On Sep 21, 2004, at 10:34 AM, Radio WC6W wrote:
On Tue, 21 Sep 2004 16:49:39 +0100 "Ian White, G3SEK"
<G3SEK@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
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