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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.firstname.lastname@example.org> <3DA9D9E1.email@example.com> <3DA9EE2A.B7CFD919@juno.com> <3DAAF96D.firstname.lastname@example.org> <3DAB71EC.DB68183C@juno.com>
Date: Tue, 15 Oct 2002 14:15:15 GMT
NNTP-Posting-Date: Tue, 15 Oct 2002 07:15:15 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
> Thanks for the compliment.
> Please educate me more.
> A CD4020/40/60 counter is a cmos counter, yes?
Yes it is but it's not directly driving a relay, and it's possible to
put well over 10K ohms between it and the relay.
> The fet is driven by the same power that drives the CMOS
> counter, yes?
Not really- you should have ac-isolation between CMOS Vdd and the 12VDC
line. This is very easy to do if the CMOS loading is uADC-)
> You can take out the 7 parts without removing the diode
> on the relay coil.
> It's all isolated by a relay and can be arbitrarily shielded
> against external influence, yes?
Not necessarily, it depends on the relay, you very well may require
isolation *from* the relay. And who says the relay is not remote-)
> The FET won't require
> any more isolation/shielding than is required for the rest
> of the CMOS stuff.
Yes it will. As a minimum you should use a gate zener, and that blows
any cost advantage you think you have gained by removing a 3-cent
transistor and sub-penny resistors-)
> If you'd said, "dont use cmos anywhere in the system," you
> would have an arguable point.
Well a single MOSFET is not CMOS-) The CMOS logic can be protected under
a single umbrella with lots of isolation between it and the outside.
> In any practical application, additional protection may be required
> to meet demands not indicated on the schematic.
> There may be lots of issues, but if the two-transistor circuit
> works, the FET will work with 7 fewer parts. And that's ALL I said.
I understand that, and I am saying that is not correct.
> I am proud to be a minimalist. That doesn't mean I don't know
> what I'm doing. Minimalist design is not convenient; it's
> VERY difficult to produce a low cost, reliable design. If you catch
> an engineer before he gets set in his ways, you can train him out of
> "brute force design mode".
This is true but what I see here is minimalist component count
suggestions devoid of application considerations.
The OP originally asked about transistor drive. It is obvious that he
does not know very much. Are you denying that the transistor drive will
be a much more durable driver than the FET given the hazard possibilities?
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