[R-390] Power supply capacitors

[kenny]

Gord Hayward wrote:
>
> 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)
>
That's really interesting what you say Gord about the extra energy 
showing itself as heat in the regulator. That didn't even occur to me. 
My particular circuit uses a 2200uF and LM317 (among other things) to 
produce a stable and regulated 12.6 volts for the filaments of the BFO 
and PTO tubes and the regulator DOES run hot...even with a good sized 
heatsink it will heat up the heatsink quite nicely. I had a nagging 
feeling 2200uF was too big....so I'm going to start playing with it 
until I feel I have a good balance between acceptable performance and 
long term reliability.

Regards, Ken (ZL3AA)