From: Winfield Hill
Subject: Re: How to get CMOS counter to drive a relay?
Date: 18 Oct 2002 06:34:41 -0700
Organization: Rowland Institute
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> <3DAC2309.email@example.com> <firstname.lastname@example.org> <3DAFFA70.email@example.com>
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Fred Bloggs wrote...
> Winfield Hill firstname.lastname@example.org wrote:
>> I'll edit your drawing to show the standard modern excellent,
>> reduced-parts-count highly-reliable recommended small solution.
>> | | |
>> | 1N4001 | _|_
>> | --- | ) | |
>> | / \ | ) | |
>> | --- | ) | 20mA
>> | | | ) | |
>> | | +---+ |
>> | | | \|/
>> | +--------'
>> ---+-----+ |
>> 4060 | |
>> | |--'
>> Q +----||<-,
>> | |--+ BSS138 etc
>> ---+-----+ |
>> | |
>> --+--------------+--- gnd
> This is bull. The BSS138 is a $0.33 item in small quantity from Digi-Key
> and only available in SOT. It is a weak little thing with ~200mA Idc,max
> and 360mW Pmax- sounds flimsy to me. Fairchild is apparently too ashamed
> to include a datasheet link in the product folder.
Oh yeah? Let's step outside and settle this like men. Bring a weapon
of choice, TO-92 or SOT-23 at 40 paces. No TO-220, etc. allowed. :-)
Hey, mine is a "modern" solution; you're still using massive ancient
through-hole parts? Pick your FET of choice, whether TO-92 or SOT-23:
2n7000 or 2n7002; BS170 or BSS138; or Zetex ZVN4306A or ZXM61N03F.
The latter part features Rds(on)max of only 0.22 ohms at 10V drive.
But back to the classic BSS138, here's a serious FET with 30pF of
input capacitance - that's no wimp. Fairchild, ON Semi, Infineon,
Zetex and others present highly-detailed curves. With 5V drive this
fine part can sink over 1A. It's rated at 3.5-ohm Rds(ON) max for
5V gate drive, and certainly is much lower at 12V. Assuming 3.5
ohms, it dissipates under 1.0mW driving our OP's 720-ohm, 17 mA
relay coil, and the 360mW rating exceeds the requirement by 370x.
That's not bad for a common 5.2 cent part (Future, single reel) and
nicely fits the need. Sorry, no bull there, just solid engineering.