[Amps] Equalising resistors with HV diodes

[R.Measures]

On Sep 21, 2004, at 7:29 AM, TexasRF at aol.com wrote:

> So we have a string of diodes with a few microamps flowing in the 
> reverse
> direction. If one of the diodes is rated for 200v and and there is 
> 300v across
> it, will it fail?

--  Not necessarily.  I have tested diodes with a 200piv rating that 
measured >700v.  I have also tested "1000piv" diodes that measured 
900v.  This is the reason why I like to test components.  
>
> I guess I haven't seen the explanation for the failure mechanism in 
> silicon
> diodes. If the reverse current really is limited will a failure 
> inevitably
> happen?

No, but the only practical way to limit reverse current to a safe value 
is to use more total piv than exists in the circuit.  For example, in 
the current project, the actual piv per leg should c. 9900v.  I used 
14, 1000v-rated 1N5408 diodes per leg that measured c. 1300piv each on 
a high-pot tester.  This will give a safety factor of c. 8000v.
>
>
Richard L. Measures, AG6K, 805.386.3734.  www.somis.org