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From: brianr@tekig5.PEN.TEK.COM (Brian E Rhodefer)
Newsgroups: sci.electronics
Subject: Re: 12V to 5V DC conversion (for TTL logic in car) wanted
Keywords: LEDs
Message-ID: <6558@tekig5.PEN.TEK.COM>
Date: 12 Jul 90 22:00:49 GMT
References: <1990Jul12.024412.122@mintaka.lcs.mit.edu>
Reply-To: brianr@tekig5.PEN.TEK.COM (Brian E Rhodefer)
Distribution: na
Organization: Tektronix, Inc., Beaverton,  OR.
Lines: 31

In article <1990Jul12.024412.122@mintaka.lcs.mit.edu> streeter@im.lcs.mit.edu (Kenneth B. Streeter) writes:
>
> ... [abridged]
>
>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.

I've seen several replies to Ken's question that missed the obvious
solution, so I suppose I can toss my .02$ in:

In the stated application, the logic ICs are the only components which
require a well-regulated power supply, and, if they are indeed all CMOS,
they could probably be supplied by a single TO092-packaged 78L05, with
NO heatsink. 

The only significant consumer of current is the bank of "5 or 6
hexadecimal LEDs".  Now, why insist on feeding the LEDS on regulated
5V power?  I presume they're being driven by open-collector devices,
so why not run them directly from the +12VDC supply?  The same amount
of power is wasted, only now it's ALL burned in the series dropping
resistors, which, of course, must be re-sized for the 12V operation.


Brian Rhodefer