[AMPS] PS diode equalizing

[Peter Chadwick]

I tend to go along with Tom here. Rich's example is fine but..........

1. If you were applying 1800v reverse, then you'd use two diodes rated at
1kV each. For an 800 piv and a 1000piv diode in series, the maximum is
really 1600volts.

2. Not having equalising resistors relies on diode leakage currents
matching with temperature, as well as diode reverse capacity - which may
apply if all the diodes are from one diffusion batch, but you can't
guarantee it.

3. The capacitor tolerance for shunt capacitors can be a problem. If the
caps are the usual +50%/-20%, the reactance of a 0.01microfarad cap can vary
from 147kohm to 318kohm at 60Hz. The resistor value needs to be small enough
that the reverse impedance across the diodes is substantially constant from
diode. Again, this suggests capacitors from one manufacturing batch. Note
the caps need to match in temp co and leakage. For 0.01 microfarad, it
suggests that the resistor shouldn't be more than 100kohm. Now you're
talking dissipation.....

I use resistors across the diodes, with no capacitors. With 1kV diodes, I
use two 500volt resistors in series, although that's probably 'belt and
braces'. If I was using capacitors, they'd be .01 microfarad 1kV working
micas or plastic film, not disc ceramic, because of temp co and tolerance.
I've never had hash caused by the diodes switching abruptly, either. With
1N5400 series, they're relatively slow, so I wouldn't expect it.


73

Peter G3RZP

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