Peter Chadwick wrote:
>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".
>
This is another of those recurring FUQs (Frequently Unanswered Questions
- what else?).
When it came around about two years ago, I checked direct with the
author of the revised Handbook chapter (a power supply engineer to MIL
standards). He said that "equalization" became necessary because the
first rectifier diodes had relatively low PIV, were too expensive to
throw in lots more diodes in series, and failed catastrophically by
arcing over or through the silicon die. All of that has changed. Modern
rectifier diodes have higher PIV, lower cost and also have avalanche
breakdown characteristics which are not necessarily fatal.
Therefore in his professional view and that of the diode manufacturers
he consulted, it is not necessary to use equalization components - so
that's what the Handbooks now say.
For me, the whole question about how close you can come to breaking down
a string is not relevant. If in doubt, use more diodes - they're cheap
enough! We amateurs cannot insist on diodes that have been specially
high-temperature-reverse-bias tested, but so long as the diodes are
reasonably well matched, eg full-spec brand-name diodes from the same
bandolier, we should be pretty safe. It *has* to be worth the extra few
cents.
However, someone also pointed out that the economics are different for
large amplifier manufacturers, who are very cost-conscious but can get
well matched ceramic disc caps much more cheaply than we amateurs can.
Then the economics may tip back towards using fewer diodes with
equalization capacitors.
There are also EMC considerations where capacitors can helpful in
reducing reverse breakdown spikes going back through the mains, so
perhaps capacitors may be making a comeback in these more EMC-conscious
days.
>They don't advise nichrome parasitic suppressors either. Just because it's
>the ARRL Handbook, it's not guaranteed to be correct.
>
Depends who's writing it... and also who's reading it :-)
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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