I have three separate 24VAC circuits that I want to energize a device when any of them is energized (logical "OR"). I tried just connecting them all to the device in parallel, and of course when one is energized it cross-energizes the other two circuits, which is not good.

How can I wire this so that the device runs when any of the three circuits is energized, but without cross energizing any of the other circuits? Is there a wiring trick, or do I need to use relays or something? (Remember this is 24VAC)

 

You need diodes in the circuit. Diodes allow current to flow in one direction only. However, I have no idea (without calculations I haven't done since college) what size diodes you would need.

 

Yea, I believe you might be able to do it. It's a little more difficult because you are dealing with 24V AC. On the end of each AC line you will have to use a full wave bridge rectifier. You can buy such things at Radio Shack. They come complete all in one package. The bridges will convert the AC to DC. You can then use some more diodes to drive the load you want ORed. That load will have to be DC though.

 

OK, so diodes keep the current from flowing backwards, but diodes only work with DC current?

And if I need to convert the output back to AC, should I use a relay or is there such a thing as a bridge "un-rectifier"?

 

Diodes will work with both AC and DC. The problem with your circuit is that you effectively have 'DC with alternating polarities' for lack of a better term, or just plain AC. In order to get isolation from the other circuits you have to resort to some fancy tricks. You never did specify just what kind of loads you were working with so I won't speculate on what else you might have to do in order to get the lash-up to work. You can turn the DC into AC again, but that takes an inverter and that's a little more expensive. It might be easier to rig some relays and do your OR'ing with the contacts. I was thinking that you might be driving the coil of a relay or a lamp. In that case you just might get away with using the bridge rectifiers because your load could tolerate the DC.

 

I'd think that a set of 3 relays wired in parallel would be the simplest. Cheap too. Here's an ebay link for an example.

Omron 24vac Relays

Each of the 24vac circuits would activate its own relay. The 3 relays are wired in parallel, supplied from the same source and feeding the same load. If 1 (or more) of the 24vac control circuits activates its relay, your load is energized.

Rob

 

Loads are under 1A for all circuits.

I am going cross-eyed trying to find a way to make this work with three 1PDT relays. Is there really a solution if any and all three circuits could be energized, and I want to prevent any cross-energization?

I can do this with one 3PDT and one 2PDT 24VAC relay. If I number the circuits 1, 2, 3, and call the relays A and B, then I would have the following setup:

1 controls relay A, and is connected in parallel through the N/O side, first pole. 2 is connected through the N/C side of relay A, second pole, to control relay B, and is connected in parallel through the N/O side of relay B. 3 is connected through N/C side of relay A, third pole, then through the N/C side of relay B, second pole.

So overall circuit 3 is Normally Closed. When 1 is energized, it disconnects the other two circuits. If 2 is energized, then 1 must be off and it disconnects circuit 3.

Now as a shortcut I guess I can connect circuit 3 to the same pole on relay A as circuit 1, since they would only share a drain at that point. So I would only need two 2PDT relays. But I have this feeling there's some trick to this business of three or more circuits that I'm missing....

 

You only need (3) Single pole, NO relays.

- Circuit 1 activates Relay A via 24vac control circuit.
- Circuit 2 activates Relay B via 24vac control circuit.
- Circuit 3 activates Relay C via 24vac control circuit.
- Run conductor from 120vac supply to Relay A NO contact.
- Run conductor from the same 120vac supply to Relay B NO contact.
- Run conductor from the same 120vac supply to Relay C NO contact.
- Run conductor from Relay A NO contact to load.
- Run conductor from Relay B NO contact to load.
- Run conductor from Relay C NO contact to load.

Remember that the control coils for the relays are independent of the load(s) you are controlling with the relay(s). The fact that Circuit 1 activates Relay A doesn't affect Relay B.

• If Circuit 1 activates Relay A, nothing is backfed to Circuit 2 or Circuit 3.
• If Circuit 2 then separately activates Relay B, you have just created a parallel path for power to flow from your 120vac source to your load.
• If Circuit 1 then opens and deactivates Relay A, your load keeps running because Circuit 2 has Relay B activated which is completing the circuit from 120vac source to your load.

Make sense?

You could use #18 for the 120vac conductors because of the 1-amp load, but would need to put a fuse in the circuit to protect the #18 - one of the little AGC glass fuses would be fine.

 

I see what you're saying. My load is actually 24VAC as well, so I was stuck in this idea of using my control circuits to supply the current for the load also. But if I use a separate 24VAC supply for the load, then I guess I'm all set using your solution.

Thank you for all of the help!

 

You only need one power supply and three single pole normally open relays with 24 volt AC coils.

 

You only need 1 power supply.

You've basically got 3 switching circuits and 1 load. Think of how your house is wired. You only have 1 "power supply" (the utility company) and you have a gazillion switching circuits and loads run from that 1 power supply.

From your 24vac power supply, feed the 3 switching circuits (you called them circuits 1,2 and 3). Run a separate set of conductors to the relays as I described. Make sure the 24vac transformer puts out enough current for the total of the loads you want to run. I'd also suggest those AGC (little cylindrical glass) fuses for each switching circuit, in addition to the load supplied by the relays.

 

Try this

<img src="http://ronj48.home.mchsi.com/Ediagram1.jpeg"

 

Oh ... in retrospect that's obvious, and it makes life even easier! Thanks again for straightening me out!

 

In answer to the original question of ORing three 24VAC supplies to drive a load when any one of the supplies is active:

Assuming that the three 24VAC supplies share a common return, this can be done with two SPDT 24VAC relays. The relays each have a NO (normally open), NC (normally closed), and common contact.

Relay #1:
- 24VAC #1 is connected to NC contact
- 24VAC #2 is connected to NO contact and coil
- Common contact is connected to NC contact on relay #2
- Other coil terminal is connected to common return

Relay #2:
- 24VAC #3 is connected to NO contact and coil
- Common contact is connected to load
- Other coil terminal is connected to common return

Here's how it works:
(1) If 24VAC #3 is active, relay #2 is energized and the load is fed from 24VAC #3 through the NO contact of relay #2.
(2) Else if 24VAC #2 is active, relay #1 is energized and the load is fed from 24VAC #2 through the NO contact of relay #1 and the NC contact of relay #2.
(3) Else the load is fed from 24VAC #1 through the NC contact of relay #1 and the NC contact of relay #2.

 

Relay "logic"--- a "Slave" circuit is to operate only when any 2 of 3 "Master" circuits--- A, B, & C---- are in operation. This appears as;

XA _______ XB________ -C ________ XS ( P1)

XA_________ -B ________ XC ________ XS (P2)

-A_________ XB ________ XC _________ XS (P3)

This is an "Or" circuit with 3 paths, P1, P2, & P3, each path consisting of 3 relay-contacts in series and each path completing a circuit to operate relay "S" when the operate-mode is satisfied.This requires that 1 of the 3 "Master" relays in any path to be "normal"--- ex., -A. This circuit as shown requires 6 normally-open contacts, and 3 normally-closed contacts.

We note that XA completes Paths 1 & 2, and XC completes Paths 2 & 3--- this allows eliminating 1 'A" contact and 1 "C" contact.


____________ XA _______ XB ________ -C ________ XS
{ { {
{ {_____ -B ________ XC____ {
{ [
{ _________ -A ________ XB_____[

This circuit can be wired with one SPDT "A" relay-contact, one SPDT "B" relay-contact, one SPDT "C" relay-contact, and one NO ( normally-open) "B" relay-contact.

 

__________ XA________ XB_____________ -C_________ XS
[ [ [
[ [_____ -B ____________ XC ____[
[ [
[______ -A__________ XB ________[

This is the correct "graphic" ( I hope!!!)

 

Wrong Again!!! Sorry, friends---the "garphic" I "submit" is distorted---