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From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: Choosing Cap Size in Power Supply
Message-ID: <1884@kitty.UUCP>
Date: Mon, 27-Jul-87 00:02:38 EDT
Article-I.D.: kitty.1884
Posted: Mon Jul 27 00:02:38 1987
Date-Received: Mon, 27-Jul-87 03:53:55 EDT
References: <5705@ut-ngp.UUCP>
Organization: Recognition Research Corp., Clarence, NY
Lines: 34
Summary: It all depends upon acceptable ripple...

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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