NNTP-Posting-Date: Sat, 19 Oct 2002 20:27:00 -0500
From: "Dave VanHorn"
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Subject: Re: How to get CMOS counter to drive a relay?
Date: Sat, 19 Oct 2002 20:27:00 -0500
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> I simply rembered a guy, 25 years ago, who had wired some
> 7-segment led displays to a clock chip, not using any
> transistors at all. The chip ran pretty hot, but it
> worked okay. I wondered what would happen - call it an
> experiment - if you tried hooking up that 20mA relay
> without a buffer.
If you added a protection diode across the relay coil, it might work.
It wouldn't be a good idea though, unless the chip is rated to sink or
source that much current, and it's internal voltage drop at that current is
low enough that the relay will always pull in.
It's the sort of thing that you see in hobby designs, where it might work
for me, with my chips and my relay, but not for you with similar chips and
If you stay within the specs, then it's guaranteed to work. If you violate
the specs, then it still may work, but the next one may not. I've seen
PhD engineers make that mistake. (and had to clean up the mess too)