ve6nap@rac.ca
-----Original Message-----
From: measures [SMTP:2@vc.net]
Sent: Tuesday, February 22, 2000 8:15 AM
To: Peter Chadwick; Steve Thompson; amps
Subject: RE: [AMPS] Diodes
>Rich says:
>
>>On each cycle, 'equalised" diodes are forced to have unequal reverse
>>currents by the "equalisation" circuitry.
>
>So what? That doesn't matter. What causes breakdown in the junction is
>excess reverse current, leading to heating which causes more current
>which....etc. It doesn't matter if the currents are unequal. What does
>matter is if the voltage across one junction gets so high that the junction
>breaks down,
Two diodes rated at 100piv minimum are connected in series. One diode
measures 101piv and the other measures 190piv. The diodes are wired in
series. No ''equalization'' is used. A mains surge of 250p-v appears.
Does either diode conduct in the reverse direction?
[Gerald Caouette]
If one designs a circuit so poorly that the equalized diodes
do not have a safety factor of at least 4 for just such an occasion
and did nothing on the mains input to protect from over voltage
they get to go back to the drawing board and forget about a raise
that year.
Two diodes rated at 100piv minimum are connected in series. One diode
measures 101piv and the other measures 190piv. The diodes are wired in
series. ''Equalization'' is used. A mains surge of 250p-v appears.
Does either diode conduct in the reverse direction?
[Gerald Caouette]
Both will conduct and which if any conducts to destruction
depends on the rise time of the surge voltage , junction capacitance, junction
chemistry, etc..
>From the examples stated, your designs and those you seem familiar with lack
input over voltage suppression and adequate voltage ratings , a fast rising
voltage transient
(dv/dt) would also be influenced by the capacitance of the reverse junctions
which may
still result in a failure of one or both of diodes . High value non inductive
resistors across
the diodes do help equalize voltages under these and several other conditions -
Good practice would seem to indicate installing some form of input overvoltage
protection,
conservatively rated components and applying equalization to the diode and
filer banks.
>.......
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