# Alternating relay

Discussion in 'Electronic Basics' started by Ryan, Jan 27, 2006.

1. ### RyanGuest

Is there a stock item which directs voltage onto one wire the first
time it is given power, and then to another wire the second time when
the item is given power?

I have two air compressor units tied together as one tank. Right now,
only one of the pumps is hooked up.

The pressure switch is pneumatically closed/opened. The contacts of
this switch power the motor on directly.

I would like to introduce a contactor that, when given this on signal,
will power on pump #1 for that cycle and then shut it off when the
pressure switch is disengaged. Next time air is low, I want a
contactor to engage pump #2 for that cycle and then turn off the motor
when it is powered off by the pressure switch.

Back and forth at infinitum so the load is distributed evenly over time.

The only "alternating" relays or contactors I have found require
either separate on signals (one for each coil) or else reverse
polarity in a single coil. I have only one signal: the on / off
state of the pressure switches contacts.

My local electical supply houses are either dumbfounded to
speechlessness by my request, or else don't have anything to fit the
bill.

With that having failed, I wonder if there are stock electronic
circuits that exist, or that I could fashion, and then relay that
signal into larger switching gear without having to invest in a rocket
science degree first. (Perhaps I could use a doorbell transformer
or something easily found to power the coils and maybe the small
electronics too.)

The circuit would perform like this:

Source Off = Motor1 off Motor2 off
Source On = Motor1 on Motor2 off
Source Off = Motor1 off Motor2 off
Source On = Motor1 off Motor2 on
Source Off = Motor1 off Motor2 off

I'm stumped.

-Ryan

2. ### Lord GarthGuest

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.

3. ### ArtGuest

Simple fix would be a power switch which you would need to manually select
as to which compressor you care to use. Otherwise you may need a custom made
switching box produced for your application.

4. ### RyanGuest

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.

5. ### Lord GarthGuest

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.

6. ### RyanGuest

A 'dry contact' means that it is not connected to any power source. You are
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.

7. ### Lord GarthGuest

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.

8. ### kellGuest

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.

9. ### RyanGuest

I used an alternating relay once that had a single coil. When
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)

-Ryan

10. ### ChrisGuest

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

http://www.automatictiming.com/pages_misc/app_alternatingrelays/alternatingrelays.html
http://www.automatictiming.com/pdf_div/ara-arb-duplexor_data.pdf

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

11. ### Peter BennettGuest

Look for a Potter & Brumfield S89R or S90R series Bistable impulse

12. ### Rich Grise, but drunkGuest

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