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Mains DC traps
Mains DC traps
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Old 19th September 2004, 03:24 PM   #1
Prune is offline Prune  Canada
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Default Mains DC traps

First there's the one that's attached to this post (first half of schematic), with the disadvantage that you need a lot of capacitance if you are drawing appreciable current from the mains, or it doesn't work very well.

But I don't understand the following two.

Why are the two capacitors in parallel? In the case of DC, one would be reverse biased and the aluminum oxide dielectric will electrolyze. The other one keeps working, but what happens when DC offset changes sign (say plugged in somewhere else)? Then you end up with two ruined caps.

How can this work with such small capacitors? All that the maker said in the previous thread where this was mentioned is:
You must use the right caps. They need to be AC rated X and Y caps so you don't start any fires. They are pretty expensive (relatively). The caps are aranged such that at AC zero cross, where the diodes aren't allowing any current to flow, the caps couple the AC line to the load. The other caps clean up the sharp transition of when the diodes conduct.
So the small blue caps appear to be just there to take care of diode switching ringing. But what about the other ones? The use of XY rated caps suggest they see at least a good part of the mains AC voltage, so this seems to be a different type of design than the attached one. Also, it was said there are 8 caps, whereas I can only see 7 in the picture.
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Old 19th September 2004, 03:35 PM   #2
SY is offline SY  United States
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Mains DC traps
First question: maybe the diodes in series-parallel might provide a clue? The e-lytics can't see more than 1.4V or so even in the case of DC.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
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Old 19th September 2004, 04:12 PM   #3
Prune is offline Prune  Canada
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By that logic there's no point in having them antiparallel. Yet the schematic explicitly shows they are not facing the same way. Indeed, you don't even need two capacitors, just one. So that doesn't fit. In any case, I think any DC over time will electrolyze the dielectric. To make a non-polar electrolytic you need to put two in series, facing opposite ways.
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Old 19th September 2004, 05:01 PM   #4
peranders is offline peranders  Sweden
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Mains DC traps
It's pretty useless to have anti-parallel caps since Lars Clausen uses only one capacitor and it seems to work, I'll gather having anti-parallel caps doesn't do any harm either. I have two in series then I won't have to worry.
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Old 19th September 2004, 05:14 PM   #5
Prune is offline Prune  Canada
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What about the last circuit? That's the one that's hardest for me to understand, as there's no schematic. I can't figure out any configuration that works with such small capacitors.
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