power a 12vdc cube relay using 120vac?

Discussion in 'General Electronics Chat' started by sdowney717, Mar 12, 2015.

  1. sdowney717

    Thread Starter Member

    Jul 18, 2012
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    I was able to run a 120vac relay coil using 15vdc.

    How about using 120vac to power a 12vdc relay coil?
    Would it need a small resistor in series?

    I am thinking match the ohms of an existing AC 120 v coil will make the DC coil work on AC power.
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
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    you would have to rectify it. might be easier to use a 120 vac input wal wart with a 12 vdc output. safer too, the walwarts are isolated from the line.
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Use a transformer psu to drop it down to 12v.
     
  4. sdowney717

    Thread Starter Member

    Jul 18, 2012
    385
    13
    Yes, I could do that, I have walwarts. I will figure out something.
    I would not have to rectify the current, why do you think that? Are you saying the coil winding insulation on a 12 vdc relay is inferior to an ac relay coil?

    It took from 15 to 17 vdc to turn on a 120vac relay coil.
     
  5. wmodavis

    Well-Known Member

    Oct 23, 2010
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    Don't let the smoke out of the relay.
     
  6. sdowney717

    Thread Starter Member

    Jul 18, 2012
    385
    13
    12vdc relay coil reads 40 ohms.
    120 vac relay coil reads 75 ohms.

    If I add 40 ohms resistance in series, wont that allow the coil to run on 120 vac?
    Alternatively, I could put two 12vdc coils in series.
     
  7. crutschow

    Expert

    Mar 14, 2008
    12,973
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    A 12V DC relay requires DC on the coil, otherwise it is likely to buzz loudly.
     
  8. sdowney717

    Thread Starter Member

    Jul 18, 2012
    385
    13
    Well, don't want it to buzz. The spring on those things is very weak, so wont take much current to pull down I think. I will find out in a few days. Adding in a walwart just adds complexity. the whole thing will be inside a steel panel breaker box.
     
  9. alfacliff

    Well-Known Member

    Dec 13, 2013
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    ac coils have copper rings on the core that fill in the magnetic when its is changing from positive to negatve. ac coils will work on dc at reduced voltage with no trouble.
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Why dont you just use a 120v ac relay.
     
  11. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,498
    2,364
    120vac coil requires much more than that, it is produced by the inductive reactance of the coil, the easiest way is to measure the coil current at the rated voltage, this will tell you the inductive reactance, in Ohms.
    Max.
     
    Roderick Young likes this.
  12. Roderick Young

    Member

    Feb 22, 2015
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    Interesting proposition!

    The Mad Scientist way of doing this would be to hook up as many 12 VDC relays as you have in series, and just see if it works. It might, and if it didn't, you could conclude that either there was too much or too little current going through the coils. A burning smell or smoke would suggest too much, and the relays not pulling in would suggest too little. In the latter case, you could simply remove relays one at a time out of the series string, and in the former case, it would be time to invest in better equipment to approach the problem more analytically. Although I did things in an unsafe manner in my own youth, I wouldn't recommend that approach.

    If you have access to an LCR meter, you could measure the inductance of the 12 VDC relay coil. The impedance at 60 Hz (assuming you're in the US), plus the DC resistance of the coil, would allow you to calculate the RMS current through the relay at 115 VAC. You could then compare this to the DC current required to pull in the relay, which would simply be 12V / 40 ohm = 0.3 A. If the current at AC would be less, try it anyway, maybe it would pull in. If the current would be significantly more, then you would want to put a resistor or additional inductor in series with the coil, first.
     
  13. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,769
    969
    Must suck to have a life thats not worth the cost of a cube relay..
     
    shortbus likes this.
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