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In article <5705@ut-ngp.UUCP>, jjr@ut-ngp.UUCP (Jeff Rodriguez) writes:
> 
> I'm building a general-purpose power supply
> ...
> How do I choose the size of capacitor to place across 
> the output of the rectifier?  I once heard that the 
> rule-of-thumb is 1000 micro-F per 1 A of current.  
> Is that reasonable?

	The size of the filter capacitor is determined by two factors:
(1) the maximum output current; and (2) the maximum permissible ripple
voltage.
	The usual design equation is given below [@ 60 Hz]:

	C = (I / dV) * (8.33 x 10^-3)

	C = capacitance in farads
	I = maximum output current in amperes
	dv = maximum ripple voltage in volts

	Hence, there is no hard and fast rule which is solely determined
by output current.  Furthermore, the above equation is valid only for a
60 Hz power supply; a more generalized equation is used for other
frequencies when dealing with DC/DC converters and switching power supplies.
	The real selection criterion is how much ripple will the application
circuit tolerate.  In the real world, that can cause C to vary from 100 to
10,000 uF per ampere.  Personally, regardless of the equation or application,
I would never use LESS than 1,000 uF for any power supply with an output
voltage less than 50 volts.

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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