[R-390] Capacitor calculation

[Gord Hayward]

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

This is correct.  The regulator has a minimum voltage where the 
transistors drop out and the output voltage also drops.
For the 78xx series this is about 2.5 volts.  There are newer low drop 
out chips available.  Too much ripple means that
the high end of the voltage cycle is too big so that shows up as heat 
too. The best is to have the minimum slightly above
the drop out voltage - allow a bit for brown outs - and the maximum as 
low as possible.

The capacitor calculation goes as follows:

RMS transformer voltage x 1.414 = peak   then  maximum voltage = peak - 
0.7 (half wave) or peak - 1.4 (full wave)
Ripple voltage = maximum voltage - dropout voltage
Capacitor in Farads =  (DC load current / Ripple voltage) x 1/60 sec for 
half wave or 1/120 sec for full wave

I've neglected the conduction angle (time) - it works as a safety factor 
in this calculation.

-- 

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