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From: strong@tc.fluke.COM (Norm Strong)
Newsgroups: sci.electronics
Subject: Re: 12V to 5V DC conversion (for TTL logic in car) wanted
Message-ID: <1990Jul13.181750.6637@tc.fluke.COM>
Date: 13 Jul 90 18:17:50 GMT
References: <1990Jul12.024412.122@mintaka.lcs.mit.edu>
Distribution: na
Organization: John Fluke Mfg. Co., Inc., Everett, WA
Lines: 31

In article <1990Jul12.024412.122@mintaka.lcs.mit.edu> streeter@im.lcs.mit.edu (Kenneth B. Streeter) writes:
}
}Some of you may recall a previous query concerning building an
}electronic odometer for my car.  (The application is TSD rallying.)
}Well, I've pursued a number of ideas, and still haven't really reached
}a decision.  However, I am having power supply difficulties with one
}of my test circuits.
}
}In order to supply the 5V needed to drive the circuits, I am using a
}pair of 7805 regulators, using the car's 12V power as a supply.  I
}have each regulator driving about half a dozen CMOS chips (uv erasable
}PALs, mainly) and five or six hexadecimal LEDS.  (The regulators
}aren't in parallel, they're each driving separate sets of chips.)
}However, I seem to be having power supply difficulties.  The
}regulators can just barely handle the current requirements, it seems.
}They get REAL hot (even with heat sinks) and can barely muster 4
}volts.  This makes the circuit VERY flakey.
}
}Driving the entire circuit from an external 5-volt supply (bypassing
}the regulators) indicates 1.26 amps of current being drawn.
}Basically, I'd like a trouble-free, reliable 12V to 5V converter that
}can push maybe 3 amps of current, and handle the switching transients
}in the circuit.  Any suggestions?

Boy, this is your lucky day!  What you need is a National Semi LM2575-5
switching regulator.  They're sampling them now; call your local NS rep.

-- 

Norm Strong  (strong@tc.fluke.com)
2528 31st S.   Seattle WA 98144