Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!kitty!larry 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 <> UUCP: {allegra|ames|boulder|decvax|rocksanne|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {hplabs|ihnp4|mtune|seismo|utzoo}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"