Avalanche diodes do not need to be identical. The diodes are in series so
the reverse leakage current is the same in all diodes. When a diode reaches
the avalanche voltage the voltage drop stabilizes much like a zener diode
does.
Once one of the diodes reaches it's avalanche voltage, the current is
limited by the reverse leakage of the remaining diodes in the string. If the
reverse voltage is high enough, a second diode will reach it's avalanche
voltage and stabilize. As before, the reverse current in the string is then
limited by the reverse leakage of the remaining diodes in the string.
The idea is that the sum of the avalanche voltages for the string is much
less than the peak reverse ac voltage applied to the string and there will
always be one or more diodes that have not reached their avalanche voltage
and thereby limiting the reverse leakage current for the entire string.
Since the reverse current is limited it does not matter what the individual
diode reverse avalanche voltage might be.
I have tested a number of 1N5408 diodes (rated 1 KV/3A) for their avalanche
voltage and a typical number is around 1300 volts. If there are enough
diodes in the string none of the diodes will ever reach the avalanche voltage
which is ok but does not matter. As the ac voltage is ramped up one can
observe each diode reach it's avalanche voltage one by one. Continuing to
increase the ac voltage will ultimately result in all diodes reaching their
avalanche value and after that the current will be limited only by the
transformer and copious smoke will ensue.
73,
Gerald K5GW
In a message dated 2/2/2016 10:01:28 P.M. Central Standard Time,
dezrat@outlook.com writes:
------------ ORIGINAL MESSAGE ------------(may be snipped)
On Fri, 29 Jan 2016 09:25:39 -0800, Jim wrote:
> Dont put ANY shunt resistors across a 6A10. They don’t need them!
REPLY:
I agree with Jim but I would qualify that a little. The 6A10s must be
identical, i.e. from the same manufacturer and same batch. Do not try
to mix and match. The safest way to purchase them is "tape and reel"
so you can be sure they are all the same.
Unless they are truly identical, the reverse voltage will not divide
equally across each diode and one or more may exceed their PIV rating.
If you can not do this, then by all means use equalizing resistors.
73, Bill W6WRT
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