On Sep 20, 2004, at 7:14 AM, DF3KV wrote:
----- Original Message -----
From: <peter.chadwick@Zarlink.Com>
Subject: Re: [Amps] Equalising resistors with HV diodes
Please explain why equalizing resistors have merit.
Because if the reverse leakage differs between diodes, the reverse
voltage
distribution also differs. The one with the least leakage ends up
with the
highest voltage across it, which could exceed the breakdown volts.
With
resistors, the voltage distribution across the diodes is fixed by
the
resistor ratios.
If the capacitances aren't matched, that will also affect the
voltage
distribution. That's one reason why it is not wise to mix diodes of
the
same nominal type from different manufacturers without some
equalisation.
Another is the reverse recovery time.
Experience also suggests that for best reliability, buy diodes from
reputable manufacturers - some of the far Eastern ones are
distinctly dodgy
froma reliability viewpoint.
Hi Peter,
If that theory is solid, all fullwave bridge rectifiers should have
equalising resistors connected across the diodes, but no manufacture
suggests that.
- Around 30-years ago, manufacturers of HV rectifier stacks stopped
using "equalizing resistors". Apparently, someone realized that the
currents in all parts of a series circuit are already exactly equal.
Adding parallel resistors can quite possibly un-equalize these
currents.
I always used triple safety on Vrrm when I used diodes in HV rectifier
stacks and never experienced failed diodes (most BY255)
73
Peter
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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