Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

120v relay logic

  1. Jan 13, 2010 #1
    I want to use 24v to break 120v loads. I understand the logic for dc circuits but im not sure how to break 120v loads.

    What relay do i use? I was looking at the altronix RAC120 or RAC24.

    Do I break just the hot leg of the 120v?

    Any help on such a beginner question would be helpful! Thanks!
  2. jcsd
  3. Jan 14, 2010 #2


    User Avatar
    Science Advisor

    As in 'digital' circuits, you just need to break the circuit and interrupt the flow of charge. In theory, you could break either leg. In practice, it's better to break the 'hot' leg, just in case someone/thing accidentally grounds out part of the circuit/case that is still hot, or cuts the cable, or some such.

    Most power relays can do the trick, as long as the 120V doesn't arc across the contacts (as may be the case when the contact and paddle/reed are particularly close together, as may be the case with low voltage relays). If there's an AC rating for the contacts, it should do the trick. Obviously, the coil must be rated for 24V (AC or DC, you don't specify).
  4. Jan 14, 2010 #3
    You want a relay with a coil rating 24 VDC and 120 V contacts, make sure that it is rated for the current that you are switching.
  5. Jan 14, 2010 #4
    Great. I have relays that will do that. Thanks fellas
  6. Dec 22, 2011 #5
    Anyone know how to do this exact same thing but use 120V (instead of 24V) to switch the relay for 230V (instead of 120V) connection.

    I want to turn on and off a 230V pump, but using a controller that already sends 120V on/off. I am just trying to avoid buying another controller that will switch on and off 230V.

  7. Dec 22, 2011 #6


    User Avatar
    Science Advisor

    Yes: find a relay with a coil rated for between 110 and 130 Vac (or AC/DC, but don't use one that's just for DC) which is capable of switching 230 Vac at the current required. For instance, I found this at Digikey by looking for relays, narrowing it down to those between 110 and 130 Vac and then sorting by carrying current:
    http://search.digikey.com/us/en/products/G7L-1A-TUB-J-CB-AC100%2F120/Z814-ND/127305 [Broken]
    Last edited by a moderator: May 5, 2017
  8. Dec 23, 2011 #7

    Ivan Seeking

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Be sure you derate the relay for inductive loads. The stated contact capacities are generally for resistive loads. The manufacturers provide a cut sheet for the derating.
    Last edited: Dec 23, 2011
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook