Newsgroups: sci.electronics Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!uupsi!rodan.acs.syr.edu!amichiel From: amichiel@rodan.acs.syr.edu (Allen J Michielsen) Subject: Re: Power supply design Message-ID: <1991Jun25.032937.27129@rodan.acs.syr.edu> Organization: Syracuse University, Syracuse, NY References: <1991Jun18.175131.8374@ux1.cso.uiuc.edu> Date: Tue, 25 Jun 1991 03:29:37 GMT In article wayned@wddami.spoami.com (Wayne Diener) writes: >In article 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. >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. Depending on your noise requirements, you may well find that the lm350 and straight rectified ac will produce so much noise at high current usage that it is unusable. You can improve this with higher input voltages (smaller zero crossing times) and larger capacitors, but you end up with a pretty big box that you may be able to buy cheaper.... Then, with higher current usage, and higher input voltages, the power dissipated in heat becomes critical to the lm350 and either massive heat sinks or fans are needed. If noise (and we can say, like 5 volts) isn't a problem, then do it. al -- Al. Michielsen, Mechanical & Aerospace Engineering, Syracuse University InterNet: amichiel@rodan.acs.syr.edu amichiel@sunrise.acs.syr.edu Bitnet: AMICHIEL@SUNRISE