[Amps] Equalising resistors with HV diodes

[R.Measures]

On Sep 21, 2004, at 10:34 AM, Radio WC6W wrote:

>
> 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...)

Kirchoff's current law is the rule.

> 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

-  Probably The Worst

> might just be that large power
> transformer (when switching off... ) located beyond the mains filter!
>
-  With a C-filter -- or a resonant-choke in,/C-out filter and a FWB or 
FWD rectifier circuit, the energy from positive as well as negative 
spikes is rectified and stored in the filter-C.
>
>>
>
Richard L. Measures, AG6K, 805.386.3734.  www.somis.org