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Old 3rd June 2004, 09:22 AM   #1
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Question Capacitor across A/C inputs in power supply?

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).
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Old 3rd June 2004, 09:40 AM   #2
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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.
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Old 3rd June 2004, 09:50 AM   #3
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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.
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Old 3rd June 2004, 10:14 AM   #4
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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.
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Old 3rd June 2004, 01:53 PM   #5
jfd_212 is offline jfd_212  United Kingdom
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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
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Old 4th June 2004, 12:58 AM   #6
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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.
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Old 4th June 2004, 02:08 AM   #7
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Have a look at Jon Risch's AC line filter for some interesting thoughts.

Regards,
Dan
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Old 4th June 2004, 04:08 AM   #8
jaycee is offline jaycee  United Kingdom
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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.
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Old 4th June 2004, 01:36 PM   #9
LBHajdu is offline LBHajdu  United States
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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 itís off. Does anyone know what this draw is, or how to calculate it?
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