The modern 1N5408`s do not require bridging with resistors&capacitors and
can be found on the internet very reasonable in price. I use series parallel
strings by matching diodes using my hipot tester to determine reverse
v.breakdown of each diode and then using matched diodes in my
FWB/FWCT,etc...Airflow is always important and strings should be arranged
for most effective air cooling.The last time I lost a diode string was in
the late 1970`s and that was using the old 2.5 amp/1kv Radio Shack diodes
with "Protection" and that during a glitch with the old 4-1000`s...I have
also tested and used many of Franks (K2AW) Diode blocks and find these test
very closely,however being enclosed in epoxy are somewhat harder to cool...I
found a local operator to test my amps....He likes to get every last
watt&then some so if things survive him I guess I did it right!
73 & happy turkey day to everyone,Jim K7RDX...----- Original Message -----
From: "Manfred Mornhinweg" <mmornhin@gmx.net>
To: <amps@contesting.com>
Sent: Friday, November 16, 2007 1:13 PM
Subject: Re: [Amps] Using HV rectifiers from microwave ovens
> Hi Angel,
>
> > I was thinking of using about 1 ohm. This would produce a voltage drop
> > of about 0.5 volts at 500 mA. Ten times more than the mismatch between
> > two individual rectifiers. Does this seem right?
>
> No, it's not enough. The problem is thermal runaway. If one diode gets
> warmer, it will reduce its voltage drop, take a larger current share,
> get even hotter, etc. You would end up with most current in one diode.
>
> To be safe against this problem, you need about 0.5V drop in the
> resistor for EACH silicon junction. Since your rectifiers seem to have
> six diodes in series, you would need about 3V drop in the resistors.
> With 0.5A total, that's 0.25A in each resistsor, thus about 12 Ohm. A 2
> Watt rating should provide enough safety margin.
>
> But all this is rather academic. I agree with Carl. 1N5408 diodes are
> cheap, rugged and plentiful. Use strings of them in series, and you
> probaly don't even need any resistors nor capacitors with them. In my
> amp I have strings of 5 of these diodes in series, in each leg, for
> 2800V, which is still really cheap and gives ample safety margin. I'm
> using just the bare diodes, with no resistors nor capacitors. So far it
> works, and has worked for years.
>
> Manfred.
>
> ========================
> Visit my hobby homepage!
> http://ludens.cl
> ========================
>
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