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Old 16th September 2004, 10:00 AM   #11
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Actually, I have, several times. I had no good results though

Since there obviously was a result here, I resolve to be more open minded and re-add that circuit to my "brain-box"
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Old 16th September 2004, 11:08 AM   #12
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Erm, the 1N5404 diodes turn on when DC > 0.6V.

Also, you only need one cap.

I don't see DC getting on the line anyway, unless you have a **LOT** of half-wave-rectified equipment running on your circuit. Consider that line voltage comes from a transformer (pole pig) which delivers about 240VCT at 200A or more... you think any DC that matters to your equipment is going to get past that short circuit of a winding?

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Old 16th September 2004, 11:34 AM   #13
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Tim, if you unsymmetrical sine wave you HAVE DC, by definition. DC in the mV range is enough to get toroids to hum. Tim, the only reason for using a DC trap is to get rid of annoying hum from toroids especially, nothing else!

My solution look like this

Lars Clausen has also a DC trap but I can't find it. Lars, where do you have it? You have it in the pricelist but the link goes to something else.
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Old 16th September 2004, 12:02 PM   #14
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Quote:
Also, you only need one cap.
Why? That cap could get a sustained up to 0.6 V (in my case 1.2 V as I used two diodes to block more DC) in the wrong direction, which over time will still electrolyze the aluminum oxide.

What I'm wondering is how two capacitors a per Peranders' design and the one I posted are supposed to protect against this. I need to know whether to go out and buy two double sized ones to replace the single polarized one I'm using.
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Old 16th September 2004, 12:25 PM   #15
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I suppose only one cap will work becuase of the very low possible reverse voltage but I don't take any chances. Lars has only one cap in his design and I'll gather that this works.
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Old 16th September 2004, 02:07 PM   #16
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According to this, back-to-back capacitors for non-electrolytic only works if the capacitors are very well matched. How the hell can one do that without a capacitance meter?
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Old 16th September 2004, 02:38 PM   #17
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Er, one pair of caps.

Anyone who's dealt with distortion knows that a sine wave can be asymmetrical without DC offset... how else does 2nd harmonic distortion get through an output transformer?

Which begs how it starts in the first place - the power company produces a quality 50/60Hz sine wave of very good frequency stability and generally good regulation (in here it stays between 120 and 125V, YMMV up to 5% I think). Noise pickup, particularly from the many DC power supplies running off it, bast@rdize the waveform but only with odd harmonics (symmetrical distortion). It would take a half wave rectifier of large capacity (or many, many small ones) on your local circuit (which includes your house and most likely a number of neighbors) to produce a significant offset.

Hm. If a 15A circuit can draw 15A with negligible drop (maybe 1VAC), it would take 9A rms DC on that circuit to produce a 0.6V offset. On a seperate circuit, you have far less, and lower resistance, wire in common with said circuit, I would expect maybe 30mV from same load on a different circuit. And none if on the opposite leg of your house circuit (US residents get 120-0-120V (240VCT) at maybe 100A, with half of the circuits running on each leg, with any 240V loads (water heater, dryer, etc.) going between legs).

If you truely have 0.5V DC offset, and it's not a measurement glitch... you honestly should have your wiring or appliances looked at.

Tim
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Old 16th September 2004, 02:46 PM   #18
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I should add--

What I mean by glitch is this: I just measured 120.2 to 120.4VAC (fluctuating a bit) at the computer here. The DMM measured DC as 260mV to 280mV (quickly alternating), however it read the same "offset" in the reverse direction. It should read -270mV if it were true, but instead it's all meter error. Even so, it's the same AC creating the error and the same meter, so half the difference in these measurements should show actual DC - but the numbers are exactly the same. So as I expected, I have no DC offset on this circuit. YMMV, but I doubt by much.

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Old 16th September 2004, 03:15 PM   #19
MikeB is offline MikeB  Germany
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Hmm, about the mainsdcblocker.gif , i might be paranoid, but doesn't
the combination of L1/L2 + C2/C3 form a nice oscillator ? Aren't
resistors in series with C2/3 a must ?

Mike
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Old 16th September 2004, 03:19 PM   #20
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My question as to how to match capacitors remains unanswered.
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