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From: wayned@wddami.spoami.com (Wayne Diener)
Newsgroups: sci.electronics
Subject: Re: Power supply design
Message-ID: <wayned.3278@wddami.spoami.com>
Date: 21 Jun 91 03:17:34 GMT
References: <1991Jun18.175131.8374@ux1.cso.uiuc.edu>
Organization: PD-News, Spokane
Lines: 30

In article <1991Jun18.175131.8374@ux1.cso.uiuc.edu> kline@ux1.cso.uiuc.edu (Charley Kline) writes:
>Hi, we're trying to build a power supply for a device that requires 3A
>at 13.8 volts. The voltage regulator will be an LM350T, a 3A-rated
>variable voltage regulator.
>
>The problem is the power transformer. I can't find anywhere a 3A
>transformer at 18 volts.
>
>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?
>
>

You also lose the forward voltage drop of the diode, but you're 
basically correct.  Try to find a 12.6 volt "filament" transformer.
It should give you about 17 volts of filtered DC...  enough overhead
to run the regulator, but low enough that you won't need a massive
heat sink.  Even then, you'll have to dissipate almost 10 watts
in the regulator.  You're also going to need pretty large capacitors
on the output of the rectifier to "hold up" the input voltage during
the AC zero crossings.

--
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|       //                  Wayne D. Diener                     |
|      //                   Spokane, WA                         |
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