[Amps] Equalising resistors with HV diodes

[Radio WC6W]

On Tue, 21 Sep 2004 16:49:39 +0100 "Ian White, G3SEK"
<G3SEK at ifwtech.co.uk> writes:

Hi Ian,

> Radio WC6W wrote:

<snip>

> >   Are your pet diodes (1N5408) characterized in avalanche?

   Longest (short version) response I've received in a while sans an
answer...

  <snip>

>But modern rectifier diodes don't have that mode of arc failure any 
> more. You may not find it on the data sheet, but all modern silicon 
> rectifier diodes are constructed so that excess reverse voltage 
> causes a "zener-like" avalanche breakdown at a constant voltage, which
is 
> lower than the voltage required to cause a destructive arc. If the
reverse 
> current is limited by the other diodes in series that have *not* gone 
> into breakdown, a brief reverse-avalanche event in an individual diode 
> is usually survivable.

   Ahh... but, by Rich's rule (Kirchoff is not on this list...) of series
circuits all the diodes must have equal currents.

   Therefore, if one diode experiences a sudden increase in current then,
they all must... though, perhaps, not all at the exact same time!

   If one actually performs the experiment with a string of diodes, one
sees a very messy response as the individual diodes in the string go in
and out of avalanche, due to their individual capacitances charging and
discharging, until enough voltage is applied to push them all over the
edge.

   > If anyone is still worried, and still can't resist the urge to throw

> money at the problem, then spend it on a mains filter.

  One of the worst transient sources might just be that large power
transformer (when switching off... ) located beyond the mains filter! 


> 73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
> http://www.ifwtech.co.uk/g3sek

73,
   Marv  WC6W






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