Will 27 VAC damage the coil of a 24VAC relay?

Discussion in 'The Projects Forum' started by botanist, May 25, 2009.

  1. botanist

    Thread Starter New Member

    May 23, 2009
    I am building a system to dose calcium, magnesium, and buffer solutions into a saltwater aquarium. I'm using a Toro Ecxtra irrigation sprinkler timer to control 3 - 24VAC relays each of which is responsible for controlling a peristaltic pump.

    The sprinkler controller is designed to work with standard 24VAC irrigation solenoids. I'm just substituting some relays with 24VAC coils in place of the solenoids. The problem that I'm running into is that the voltage from the controller is 27 volts instead of 24. I'm using an Omron G2R-1-T relay and the data sheet for this relay says the maximum votage is 110% over rated which would be 26.4. The rated current is listed as 37.5 mA and they are drawing 42 mA. I'm concerned that the current and voltage values above the rated will cause premature wear and failure of the relay coils. Is this a reasonable concern or are the values close enough? The coils will be energized for 15-30 minutes 2-4 times a day.

    If this is a legitimate concern, how can I lower the voltage being delivered to the coil? I considered a voltage divider circuit, but most of my previous experience with control circuits has been DC, not AC, so I'm looking for some advice before I proceed.

  2. Bernard


    Aug 7, 2008
    Probably not a problem if coil does not get too hot to hold, or add 82Ω 1/2 W resistor in series with supply & coil.
  3. eblc1388

    AAC Fanatic!

    Nov 28, 2008
    Well 110% overvoltage rating is kind of small so I also check up on the relay datasheet.

    According to the Omron datasheet here:

    Omron G2R-1-T relay datasheet

    The overvoltage rating can be 140% at 23°C so your 27V is acceptable to the relay regarding your low duty cycle per day.

  4. botanist

    Thread Starter New Member

    May 23, 2009
    Thanks guys, I think I will proceed without adding any resistors and just monitor the temperature of the relay closely during the testing phase of the project.

    The G2R-1-T data sheet that I looked gave the rating for a temperature of 70C instead of 23C. I guess that accounts for the difference between the 110% limit that I reported and the 140% that eblc1388 found. I'll also be sure that the enclosure is well ventilated and the enclosure temperature does not get too high.

    Thanks again for the quick response.