Hi, I’m looking for suggestions on what component to use for a specialized HVAC project I’m working on. Since it’s a lot easier to draw my situation than to describe it, please see the diagram below:

Essentially, I’m looking to control a load powered by Transformer “B” by means of Sw III which is connected to a different power source, Transformer “A” (both 24 VAC).

Under normal circumstances, a relay would do this job perfectly. The problem is, as you can see in the diagram, there is no line from Sw III back to the negative side of Transformer A to complete the circuit, and due to the design of the building and project, we can’t run such a line. So essentially what I’m looking for is a box that can be wired across Sw III (brown wires) and would sense just contact closure/continuity, not voltage. It would have to not bother or be bothered by the fact that the “+” side of Sw III is connected to the + side of Transformer A (blue wires, the split of the 24 VAC to the three switches in internal to a PCB based device, so I can’t disconnect it). When there is continuity across Sw III, it should complete the circuit on Transformer B (green wires) so as to power a load, and disconnect that circuit when Sw III does not have continuity. Does such a gizmo exist?

An added benefit would be of the device could be powered off the existing wiring. It could either power steal from Transformer A across SW I or II (I’ve confirmed that a standard relay will work fine in this way), or power steal from the load it is controlling on Transformer B. A separate power supply can be arranged, but would be less ideal.

It would be absolutely fantastic if this would be a small, self contained device (like a relay in a box), and if it would operate fairly quietly.

All suggestions much appreciated. Thanks in advance!

 

If you can access the transformer side of either relay one or relay two, your yellow box could be an ordinary relay.

 

You can complete the circuit for the relay coil by just connecting the other side of the coil (the one is connected to the switch) to the minus terminal of your transformer A, according to the diagram.

 

Thanks for the replies! Actually, I'm aware that a relay would do this job cheaply and perfectly by simply running the coil circuit from the unconected end of Sw III to the transformer -. The problem, as mentioned, is that we can't run such a line. In the real world, the transformer and the switch are several floors apart, and the transformer is difficult to get at to boot. So, I'm looking for a more complex solution which will let me work with what I have, namely continuity across Sw III when it is closed, and the circuit on Transformer B that needs to be controlled.

Any ideas? Thanks again!

 

Can you use the B transformer to power the relay?
If not, then add a small transformer in series with the relay to provide power.

 

Even with any other sensing device you will need two wires going to the relay to connect the circuit. Use another small relay near transformer A which will be switched by switch III. For the yellow box on the diagram use a 240VAC coil relay and use the small relay near the transformer A to drive the 240VAC relay (connect the mains supply on the common pin of the small relay and the brown wire going to the 240VAC relay to the normally open contact of the small relay). Thus you will need only one wire going to the 240VAC relay and you can take a return wire to neutral from a point near where the 240VAC relay is located.