On Sep 20, 2004, at 7:38 AM, Steve Thompson wrote:
On Monday 20 September 2004 14:08, peter.chadwick@Zarlink.Com wrote:
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.
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.
If I understand it correctly, equalising resistors might hurt you only
if you
get a reverse voltage spike that exceeds the specified breakdown of
the diode
chain. In many, if not most, power supplies will a modest mains filter
all
but eliminate that likelyhood?
Steve - In the example I gave, adding equalizer resistors would cause
the lesser piv diode to fail and take out the other diodes in the
series string without any spike having come along.
Steve
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Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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