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Old 16th September 2004, 03:30 PM   #21
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Quote:
Originally posted by Geek
The mains DC is from an asymmetrical waveform due to incorrect power distrobution, loads or interference on the lines. Because of that, the circuit is, IMO, useless.

Funny circuit. I would think it actually CREATES offset (low at that) what with the diodes which are probably not matched. The caps across the diodes are pretty useless, no? the waveform across the diodes looks very much a square wave with 1.2V amplitude or so. That means that the voltage across the transformer, which is the mains minus the voltage across the diodes, looks like sine with the middle part missing, that's disturbingly similar to good old xover distortion waveform. Why would that stop a toroid from buzzing?
Strange.

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Old 16th September 2004, 03:37 PM   #22
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I ran this in LTSpice. If you simulate it, the sine wave goes through just fine. Without the capacitor that does not happen. The cap lets through part of the waveform until the diodes begin conducting, omit it and the waves get distorted.
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Old 17th September 2004, 11:50 AM   #23
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As long as the reactance of each is well under the resistance of the load (so it produces less than a volt AC drop), it won't matter. As a plus, you can also use 6.3V caps, odd as it may seem on a 120V line.

The L+C is a filter, even if it did oscillate (read: ring due to an external source) it would be at some random frequency, depending on leakage inductance.

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Old 17th September 2004, 02:35 PM   #24
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Quote:
Originally posted by MikeB
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
If you have a LC-filter you can always get a resonance, but not an oscillator! You must have an active element for this. But nevertheless the filter designer must know how much Q the circuits has in order to determine the filter characteristics.
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Old 17th September 2004, 02:37 PM   #25
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Quote:
Originally posted by janneman

Funny circuit. I would think it actually CREATES offset (low at that) what with the diodes which are probably not matched.
The diodes are for inrush current transients. If the DC level is more than 0.6 V you must either change the caps to a bigger value or add more diodes in series or use Zeners.

The cap values is determined by the max steady state current. If one diode is needed the max voltage drop over the cap is 0.5 - 0.6 V.

4 A, 50 Hz, 0.6 V max = 21221 uF
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Old 17th September 2004, 02:48 PM   #26
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Quote:
Originally posted by peranders

If you have a LC-filter you can always get a resonance, but not an oscillator! You must have an active element for this. But nevertheless the filter designer must know how much Q the circuits has in order to determine the filter characteristics.
Yes, that's what i've meant. Anytime a spike comes in, the LC is
pushed, and could create a HF-ringing. I am missing a lot of english
words for these things. In german the combination of L & C is called
"Schwingkreis", this is normally used for AM-receiving.
My problem with the filter is, that these Spikes appear often in the mains.

Mike
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Old 17th September 2004, 03:21 PM   #27
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Any tank can be considered an oscillator (an "oscillating system", pendulum, spring and weight, inductor and capacitor, etc.), but us electronics-heads take it to mean an active device.

Tim
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Old 17th September 2004, 04:06 PM   #28
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Peranders, what's the formula you used for that calculation?
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Old 17th September 2004, 04:20 PM   #29
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Quote:
My question as to how to match capacitors remains unanswered
Put them in opposite arms of an AC bridge (easily thrown together if you don't have one).
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Old 17th September 2004, 05:20 PM   #30
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Peranders, did you solve for the -3 dB corner, as that's what it looks like? Don't you think that the -3 Db is a rather arbitrary choice? Why not -1 dB or -5 dB instead?

I wonder if the distortion added by insufficiently large caps may be worse than a little DC.
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