[Amps] Equalising resistors with HV diodes

[R.Measures]

Hello, Roger -- In early times, it was thought the "equalizing" 
resistors might be a good idea.  If all of the rectifiers in a 
series-string have the same, measured PIV, equalizer resistors will 
cause no harm.  However, what if the rectifiers have different PIVs?  
In your FWB power supply, the actual PIV across each of the 4-legs =s 
the no-load V on the filter-C. -- which will probably be c. 2400v.,  
Let's say that three rectifiers are used per leg, for a total of 12 
rectifiers.  11 of the rectifiers measure 1000PIV and one of the 
rectifiers measures 600PIV.  Thus, one of the legs will have a total 
PI-capability of 2600V and the other three legs will have a total 
PI-capability of 3000V.  Since the total PIV of the weak leg is not 
exceeded during operation, no failure should occur.  Next, add 
"equalizer" resistors.  This causes an equal 800v to be applied to each 
rectifier.  This causes no problem for the 1000piv units, but 800v 
reverse causes the 600v rectifier to avalanche and short, which in turn 
applies 1200v to each of the remaining 1000v rectifiers, which causes 
them to short -- sort of like falling dominoes.
-  note - the new ARRL Handbooks explain this problem - but in less 
detail.
cheerz

On Sep 18, 2004, at 8:55 AM, Roger B wrote:

> Hello to all
> I am designing a 2kV PSU for a 1kW amp (bridge not
> doubler).
> About equalising resistors and capacitors with HV
> diodes, some says it is absolutely necessary. Others
> say it is a waste of time with modern diodes.
> Any comment will be appreciated. 73 Roger
>
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org