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Re: [Amps] Equalising resistors with HV diodes

To: Radio WC6W <wc6w@juno.com>
Subject: Re: [Amps] Equalising resistors with HV diodes
From: R.Measures <r@somis.org>
Date: Mon, 20 Sep 2004 12:39:52 -0700
List-post: <mailto:amps@contesting.com>

On Sep 20, 2004, at 8:53 AM, Radio WC6W wrote:



On Mon, 20 Sep 2004 14:08:51 +0100 peter.chadwick@Zarlink.Com writes:


Good morning Rich & Peter,


Rich said:


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.

Thus, creating a known operating condition. Without the R's, one is
relying upon the random reverse resistances & the unspecified avalanche
characteristics of common diodes to set the voltage distribution.


Equalizing resistors exacerbate the problem of using less than perfectly matched diodes.

Without the R's, a string of unmatched and/or non-avalanche rated
diodes might, or might not, exhibit a higher reverse breakdown capability

The avalanche characteristic is built-in at the time of epitaxy.


but, this figure would wander around with temperature as it effects
leakage resistance.  One diode in the string (likely the best, lowest
leakage, unit!) would be the most stressed and prone to early failure.

Stress is caused by reverse current. If the reverse currents are equal, how can one diode be stressed more.


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.

C's are used for transient protection in a low frequency rectifier
application. They should be selected to be large enough to swamp the
diode's intrinsic C to but, not so large as to drive excessive avalanche
energy into the individual diodes during an "event".

Why wouldn't a C-filter store the rectified energy from a transient.


Richard L. Measures, AG6K, 805.386.3734. www.somis.org

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