Re: Diode Strings

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

Original poster: "S&JY" <youngsters-at-konnections-dot-net> 

Steve C. Is correct - I was wrong.

If one draws a bridge rectifier and, for a given half-cycle of operation,
you replace the two conducting diodes with
wires, then it is easy to see that the remaining two diodes are in parallel
and reverse biased by 1.4 RMS AC input voltage.   For a 14 KV pig, that is
about
20 KV PIV across each bridge leg.  A factor of 2 safety factor then requires
40 diodes per leg.  60 per leg would be very conservative.

Here is more diode wisdom, from the ARRL Handbook for Radio Amateurs.
Because the rectifier never allows current to flow more than half the time,
when it does conduct it has to pass at least twice the average direct
current.  With a capacitor-input filter, the rectifier conducts much less
than half the time (only when the supply voltage is more than the capacitor
voltage), so that when it does conduct, it may pass as much as 10 to 20
times the average DC current.  Two useful diode rules of thumb are 1) The
maximum repetitive pulse current rating can be assumed to be about 4 times
the steady state rating, and 2) the maximum surge rating (e.g. when supply
is first turned on and charges the filter cap in a half-cycle) can be
assumed to be about 12 times the steady current rating.  Good information to
know when choosing diode current ratings.

There is a lot of practical information, applicable to coilers, to be found
in the Radio Amateur's Handbook.  Used ones show up on Ebay and other used
book sites.

--Steve Y.
----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Friday, August 20, 2004 7:42 AM
Subject: RE: Diode Strings


 > 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.
 >
 >
 >