Connect with us

help designing a 24 VAC circuit

Discussion in 'General Electronics Discussion' started by blaireger, Aug 26, 2011.

Scroll to continue with content
  1. blaireger

    blaireger

    5
    0
    Aug 26, 2011
    Hi all

    I was wondering if anyone knows of a relay that I can purchase so that I can have two 24 volt AC circuits coming in to a relay that will isolate them so only one of the two inputs sends a 24 vac signal downstream in the circuit. I have two zones in my boiler and I want to have either zone 1 or zone 2 start the boiler pump. My only issue is that it is possible for both zones to try to power this relay at the same time so I need something that wont burn up in that condition
     
    Last edited: Aug 26, 2011
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,386
    2,772
    Jan 21, 2010
    So you have 2 sources of 24VAC and you want to power something by 24VAC from one of these sources (presumably switching over to the other if one fails)?

    Or is it something else?
     
  3. blaireger

    blaireger

    5
    0
    Aug 26, 2011
    No. I have two boiler zones in a home heating system and I want to start the boiler pump when either zone 1 calls for heat or zone 2 calls for heat. THe only problem is if both zones call for heat at the same time, I will burn up the next component in the circuit. Therefore, I am looking for a relay or diode or something that I can wire in between the two inputs
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,386
    2,772
    Jan 21, 2010
    OK, how do they call for heat? Is it by applying a 24VAC signal?

    Why will something burn up if both zones call for heat? Is it because you can't supply both zones simultaneously, or is it an interaction of the control signals?

    If it is the latter, and they call for heat by applying a 24VAC signal, then each 24VAC signal could actuate a relay, and the relay contacts in parallel would ensure that the boiler pump would start if either or both relays were pulled in. Presumably you would need either another 24VAC power source to power the pump.

    Forgive me. The most heating I have here is "put on a jumper" (It's the middle of winter here and I ride to work wearing not much more than shorts and a t-shirt).
     
  5. blaireger

    blaireger

    5
    0
    Aug 26, 2011
    A 24 volt AC signal comes from either of the two zones in order to turn on the boiler pump. I assume if the two zones power at the same time they would try to short out a component. At present, I have zone 1 conected to a relay that has a 24 VAC coil in it that closes a 120 VAC contact which powers the 120 VAC boiler pump. I need to connect zone 2 to this relay as well but I am afraid when both zones power at the same time it will cause a short. This is why I was wondering if I could maybe use some type of diode that allows for two inputs and only one input and also protects from shorting of the two inputs, or a relay that basically does the same thing. I am looking for suggested part numbers from somewhere like Digi-key or Omron. I better understand DC circuits than AC circuits and if I was doing this in a DC circuit I would need to protect the DC inputs from shorting. Is this not the case in AC circuitry where you have two inputs tied together?
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,386
    2,772
    Jan 21, 2010
    Ah, you presume bad things will happen.

    I think it's likely that they won't, however the safe solution is to have 2 relays either of which will turn on the pump. So place their contacts in parallel.

    It's quite likely that both zones actually have the same source of 24VAC so connecting them together would be safe. But you wouldn't want to swap the leads around, so 2 relays is guaranteed to be safer.
     
  7. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
    Steve,

    It might be safe, but you would then be turning on both zones if either one came on. This is not likely what is wanted.

    Bob
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,386
    2,772
    Jan 21, 2010
    My understanding is that a single pump needs to be turned on when one of two 24VAC signals (actually power from the sound of it) is present.
     
  9. rootboy

    rootboy

    22
    0
    Aug 26, 2011
    What I would do is to buy two relays, one for zone 1, and one for zone 2 and have each of them come on individually when that zone calls for heat.

    Then I would "OR" two N.O. contacts together and instead of powering the pump control circuit from either one of the zones, I would purchase a "doorbell" transformer and use that instead (it's commonly used for this in addition to powering doorbells).

    Just make sure that you size the transformer based on what the load on the pump control circuit is.


    Enjoy! :)
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,386
    2,772
    Jan 21, 2010
    Well that's 2 people opting for that solution. :)

    I am still wondering if there is any need for an additional 24VAC signal. If you read blaireger's most recent post, it appears that the relays directly switch 110V to the pump.
     
  11. rootboy

    rootboy

    22
    0
    Aug 26, 2011
    What is the voltage on the coil that controls the 110V?
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,386
    2,772
    Jan 21, 2010
    24VAC. The OP said that the signal was originally used to close a relay.

    So, presumably, a second of the same sort of relay would be an option.
     
  13. rootboy

    rootboy

    22
    0
    Aug 26, 2011
    Yup, so he did. Can't trust my own darn eyes. :)

    Yeah, it's a simple fix, and he wouldn't necessarily need the new transformer, but it would keep the design cleaner, and he would be able to turn either of the zones completely off and still be able to control the pump with the remaining zone.
     
  14. blaireger

    blaireger

    5
    0
    Aug 26, 2011
    Guys
    I like the idea of hooking both zone signals to the one relay that is presently in the circuit and if the two zone outputs wont blow the relay when they both energize at the same time I should be good? No? Can I use one way diodes to prevent each circuit from trying to back feed the other zone?
     
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,386
    2,772
    Jan 21, 2010
    No, we're saying: "Use 2 relays"

    edit: 2 lots of 24VAC won't blow the relay. However it might feed back to the other zone with unknown effect. If the AV power is from different sources, or the same source but one is wired up in reverse you are at some risk of damaging the source of the 24VAC. 2 relays, each independently wired to activate the pump is the safest solution given that we don't have perfect knowledge of the rest of your system.
     
    Last edited: Aug 31, 2011
  16. blaireger

    blaireger

    5
    0
    Aug 26, 2011
    OK guys

    I will go buy another 24VAC/120VAC relay and wire one to zone 1 and one to zone 2 and wire them in parallell and go from there. Thanks for your help. Blair
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-