Alternating relay

Is that pneumatic pressure switch a set of dry contacts? I'm thinking you can use a T flip-flop with a relay driver on each output to activate the compressors. You will have to detect that pressure switch open condition as a sort of master off for both relay drivers.

I don't think I understand what are "dry contacts."

I can tell you that the pressure switch does not have an electromagnetic coil to actuate the moving parts to which the contacts are connected. Therefore the pressure switch does not use a power supply, nor do I think it uses the wall voltage in any way on the "supply" side.

The change in air pressure makes the contacts snap shut and then a spring makes them click open.

It's a 220v motor, so it is a double pole singe throw.

A 'dry contact' means that it is not connected to any power source. You are free to use it in any way you require.

All that is needed is a single pole switch so you are in good shape.

Can you access alt.binaries.schematics.electronic ?

I can work up a drawing and place it there.

Then I think yes. Right now there is wall voltage to one side, and the motor connects on the other side.

I can disconnect both and use it for something else. I can then run the wall voltage to some-other-thing, and power the motor using that some-other-thing.

I prefer to automate the system rather than use a manual switch.

Yup, I see it. I see only 9 posts there, so if that sounds incorrect then its expiration may be very short.

Thank you to everyone who is offering tips.

are

Your welcome!

Are you able to obtain digital logic gates? There will be a few required in addition to two relays and some discrete components. Of course, you will need a power supply to create the 5 volts needed to operate the circuit. You may need a 12 volt power supply depending upon whether you can get 5 volt relays that can switch the compressors.

I used an alternating relay once that had a single coil. When actuated, the solenoid pulled on an alternating mechanism with two latching positions, which would reverse every time you actuate the coil. Run the motor off the contacts of the alternating relay. This way you could switch motors every time the pressure switch turns on. Of course the motor would stay on even after the pressure switch turns off. You would still need a way to turn the motor off when the pressure switch turns off, but it's easy enough to put an ordinary relay in series with the alternating relay.

So this altnerating relay would flip and flop based upon a single on/off input voltage? If so, then I think I could start with that.

Do you have any idea what brand or model this was?

My web searches for alternating relays have yielded products which have very sparse descriptions. I think they only flip if the polarity is flipped.

Makes sense I think. I'd power the coils of the contactors with the outputs of the alt relay.

The pressure switch needs to feed the supply side of the contactors so that power will shut off even though a coil is still engaged.

If I do it this way, I need an alt relay whose coil uses 220v, correct? (Or at least 120)

I'll keep thinking about this, but I'm curious how else it might be done.

-Ryan

Hi, Ryan. ATC makes an inexpensive alternating relay (also called a sequencing relay)that will fill the bill for you directly. Look at these links:

Elsewhere in the thread you say you've got a 220VAC compressor setup, so you probably want the ATC P/N ARA-240-ABA. Look at the first example in the first link for a suggested wiring diagram. By the way, make sure to wire it up so pins 4 and 5 are always hooked up to the

240VAC to ensure proper operation. Also make sure your pressure switch has enough hysteresis to prevent switch chatter, which will mess up the alternating relay operation.

If your setup is 3-phase, and/or if the compressor is more than 1/4 h.p., use only one of the phases for control with your pressure switch, then use the relay to drive two 240VAC contactors that can drive the two motors.

For industrial control applications, it's usually far better to purchase a one part solution made for the job if you can, than cobbling something together with digital logic. Not only does it make maintenance of your solution possible, but it also helps avoid problems with relay arcing causing EMI/RFI that interferes with the control logic.

You can purchase the part directly on the ATC website for $61.30 plus shipping and handling. Even if you cost your time at minimum wage, you're going to be hard-pressed to beat that price for an installed cobbled-up solution.

Good luck Chris

Look for a Potter & Brumfield S89R or S90R series Bistable impulse relay. About Can$40 from ElectroSonic in Canada.

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D'oh! Of course! A racheting relay - there's a solenoid that pulls in a pawl that engages a cam on a wheel with a detent. Every time the solenoid is energized, the cam advances by one setpoint.

You can control an arbitrary number of functions this way, but all you really need are the two contacts - to use a linear solenoid to drag a circular cam around, you'd probably want at least 4 or more positions.

I have no idea if anybody makes anything like that anymore, but if you're adventurous, and good with tools, and have parts and materials on hand, you could make one in a day. :-)

Good Luck! Rich