[R-390] Power supply capacitors

[Gord Hayward]

> I had read in an article for a 13.8V @20A power supply that 'a good 
> rule of thumb when selecting the filter capacitor is to use about 
> 5000uF per Amp'. This article was written by a ham and he also used a 
> regulator with  pass transistors.


It depends on the voltage droop (ripple) that you can tolerate.  16,666 
uF will drop 1 volt in 1/60 sec at 1 amp (half wave rectification)
and 8,333 uF will drop 1 volt in 1/120 sec at 1 amp (full wave 
rectification).  You want the smallest capacitor that will give the minimum
voltage for the regulator to work.  A bigger capacitor gives a higher 
minimum voltage (less ripple) but the extra energy just shows up as
heat in the regulator.  Bigger capacity also increases the inrush 
current - the diodes only conduct for a short time so the instantaneous
transformer current  is a lot larger than the average DC load current.

Cheers, Gord (VE3EOS)

-- 

Gordon L. Hayward, Ph.D., P.Eng.,
Associate Professor, Biological Engineering,
School of Engineering, University of Guelph,
Guelph, Ontario, N1G 2W1.