From: Fred Bloggs
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Subject: Re: How to get CMOS counter to drive a relay?
References: <0001HW.B9CEF278023A90B81662EAD0@news.covad.net> <3DA9D6CC.email@example.com> <3DA9D9E1.firstname.lastname@example.org> <3DA9EE2A.B7CFD919@juno.com> <3DAAF96D.email@example.com> <firstname.lastname@example.org>
Date: Tue, 15 Oct 2002 14:25:42 GMT
NNTP-Posting-Date: Tue, 15 Oct 2002 07:25:42 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
John Fields wrote:
> A gate resistor from the 4060 to the FET, a Zener from the gate to
> ground, and a catch diode across the relay coil, and the thing will
> be pretty much bulletproof. Maybe a Zener from the drain to ground
> just in case and that should just about do it, no?
Well add all that and you are no better off.
> Then, what about those dopey two-transistor switches of yours.
Only one-transistor is a switch, the other is an emitter follower which
goes from cut-off to active.
> Even an old-timey RCA CD4060B can source 360µA at 125°C with a 5V
> supply and still supply an output voltage of 4.6V. Something like a
> 2N4401 with a guaranteed Beta of over 100 with the relay coil
> current of 20 mA flowing through it means the base current only
> needs to be 200µA. More reasonably, at room temp with a 10V supply
> we're looking at something like 1.3mA out of the chip, so forcing
> the Beta to about 15 would get rid of everything except the base
> resistor, one transistor, the relay, and the clamp diode.
The OP says HVAC. Trying running your reasoning through -40C to 85C and
then come back with something real. Your room temp and typical spec
reasoning is okay- except maybe you really do not allow enough margin
for the relay drive.