Power Supply Filter Capacitor Size

[Martin Czech]

It may appear that the filter cap should be as large as possible.  But in
this case the trafo sees a very low load, ie. almost a short when the
coil voltage rises.

So, one has also to calculate the cap ac current, we say "Brummstrom"
over here , "humm current?". Some caps get really hot due to those currents.
It was one of my sins of youth to buy a cheap 10000uF cap for
a 8V*10A trafo, yes the voltage rating was ok, but the ac current for 8A
load was much too high, too much heat, had to change the circuit...

There are special caps that allow for large ac currents, but maybe an
array of cheaper and smaller caps will also be a solution. Of course,
the trafo must be able to carry that peak current, too. Sometimes a
current limiting resistor or a choke is used.

There are formulas for this, but I don't remember now....


m.c.