[MAMS] relay driver

[Dr. Gerald N. Johnson]

He's generating his pulses with a series couple capacitor to a higher 
impedance circuit than the relay coil. Lot of parts for what an IC could do.

73, Jerry, K0CQ

On 9/8/2012 9:50 AM, Danny Pease wrote:
> 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
>>
>>
>>
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