Drive 120vac coil relay with full bridge rectified DC volts?

Discussion in 'General Electronics Chat' started by sdowney717, Dec 8, 2014.

  1. sdowney717

    Thread Starter Member

    Jul 18, 2012
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    I have an issue with a 35 amp 4 pole 4PDT 120vac relay.
    If the power drops under a big load, the relay buzzes. I suppose due to not enough magnetic strength being generated, also due to the shore power supply not all that great.

    Can I use a bridge rectifier convert to DC volts and power the AC coil?
    That way the volts wont be cycling past zero so I think with even lower voltage the thing wont humm loudly? Due to the magnetic field being more stable that pulls the relay closed.

    Your thoughts on doing that?
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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  3. sdowney717

    Thread Starter Member

    Jul 18, 2012
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  4. sdowney717

    Thread Starter Member

    Jul 18, 2012
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    It has a 14va, 4.4 watt coil rating it seems
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Your best bet is to get a DC coil for the relay/contactor if available or swap it for a DC version and use a bridge rectifier off the relay coil AC feed.
    Do not use the AC coil in this way, the resistance is far too low.
    Max.
     
  6. sdowney717

    Thread Starter Member

    Jul 18, 2012
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    How about running the coil on 12vdc from the battery?
     
  7. MaxHeadRoom

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    You may get away with that configuration, takes a couple of minute to confirm it.
    Check for coil current, (resistance of the coil) and overheating.
    Max.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    You could use the AC coil on DC but you would have to reduce the voltage so that the rated coil current is not exceeded. You can determine the required voltage by measuring the DC resistance of the coil.
     
  9. sdowney717

    Thread Starter Member

    Jul 18, 2012
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    Ok, I will report back tomorrow the ohms in the coil.
    I can also measure the current flow with the clamp on meter. It will do AC and DC current.

    Is it you dont want DC current flow to exceed AC current flow?
     
  10. alfacliff

    Well-Known Member

    Dec 13, 2013
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    120 vac coils usually run on 24 vdc ok.
     
  11. sdowney717

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    Jul 18, 2012
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  12. sdowney717

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    Jul 18, 2012
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  13. BobTPH

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    Jun 5, 2013
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    No, because there is also inductance that limits the current for AC . This is why you cannot use the same DC voltage to operate the coil.

    Bob
     
  14. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Like I said, it takes a few minutes to check it, it is not going to damage it on DC just to asses whether its possible, That only 100ma on 12VDC.
    May not be enough if it is a large contactor/relay coil.
    One advantage with a DC coil, it takes much less power to retain it once pulled in.
    Max.
     
  15. sdowney717

    Thread Starter Member

    Jul 18, 2012
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    it is interesting only 100ma will it draw.

    How about using an AC power brick with a higher voltage like 15- 18 vdc output. Would that regulate the DC output to a closer tolerance if the AC line volts dropped under a load? Their higher volts might make the relay work better.
     
  16. MaxHeadRoom

    Expert

    Jul 18, 2013
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    How about it, give it a shot, not much to lose!
    Whether the Wall-Wart voltage will remain high depends on the type of regulation it has, if any.
    Max.
     
  17. excaliber

    New Member

    Dec 6, 2014
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    That is a common practice in industry to make a latching cct that will drop out and not start again if there is a power failure. 120 volt contactors with a mechanical linkage so that when the linkage is pressed the points are pushed closed powering up the machine, and a step down transformer that has its output full wave rectified to 10-12 volts dc. this dc voltage is applied to the coil and holds it in...not sure if this will be helpfull in your case.
     
  18. crutschow

    Expert

    Mar 14, 2008
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    You are missing the coil impedance increase due to the inductance. Note that the spec also states the operating power is 14VA. That means at 120Vac the coil current will be 14VA / 120V = 117mA. Thus to get the same coil current using DC would require a voltage of 117mA * 120Ω ≅ 14V. So I wouldn't apply more than about 15V to energize the relay.
     
  19. sdowney717

    Thread Starter Member

    Jul 18, 2012
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    I have a switching walwort 15vdc 1 amp and I measured it at 15v.
    I also have some heavier transformer warts at 12 v at half that power and they measure 19v. At their rated load they must drop volts.
    Maybe they would keep higher volts with smaller loads like the coil.
    I also read the switchers run cooler and work better over a wider range, so that might just be perfect.

    About adding a piece of mylar tape, I wonder if something like a piece of mylar computer bag plastic glued on or taped on the side would work ok.

    I will need to look at how the relay works, there might be a gap there when shut, although the buzzy hum likely means a metal on metal contact point.
     
  20. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The armature on a relay should always contact the pole face, often AC relays buzz even when the correct voltage is used, and can be caused by insufficient mating or poor contact of the pole face and armature.
    Modern Wal-Wart are usually regulated, the older style was linear transformer supply unregulated, the open circuit voltage was high and relied on the rated load to bring the voltage down.
    Max.
     
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