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Old 30th March 2010, 05:09 PM   #1
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Default eliminating pop on power up?

Hi - this is a total noob question, but I don't want to kill myself, so I'll ask: I've read that it's recommended to put a capacitor between the mains electricity and the xformer. (.1 micro-farads, I guess.) Where exactly does this go? Do I put it across the + and - contacts on my power switch? I mean the contacts that are seeing power only when the switch is in the ON position, of course.

My gut feeling is that capacitor is going to explode in my face. Is that just anxiety?

thanks
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Old 31st March 2010, 06:10 AM   #2
ratza is offline ratza  Romania
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In my case the cap is 22nF rated at 1kV placed over the power switch contacts.
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Old 31st March 2010, 08:28 AM   #3
Mooly is offline Mooly  United Kingdom
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Any capacitor used on the mains must be rated for use as such for safety. That means using a Class X or Y type. Remember the mains is 240 volts RMS (230 officially) which is 680 volts peak to peak.
Also you never connect a capacitor between the primary (mains) side and secondaries of the transformer. It's always across L-N or use that together with L-E or N-E for more complex filtering often with an inductive filter too. A small cap as previously mentioned across the switch contacts can help... I would also include a small (10 ohm 1 watt carbon etc) in series with any cap placed across a switch contact to try and prevent arcing/burning of the contacts.


If the switch on noise is more of a thump then it's more likely a design issue with the amp. Mains switches tend to produce a sharp burst of energy creating a very sharp "click" sound.
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Old 1st April 2010, 09:38 AM   #4
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Thanks for all the info. The capacitor I have was salvaged out of a VCR that I actually got just for the case. The cap. says "250V X2". It was on the power supply board, right next to the mains power coming in, so I assume it was being used for a similar function in it's previous life.

The good news is that I ran some power through it last night and no one got hurt! The switch noise I heard was a big thump, but that was when I had one channel mocked up without much solder, or a case. In terms of "design" I'm gleaning as much as I can off the internet. I bought two small kits from Maplin that just have the TDA2050 chip and some capacitors and resistors. I'm improvising the rest.
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Old 1st April 2010, 05:55 PM   #5
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Quote:
Originally Posted by Mooly View Post
Any capacitor used on the mains must be rated for use as such for safety. That means using a Class X or Y type. Remember the mains is 240 volts RMS (230 officially) which is 680 volts peak to peak.
Also you never connect a capacitor between the primary (mains) side and secondaries of the transformer. It's always across L-N or use that together with L-E or N-E for more complex filtering often with an inductive filter too. A small cap as previously mentioned across the switch contacts can help... I would also include a small (10 ohm 1 watt carbon etc) in series with any cap placed across a switch contact to try and prevent arcing/burning of the contacts.


If the switch on noise is more of a thump then it's more likely a design issue with the amp. Mains switches tend to produce a sharp burst of energy creating a very sharp "click" sound.
Uh a cap on the mains only sees 1.41 x the mains vac voltage. so a 240vac mains needs at least a 340v cap. Nothing ever sees PP because you cant produce both sides of the wave simultaneously.
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Old 1st April 2010, 06:12 PM   #6
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by nightanole View Post
Uh a cap on the mains only sees 1.41 x the mains vac voltage. so a 240vac mains needs at least a 340v cap. Nothing ever sees PP because you cant produce both sides of the wave simultaneously.
These days only Class X and Y types rated for 250vac should ever be used.
I mentioned the peak to peak value to highlight the dangers... I didn't suggest it as a value to choose.
A cap rated at 340 vdc would be asking for disaster... the mains is full of transients and spikes, that far exceed that.

The proper caps mentioned (Class X and Y rated for mains use) are self healing types that, should a breakdown in the dielectric occur, then the foil is vapourised internally and the "short" cleared. The capacitor functions as normal after this event.

680v dc caps were used years ago in the UK and were a constant source of failure. The cap "sees" that voltage.
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Last edited by Mooly; 1st April 2010 at 06:39 PM.
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Old 1st April 2010, 06:58 PM   #7
Mooly is offline Mooly  United Kingdom
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This might go some way to explaining the problems,
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