[R-390] Capacitor calculation

[Kenneth G. Gordon]

On 11 Jun 2008 at 12:08, Gord Hayward wrote:

> >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.

For those for whom this may be unclear, what Gord is saying is that the 
voltage input to the LM-78XX should be at least 2.5 volts above the 
desired regulated voltage. I.e., 11.5 V for a 9 V regulator. This is 
sometimes, and perhaps a bit inaccurately, called "overhead".

>  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.

Thanks very much for this, Gord. I'll add it to my "bag-o-tricks" :-)

Ken Gordon W7EKB