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From: rstevew@armory.com (Richard Steven Walz)
Subject: Re: SN7407, relays, chips, etc...
Organization: The Armory
Date: Thu, 13 Oct 1994 12:55:49 GMT
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References: <CxBy7D.M0B@nyongwa.montreal.qc.ca>
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In article <CxBy7D.M0B@nyongwa.montreal.qc.ca>,
Albert Yale <matrix@nyongwa.montreal.qc.ca> wrote:
> 
>HELP!
> 
>I'm sure one of the estimated 15M internet users could answere this
>question...
> 
>My parralelle port only suplys +5v (+5.2v actully) in 13mA (no kiding!) :(
>What kinda relay will switch on that? Or, how can i higher the amps?
> 
>also, what's a SN7407? (what does it do and how much does it usuly cost?)
> 
>tnx... and please reply as soon as possible! PLEASE! (email prefered)
> 
>'Matrix'
----------------------------------------------
You need a simple buffer to make the signal a bit stronger. Well, it
depends on what relay you wish to drive, but most of them can be driven off
the +12VDC line from the computer, (with a freewheeling diode across them
backwards to the flow of current as the diode symbol points, current being
positive to negative here). You can put a 1K resistor, or even maybe 10K
on the port data pin and run it to the base of an NPN transistor, like a
2N2222 or a 2N3904, and then hook the emitter of these transistors to
ground and the collector tothe relay (rated at +12VDC of course!), and
the other side of the relay coil to +12VDC, and then when the data line
goes HI, about 3-5 VDC, it will be able to turn the transistor on, which
turns the relay on, and the poles of the relay and its armature will allow
you to control up to whatever voltage and current the contacts are rated
for, note the coil may be 6 or 12 VDC, but all it does is pull the armature
down, which is the throw(s) of however many contacts (poles) the relay is!
If your relay is 6 VDC and you need to use the computer power supply, I
would at least put a big diode pointing outward on the +12VDC line of the
computer power supply and then a big capacitor, at least 16VDC or more and
at least and at best more than 1000 uF's between the cathode of that diode
and ground on it's negative end, (note electrolytic caps will explode if
reversed!!, that's why the big row of - or + signs flowing in an arrow to
point demandingly toward one end or the other! they are pointing at which
end is THAT sign!!!), and then, for the use of six volts, measure the
resistance of arelay coil in your hand and get a resistor the same size to
put from the +12VDC line to the relay, thus dividing the voltage between
them and limiting the current! Now if you wish to know how to do this with
opto-couplers and then big transistors (for DC) and TRIACs (for AC), then
write back, as my fingers are getting tired. I only use three of them to
type!!! And I do it very fast!!! I had to put bandaids on them last week,
as the pads on their ends got bruised from too much typing!! I am breaking
down in my old age!:) By the way, use the largest VALUED resistor to the
bases of the transisitors from the data line that will solidly engage the
relays! This will reduce power out of the port and spare it loading.
-Steve Walz   rstevew@armory.com

