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From: depolo@eniac.seas.upenn.edu (Jeff DePolo)
Newsgroups: sci.electronics
Subject: Re: 12V to 5V DC conversion (for TTL logic in car) wanted
Message-ID: <27042@netnews.upenn.edu>
Date: 13 Jul 90 06:33:20 GMT
References: <1990Jul12.024412.122@mintaka.lcs.mit.edu> <1163@gold.GVG.TEK.COM>
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In article <1163@gold.GVG.TEK.COM> grege@gold.GVG.TEK.COM (Greg Ebert) writes:
>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. 
>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.

Both good ideas for quick and dirty fixes.  I'll throw in another one.
Buy both a 78H09 and a 78H05 and run them serially.  It's easier to
mount and heatsink two regulators than worry about dissipation from
the power diodes.  Cascading the '09 and the '05 mounted with decent 
heat sinks should be able to provide 3 amps without a problem.  I have
a cheap-and-dirty 13.8 to 5 converter in the shack for powering small
digitial circuits just made up with a 78H05 mounted right to the back
of a small (2"h x 3"w x 2"d) aluminum box with just a heat sink pad (whatever
those things are made of) with no heat sink and I can run it at 2 amps
continuous (1 hour+) and it gets slightly warm (probably around 125 deg. F),
which isn't all that hot for a regulator.  It's been in service for
a couple of years and I haven't baked it yet.  The cost of a 78H05 is
about $2.50, so what the heck.

								--- Jeff



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| Jeff DePolo N3HBZ/AA  | Internet: depolo@eniac.seas.upenn.edu               |
| Univ. of Pennsylvania | RF: 146.685- 224.40- 442.70+ 144.455s (Philadelpia) |
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