Tweakers' Asylum

RE: Torroidal transformer hum and DC Blocker DIY

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Tweekeng

In Reply to: RE: Torroidal transformer hum and DC Blocker DIY posted by HumanMedia on March 07, 2009 at 18:37:57

At first glance I would say it looks like the diodes are just going to let the DC go right on passed the caps.

No matter what the polarity of the DC you have two forward biased diodes for it to pass through.

Better check your design again, because I don't see this doing anything.

Follow Ups:

RE: Torroidal transformer hum and DC Blocker DIY - Ugly 09:36:03 03/08/09 (2)
I think the concept relies on DC being smaller than the forward threshold voltage of 2 diodes in series, about 1.4VDC depending on the diodes. I believe it would be fairly uncommon to find DC much higher than that in typical environements. Any DC voltage less than the series combination of threshold voltages of the diode would be blocked.

The thing that worries me about this circuit is the low voltage non line rated caps they seem to be using. Sure while the diodes work properly the caps are fine. If those diodes start failing (lightning etc.) there is a real fire hazard there.

Hopefully some of the pro electricians can pop in and comment on this. I suspect that by the time you make it safe with line rated caps the cost goes so high you can no longer afford to build it.
- RE: Capacitors - rick_m 10:33:09 03/08/09 (1)
  I would say it's OK.
  
  Since the filter is all in series if parts were to fail short, say the capacitors due to a diode failure, no risk would ensue as the load is rated for the full line voltage and provides impedance limiting. The worst case would be a small fire hazard from the cap blowing which could be ameliorated by putting the assembly in a metal box.
  
  Regards, Rick
  - RE: Capacitors - Ugly 15:02:27 03/08/09 (0)
    Ah sure I see. A shunted failure would be much worse than this series stackup. Makes sense.

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