Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!sdd.hp.com!hplabs!hpfcso!hpfcdj!hpfcrlm!myers
From: myers@hpfcrlm.HP.COM (Bob Myers)
Newsgroups: sci.electronics
Subject: Re: Power supply design
Message-ID: <17660177@hpfcrlm.HP.COM>
Date: 19 Jun 91 16:20:18 GMT
References: <1991Jun18.175131.8374@ux1.cso.uiuc.edu>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 25

>My question is, am I guessing the secondary voltage wrong? Would a 14V
>secondary be okay since that is the RMS value and the
>rectified/filtered DC out of that would be sqrt(2) * 14 = 20V?

No, you're probably pretty close.  Remember that that "20V" output could only
be had from such an arrangement if there is no load - that's the peak voltage
of the transformer secondary, and the filter can't maintain that if you're
pulling charge out of the caps and sending it to the load.

An 18V secondary may be hard to find, but I would guess that there are 24V
transformers all over the place.  That might be a bit much, but if you don't
mind wasting some extra power in regulator you'll be OK.  And, as Micheal C.
already said, be conservative.  If you're really expecting a 3A load, for 
example, look for a transformer, regulator, etc., rated at, say, 5A.  5A
fixed regulators are out there, but may be harder to find.  If so, use a
smaller one and a pass transistor.  Also, don't forget to protect the
regulator by putting a hefty diode "backwards" from the regulator's output
to the input.  (That way, when the input is shut off, you won't wind up
with 13V or whatever the "wrong way" across the regulator, as you wait for
the output filter caps to discharge.)


Bob Myers  KC0EW   HP Graphics Tech. Div.|  Opinions expressed here are not
                   Ft. Collins, Colorado |  those of my employer or any other
myers@fc.hp.com                          |  sentient life-form on this planet.