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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.email@example.com> <3DA9D9E1.firstname.lastname@example.org>
Date: Sun, 13 Oct 2002 22:21:00 GMT
NNTP-Posting-Date: Sun, 13 Oct 2002 15:21:00 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
John Devereux wrote:
> Fred Bloggs wrote:
>>>What's the simplest method to get a Fairchild CD4020/40/60 counter to
>>>drive a relay with one of its Qx outputs? Is simply driving a
>>>transistor to drive the relay OK?
>>>Data sheet says output is low-power TTL compatible: fan-out 2x for 74L
>>>or 1x for 74LS.
>>>The relay coil is 12v dc, 720-ohm, 16.70 mA (Omron G6B-2114P) used to
>>>switch a 150 mA(?), 30vac HVAC control circuit.
>>>Please give component p/n's and values.
>> | | | +-+-+
>> | | 1N4001 | | ) | |
>> | | --- | ) | |
>> | | / \ | ) | |
>> | / --- | ) | 20mA
>> | 1.2k | | ) | |
>> | / | +---+ |
>> | \ | | \|/
>> | | +--------+
>> ---+-----+ | |
>> 4060 | | |
>> | | |
>> | 15K |/ |
>> Q +--/\/\--+---| 2N4401 |
>> | | |\| |
>> | | - |
>> ---+-----+ | | 330 |/
>> | | +-/\/\-+---| 2N4401
>> | | | |\|
>> | / / -
>> | 5.6K 560 |
>> | / / |
>> | \ \ |
>> | | | |
> This is still excessive; surely you use a single transistor
> for this? If it's only 20mA, your 15k feeding the base of a
> common emitter driver transistor would be sufficient. Just 3
> parts, 15k base resistor, transistor, diode. Speed is not an
> much of an issue with a relay. The CMOS is rail-to-rail
> isn't it? So it should switch the transistor off OK.
Hmmm- maybe, but Ic/Ib=5 is not excessive overdrive. I was also allowing
for the possibility that the 4060 Q output remain within a valid logic
level band- to be used as input for other logic in the circuit. The
voltage level and current is high enough that you want to keep output
transistor switching time out of the microsecond region. The impedances
in this circuit will keep you in the <100ns region. I suppose you can
shed the collector resistor shown- and change the clamp diode to 1N4148.
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