Re: Diode Strings

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Richard W." <potluckutk-at-comcast-dot-net> 


----- Original Message -----
From: Tesla list <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Saturday, August 21, 2004 7:57 AM
Subject: RE: Diode Strings


 > Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz>
 >
 > Perhaps I might be permitted to expand on this slightly:
 >
 > On 20 Aug 2004, at 7:42, Tesla list wrote:
 >
 >  > Original poster: "Steve Conner" <steve.conner-at-optosci-dot-com>
 >  >
 >  >  >Yes, the PIV is 40 KV.  But it will be distributed across
 >  >  >two legs of a bridge
 >  >
 >  > I have seen quite a few people mention this now so I couldn't let it
 >  > pass. it's wrong!
 >  >
 >  > The reverse voltage seen by any diode (or diode stack) in a bridge
 >  > rectifier is equal to the DC output voltage, ie 1.4 times the RMS AC
 >  > input voltage. The reason is that when one diode is conducting, the
 >  > voltage across it is "negligible" therefore the whole DC output
 >  > voltage must appear across the other diode on that side of the bridge.
 >  > (If you draw the diagram you can see that the two diodes are in series
 >  > across the DC bus.)
 >  >
 >  > So if you're making a rectifier to have 40kV DC output, your diodes
 >  > must be rated at least 40kV PIV each. Preferably 1.5 to 2 times more
 >  > for safety.
 >  >
 >  > Steve C.
 >
 > It actually depends on whether the bridge is feeding a capacitor or
 > not which, if it is, the above arguments rigidly apply since the
 > reversed diodes have to hold off the voltage across the capacitor
 > plus the peak voltage of the transformer.
 >
 > Malcolm
 >
 >

I setup a low voltage bridge using a filter cap  and using an O-scope looked
at the voltage across each diode. There was only 1/2 of the peak-to-peak
voltage across any one diode. In other words the diode in any leg needs to
be 1.414 x RMS even with a filter cap.

Of course add some voltage rating for safety.

Rick W.