some comments on linear power supplies: was : RE: [sdiy] Power Supply Design Questions

[Ingo Debus]

Czech Martin wrote:
> -one could think that a very large power supply capacitor
> (infinitely large) would be best. This is not the case!
> A very large cap will look like a short for the poor transformer.
> So a very high current will flow.  The ugly thing is that a
> real large cap will allow charging only during the very peak
> of the rectified sine wave, so the large charge current
> will only flow during a short moment, i.e. very high peak.

Are you sure about this?
First, the cap does not look like a short, it looks like a voltage 
source (only if discharged completely it would look like a short). 
With a lower cap, i.e. higher ripple, the cap is at a lower voltage 
when the rectifier starts conducting again.
Yes, the higher the cap value, the shorter the time the rectifier 
conducts. The same amount of energy has to be transferred in a shorter 
time. But does this mean that the peak current is really higher?

There are some formulas in Tietze-Schenk for the peak current in a 
full wave rectifier circuit with filter cap. These formulas do not 
contain the cap value.

OTOH, H&H say "An oversize capacitor [...] increases transformer 
heating (by reducing the conduction angle, hence increasing the ratio 
Irms/Iavg)." (page 330)

Ingo