[AMPS] Diodes

[Peter Chadwick]

Rich says:

>On each cycle,   'equalised" diodes are forced to have unequal reverse 
>currents by the "equalisation" circuitry. 

So what? That doesn't matter. What causes breakdown in the junction is
excess reverse current, leading to heating which causes more current
which....etc. It doesn't matter if the currents are unequal. What does
matter is if the voltage across one junction gets so high that the junction
breaks down, effectively removing that much PIV capability from the chain,
and stressing the other junctions further. Junction breakdown may be
straight breakdown of the junction, or a surface effect, or even exacerbated
by defects in the silicon crystal structure. Either which way, it's volts
that initiates breakdown.

> Unequalised diodes always have identical reverse currents.  

Bot not identical voltages across them.

If you have branded diodes from the same date code, and/or guaranteed
controlled avalanche characteristics, then probably it's OK. If not, I still
maintain voltage equalising resistors are a good idea. But not the cheap any
source resistors - a voltage equalising resistor across a 1kV diode should
be rated as a 1kV resistor. (I think Rich has made this point in the past,
saying that Mouser do suitable resistors at low price). Disc ceramic
capacitors across each diode do need matching in capacity - the +80/-20%
tolerance typical on a 1000pF 1 kV disc could make things worse, rather than
better. Do such disc caps drift the same way with time and temperature, does
anyone know? If not, they should be avoided.

	> The 1995 to 2000 Handbooks advise to not use "equalization".  

They don't advise nichrome parasitic suppressors either.  Just because it's
the ARRL Handbook, it's not guaranteed to be correct.



73

Peter G3RZP


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