Path: utzoo!attcan!uunet!zephyr.ens.tek.com!gvgpsa!gold!grege
From: grege@gold.GVG.TEK.COM (Greg Ebert)
Newsgroups: sci.electronics
Subject: Re: 12V to 5V DC conversion (for TTL logic in car) wanted
Message-ID: <1163@gold.GVG.TEK.COM>
Date: 12 Jul 90 15:46:28 GMT
References: <1990Jul12.024412.122@mintaka.lcs.mit.edu>
Distribution: na
Organization: Grass Valley Group, Grass Valley, CA
Lines: 28

In article <1990Jul12.024412.122@mintaka.lcs.mit.edu> streeter@im.lcs.mit.edu (Kenneth B. Streeter) writes:
>
>
>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?
>
Try an LM78H05, that sucker will crank-out 5 amps. It appears that the
regulator you are using is 'folding-back', ie lowering it's output voltage
to keep the internal heat dissipation lower [ these guys are almost blowout
proof !!]. You could also try a larger heat sink, or try dissipating the
heat elsewhere. For example, you can hook a resistor in parallel with the
regulator. You choose the resistor based upon the MINIMUM load cuurent and
the MAXIMUM supply voltage. Ooops! This is in a car - very nasty electrical
environment. You can also add some power rectifiers in series with the
supply lead - they will gobble-up about 0.8 volts apiece. Four of them will
get you down to 8-9 volts into the regulator, which will reduce the heat
dissipated by the regulators. Be advised, though, that the diodes will be
dissipating heat.

You could build a DC-DC converter, but I honestly don't think it's worth
the effort because you really don't have a tight power budget. The band-aid
fixes I propose will work, waste a few watts, but are cheap and easy.

By the way, I've hooked-up regulators in parallel (up to 4), without any
trouble. My floppy drives are alive and well after 10 years of (ab)use.