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I have an HVAC system with a powered open/powered closed supply damper, Ultra-aire 120 V dehumidifier and ventilation damper that is powered to open. I have everything wired up and it is working, however, since I have two 24 V signals going to open the powered supply damper they backfeed each others circuits when they are supposed to operate independently. Since they are wired together they turn each other on.

What do I need to do to isolate two 24 VAC inputs to one 24 VAC output? Both inputs need to operate independently at not complete each other’s circuit. Everything is AC current. I think diodes would fix issue if this was DC. Maybe some special relay?

The supply damper needs to be powered to close and open. So, our initial relay would apply power to open when both or either of them have voltage. I will need to also apply 24 VAC to the power close, but only when both of the other inputs are off. If one or both of the SW1 or SW2 have power, power to the powered close turns off. What would be the configuration for this scenario?

I’m at a loss. I’m no expert when it comes to this.

Null
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micron
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1 Answers1

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You can do this with an AC coil SPDT relay.

schematic

simulate this circuit – Schematic created using CircuitLab

When neither of the AC inputs are enabled, nothing comes out the output. When SW2 is closed, the output goes to 24V AC. When SW1 is closed, the coil is energised and transfers over to the other input, passing that 24V AC through. If both inputs turn on, only the SW1 input is used. This effectively creates an AC OR gate.

Note that the relay must be an AC coil type, otherwise this won't work. You can also make it work with a DC coil type, using a bridge rectifier and a capacitor, but it's more work.

Edit: Here's a schematic for the power open and power close.

schematic

simulate this circuit

With this setup, a constant source of 24VAC is connected to the pole of relay 2, meaning that power close is constantly fed 24VAC by default, until relay 2's coil is energised, at which point it drives the power open instead. In this configuration, there's no way for open and close to be powered at the same time. The remainder of the circuit is the same as before and handles the AC "OR" logic for the two inputs.

Polynomial
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  • Just to make sure I understand. So, if I energize SW1 it passes 24VAC through to output. Same for SW2? If both are powered at same time, it will also send 24VAC to output? The other side of coil goes to common? SW1 and SW2 will be isolated and never “connect” to each other? – micron Feb 25 '22 at 03:16
  • Also, would you be so kind to let me know which of these is the correct one? https://supplyhouse.com/sh/control/search/~SEARCH_STRING=AC%20coil%20SPDT%20relay – micron Feb 25 '22 at 03:20
  • Yes. If you look at how the switch is set up, only the SW1 or SW2 input can feed through to the output, never both at the same time. If either SW1 or SW2 are closed, 24VAC will appear at the output. – Polynomial Feb 25 '22 at 03:21
  • I get a 403 Forbidden error from that site; it might be geofenced to block UK visitors for some reason. – Polynomial Feb 25 '22 at 03:21
  • Bummer. Would you be able to provide a link or model number to the correct relay? So many to choose from. I know I can get White Rodgers relays….I do need something heavy duty to keep it from overheating – micron Feb 25 '22 at 03:24
  • If you're looking for a part that's easy to deploy, a DIN mounted one like this should work: https://www.digikey.com/en/products/detail/finder-relays-inc/38-51-0-024-0060/10055806 - I'm assuming the 24VAC is just for signalling and you're not pulling any heavy load off it (that relay is rated for 6A max) – Polynomial Feb 25 '22 at 03:30
  • It will have a load to open the damper…..probably around 2-3VA max. Would this one work? White Rodgers 90-372 or a JARD 92370 – micron Feb 25 '22 at 03:37
  • That one won't, since it needs 120VAC on the coil, but the White Rodgers 90-370 should work and it has an 18A switching rating so it's more than capable of handling the damper load. Datasheet here: https://www.mouser.co.uk/datasheet/2/459/90_360_thru_90_486_relays_catalog_en_1594898-2308989.pdf – Polynomial Feb 25 '22 at 03:42
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    Thanks again for your help! It’s greatly appreciated. I’ll try the 90-370 – micron Feb 25 '22 at 03:48
  • I may need to complicate things a little. The supply damper needs to be powered to close and open. So, our initial relay would apply power to open when both or either of them have voltage. I will need to also apply 24VAC to the power close, but only when both of the other inputs are off. If one or both of the SW1 or SW2 have power, power to the powered close turns off. What would be the configuration for this scenario? Thanks again for your assistance! – micron Feb 25 '22 at 11:37
  • Assuming you've got a constant source of 24VAC that you can tap into, you can get a second of the same relay and wire it up so the "OUT" from the existing circuit drives the coil of the second relay, which then selects whether open or close is powered. I've added a second circuit diagram to show how this works. – Polynomial Feb 25 '22 at 18:25
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    I'll try version one first. I ordered two relays. You don't understand how much of a help this has been! Thank you! Will keep you posted if I need any further assistance. Have a great day! – micron Feb 25 '22 at 20:11