[R-390] Power supply capacitors - ripple

[Kenneth G. Gordon]

On 11 Jun 2008 at 23:29, kenny wrote:

> 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.

There is another issue with 3 terminal regulators that is important: TOO 
MUCH ripple can ALSO cause them to dissipate too much heat.

Ken Gordon W7EKB