[MAMS] relay driver

[Danny Pease]

Lots of ways to go about it, but I am after simple. If I do anything more
complicated, I will order the pulse relay driver board kit from W6PQL, it is
only $15.00 plus postage. I am looking for a way to do it out of my junk box
(which is getting really sparse).

Thanks

Danny NG9R



-----Original Message-----
From: mams-bounces at mailman.qth.net [mailto:mams-bounces at mailman.qth.net] On
Behalf Of Dr. Gerald N. Johnson
Sent: Friday, September 07, 2012 7:39 PM
To: mams at mailman.qth.net
Subject: Re: [MAMS] relay driver

Yes, that should work, rather hammer and tongs though.

You want the capacitor to not really get much charge in the first 20
milliseconds. So the relay coil resistance and inductance  with the
capacitance want a time constat about 40 to 60 milliseconds. The bleed off
resistor and capacitor needs a much longer time constant. Probably two RC
time constants for the shortest period you expect to stay in RX or TX. Like
a second or two.

While the relay takes 15 ms to switch, what length pulse do they require? It
might be a hair longer. One spec shet I saw said 20 ms pulse.

A hair more complex and probobly a lot smaller space could use a pair of
555 timer chips or a 556 dual timer. Wire each to be a 20 ms one shot pulse
generator, one to trigger on rise, one to trigger on fall.

Or getting fancier, a 74hc221 one shot, and a dual interface driver chip in
the 75460 family. The 75460 have a dual input logic gate input, so you could
use one NAND and one NOR with the 221 triggered by rise or fall and that
same trigger signal going to the two drivers, one NAND and one NOR so the
output of one 460 would be enabled on a high and the on a low. Providing
these chips can be found these days.

Or you could pick a PIC chip with a couple sturdy power outs, and use an
input to sense the relay control line, high or low logic level (3.3 or 5
volts depending on the PIC chip supply) and program it to produce a 20 ms
pulse on the appropriate output. Or the PIC chip could drive some power
transistors, bipolar or MOSFET. The PIC chip could probably be one with 8
leads, maybe 6 using an internal oscillator and if in a surface mount
package would be tiny. I'd tend to use a DIP package because its easier to
use on a RS universal PC breadboard. The breadboard will cost more than the
small PIC chip, which might be as costly as 79 cents in single lots. And if
you buy enough I make money because I've invested in that company's stock
which has risen at least 15% since I made my investment. Where it may be
time to start taking profit by selling the stock, soon as this month's
dividend arrives.

Op amps can also be made into level triggered one shots, and an audio power
output chip with lots of gain and usually operated with lots of feed back is
simply a power op amp and ought to be able to drive the relay coil directly.

Look up indices to CSVHF and other VHS society proceedings as well as MUD
and I'd expect circuits for driving pulsed relays have been published in the
past decade or so. Not having any such relays yet, I've not bothered to
remember such circuits in detail.

Your link to dowkey didn't work for me until I appended the hp.

If you were to add enough capacitance in parallel with each coil to have a
discharge time constant of 10 or 15 milliseconds, you might be able to use
the indicator contacts for steering, so you apply power once to change to TX
and again to change to RX with the indicator contacts selecting the coil to
pulse and disconnecting power as the contacts change position. It probably
will disconnect before 10 or 15 milliseconds so the capacitor on each coil
is needed to supply power to the coil just long enough after the indicator
contacts disconnect.

8 ways to build a clock, beats RIW's 6 ways.

73, Jerry, K0CQ

On 9/7/2012 4:44 PM, Danny Pease wrote:
> I wonder if I can simply put a rather large capacitor on series with 
> each coil and switch the DC from one to the other when going from TX 
> to RX? The discharged cap should look like a short for a few 
> milli-seconds, allowing the pulse to activate the coil and then when 
> the cap is charged, no more current. Of course there would have to be 
> a discharge path to bleed the cap off. The coils are about 60 ohms and 
> the specs say the relay takes 15 milliseconds to switch, so maybe a 
> 470 uF cap with a 610 ohm resistor in parallel?
>
> Danny NG9R
>
>
>
______________________________________________________________
MAMS mailing list
Home: http://mailman.qth.net/mailman/listinfo/mams
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:MAMS at mailman.qth.net

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html