Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!lll-winken!uwm.edu!wuarchive!zaphod.mps.ohio-state.edu!ub!dsinc!netnews.upenn.edu!eniac.seas.upenn.edu!depolo From: depolo@eniac.seas.upenn.edu (Jeff DePolo) Newsgroups: sci.electronics Subject: Re: WAIT! - Re: 12 to 9 volts dc (kinda long) Keywords: safety Message-ID: <38498@netnews.upenn.edu> Date: 1 Mar 91 01:24:17 GMT References: <1991Feb10.220719.17078@cbnewsl.att.com> <1991Feb26.235009.1389@cbnewsj.att.com> <5825@optilink.UUCP> <4582@eastapps.East.Sun.COM> Sender: news@netnews.upenn.edu Reply-To: depolo@eniac.seas.upenn.edu (Jeff DePolo) Distribution: na Organization: University of Pennsylvania Lines: 37 In article <4582@eastapps.East.Sun.COM> gsteckel@east.sun.com (Geoff Steckel - Sun BOS Hardware CONTRACTOR) writes: >The LM317 is a good chip for this sort of linear stepdown application. >BUT: > the automotive alternator/battery/voltage regulator combination can produce > violent transients, AND > most power semiconductor devices fail shorted under these conditions > >Do you really want to send the 140 V load dump or the 1500 V spike through >your homebuilt regulator into your $200+ portable stereo? I didn't think so. >(:-) The monolithic voltage regulator can only do so much. > >You want at least a series fuse, a zener/MOV overvoltage protector, and some >RFI/EMI protection - the little microprocessor in a Discman gets confused easily. >Reverse voltage protection is nice, too. Only about $2 for parts, too. I think this is getting a bit silly. First of all, the adapter that Sony sells doesn't have that much stuff in it - probably a simple regulator, maybe a filter cap for minor de-spiking, reverse polarity protection (diode), and a fuse. That's it. There is no RFI/EMI protection, no MOV, _maybe_ a zener for OV. How much do you think they're going to give you in a $20 adapter (maybe $5 worth of parts)? The easiest way I can think of is also the simplest (KISS). Take a 78H09 regulator, mount it to something (a very small project box), put a back-biased diode across the 12v input, and a fuse in-line with the output. Total cost: $4.00 + cost of cigarette lighter plug. EMI/RFI - forget it. An MOV is your only hope of protecting the unit from high-voltage spikes, but again, I've never seen the need. Look at all of the car electronics that don't use them - is everyone else wrong? I don't think so. It's overkill, IMHO. -- -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- Jeff DePolo N3HBZ Twisted Pair: (215) 386-7199 depolo@eniac.seas.upenn.edu RF: 146.685- 442.70+ 144.455s (Philadelphia) University of Pennsylvania Carrier Pigeon: 420 S. 42nd St. Phila PA 19104