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From: phil@brahms.amd.com (Phil Ngai)
Newsgroups: sci.electronics
Subject: Re: 12V to 5V DC conversion (for TTL logic in car) wanted
Message-ID: <1990Jul13.192209.8178@amd.com>
Date: 13 Jul 90 19:22:09 GMT
References: <1990Jul12.024412.122@mintaka.lcs.mit.edu>
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In article <1990Jul12.024412.122@mintaka.lcs.mit.edu> streeter@im.lcs.mit.edu (Kenneth B. Streeter) writes:
|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
|...
|Driving the entire circuit from an external 5-volt supply (bypassing
|the regulators) indicates 1.26 amps of current being drawn.

3 amps times 7 volts drop = 21 watts. I don't think you can do that
with the To-220 case the 7805 comes in, especially without a heatsink,
which you didn't mention you were using. You'd have a better chance
with a To-3 but you still need to put some effort into heatsinking.

This might be overkill but way back in 1979 I built a DC-DC switching
converter to go from 5 to 12 volts. It worked very well and would
avoid this heat dissapation problem you have. Lower temperatures==
better reliability. There's a lot of single chip solutions available
now. I did wind my own toroidal transformer but that wasn't hard.

--
--
Phil Ngai, phil@amd.com		{uunet,decwrl,ucbvax}!amdcad!phil
PALASM 90: it's not the same old PALASM any more!