Re: Diode strings - equalizing resistors

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

Original poster: "Peter Terren" <pterren1-at-iinet-dot-net.au> 

It is easier to use more diodes, but you must use enough.  Save the 
resistors and use more diodes. I've blown 40 x IN4007's with resistors with 
diodes as easily as 40 x IN4007's without.
Use more diodes than you think. For a 15kV NST the voltage a single diode 
and capacitor will be exposed to is 42kV. But allow for 10% variac 
increase, and a 50-100% allowance for uneven voltage sharing at the time of 
reverse bias plus transients and you should use 70-90kV.  This is up to 6 
times the intuitive level you might expect for a 15kv NST.
Similarly microwave ovens use an 11kV rated diode for a 2kV transformer ie 
nearly 6 times.
My string of 300 diodes.
http://tesladownunder.iinet-dot-net.au/Diode300kv.jpg
Peter

>Original poster: "Richard W." <potluckutk-at-comcast-dot-net> Hi List,
>I have a question concerning the use of equalizing resistors on diode strings.
>I've heard it said the reason for the resistors is that if one diode turns 
>off before the others then all voltage may be seen across that one diode. 
>That makes sense if the diode turns off anywhere else but close to zero 
>crossing. But the diode/s won't trun off until the voltage is near or at 
>zero crossing. At that point there really shouldn't be enough voltage 
>anywhere in the string to overwhelm the PIV spec of any one diode if the 
>diodes are rated at 700 vrms or 1000PIV.
>Is my thinking amiss?
>Rick W.
>Salt Lalke