location: Cardiff	Hi, I don't normally use it in my design. I don't find that it effect the sound quality. If youre really serious about reducing line noise then I suggest that you build a proper line conditioner. Normally, its for AC line noise and radiation countermeasure. cheers. JFD_212

4th June 2004, 02:08 AM	#7
dantwomey diyAudio Member Join Date: Feb 2002 Location: Canada	Have a look at Jon Risch's AC line filter for some interesting thoughts. Regards, Dan

4th June 2004, 04:08 AM	#8
jaycee diyAudio Member Join Date: Apr 2004 Location: Norwich, UK	A good source for AC line conditioner parts is a PC power supply - nearly all of these have one X2 rated cap, two Y2 rated caps and some even have a line choke. I have a dead PC PSU that has a 2 stage AC filter which I'm thinking of using in my integrated amp. I'm not sure if it'll handle the current though.

4th June 2004, 01:36 PM	#9
LBHajdu diyAudio Member Join Date: Feb 2002 Location: NY, USA	If putting a cap across the switch extends the life of the switch is seems like a good thing to do. However, putting a cap across the switch would mean that a little current, is always flowing through it when the switch is open. So the amp always consumes some power even then it’s off. Does anyone know what this draw is, or how to calculate it?

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)