Paul Sanchez wrote:
How much is "too much" capacitance for the primary filters, using
full-wave-bridge rectifiers?
Thanks

There isn't a simple answer, I can't say "Oh, you know, anything over
100 uF is too much." When using capacitance-input filters, there will be
a high inrush current at every wave peak, while the capacitor recharges.
This inrush current depends essentially on the total series resistance,
including the transformer's "effective series resistance," diode
resistance, and capacitor's ESR.

This has a number of effects:

1) It can exceed the ratings of the rectifiers. This is especially an
issue with vacuum rectifiers, and the spec sheets will therefore give a
maximum recommended capacitance for a given voltage and transformer
effective series resistance.

2) It will significantly increase the heating of the power transformer
for a given output current. This is because the power dissipated is
proportional to the SQUARE of the current, whereas output current is
merely the AVERAGE over a half-cycle.

3) It can cause excessive buzzing in amplifiers, especially if you're
not very careful to avoid noise being picked up via the ground returns.

3a) It can significantly increase the mechanical noise of the
transformer itself.

What's more, the duration of this current becomes longer with higher
input capacitance. So, even though beyond a certain point there won't be
an effective increase in peak current, the duration of this current will
increase with possibly harmful effects.

The trade-off in the other direction of not enough input capacitance, is
poorer regulation (more voltage sag under load), and the need for higher
inductance filter chokes (if using CLC) or higher value resistor (CRC).
With CRC the regulation gets even worse due to the higher resistance
value required to keep ripple down low enough.

Practically speaking -- for the usual supplies around 400 volts, 50-100
mA output current, I'd say that 100 uF is about the limit for
solid-state rectifiers, although there are exceptions. (For instance,
when building supplies demanding higher output current, you'll often
need higher input C). The maximum is usually significantly lower (20-50
uF, depending on the rectifier) for vacuum rectifiers.

Cheers,
Fred
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