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Using 120VAC to actuate a 100VAC relay coil...

Discussion in 'Electrical Engineering' started by Robin S., Jun 6, 2004.

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  1. Robin S.

    Robin S. Guest

    I bought a relay to make a simple motor controller (w/o the overload
    protection). The relay's coil is rated for 100VAC and I want to run it on
    120VAC. Is this a big issue? I've been told that because a coil is a highly
    inductive load, one cannot simply find the correct value for the resistor
    (guess and test would be a good idea).

    This is for a hobby machine tool, not an industrial installation.



  2. John Gilmer

    John Gilmer Guest

    It might be.

    There are several posibilities but most can be "evaluated" by just
    connecting 120 volts to the relay for a few moments while measuring the
    current. If the current stays at a fraction of an amp then "fire for
    effect" and leave power on for some minutes to see how hot things get and
    whether the relay passes the "smoke test."

    The trouble with a resistor in series is that the current flow is MUCH
    highter before the relay operates than after it does. A resistor large
    enough to prevent overheating might keep it from operating.
  3. Robin S.

    Robin S. Guest

    Sounds like a good idea. The relay is made by Omron in Japan so I'm assuming
    it should be half decent. How hot should it normally get? I understand you
    can't exactly tell me a number, but should it be warm, hot, too hot to
    touch, etc.?
    Sounds something like a locked rotor current situation? If the relay gets
    *too hot*, do I have any options (other than to go and buy a real

    Thanks for your swift reply.


  4. SQLit

    SQLit Guest

    If it is an icecube relay, then why can t you get the right one for your
    My applications provide for the relays not to be EVEN warm because of
    Please notice relays not motor starters.
    Relays are not designed as motor starters. Motors draw 6X running current
    when starting and they can be overloaded that is why the NEC requires a over
    load protective device to be installed for protection.
    Have fun and I hope this is not a critical application.
  5. Robin S.

    Robin S. Guest

    I bought it surplus. It's not exactly an ice cube relay. It's got the 1/4"
    flat connectors on it and it's certainly beefier. Granted, it's not a
    contactor, but the contacts are rated for 20A at 115V.
    Yup, you're right.
    This is a motor starter for a 1/4HP fan motor that I'm using to run a small
    lathe. It is in my garage and I will be there 100% of the time in which it
    is operating.

    I think the worst that will happen is that the contacts will wear out and
    I'll have to splurge for a contactor.

    Thanks for all the comments. As a tool and die apprentice, it kills me to
    see people not doing it the "right" way so I understand your concern. I'm
    just too cheap to buy the correct product for my application. Because I can
    supervise it and I'm not to concerned if it fails, I think I should be OK.


  6. Robin S.

    Robin S. Guest

    This relay only has NO contacts...
    I don't think it said. It's just got a painted "100VAC" which looks larger
    and rougher than the other ratings and info on the relay. Odd, but I guess
    they can do it any way they want.

    I ran the relay for about 5 min today. It got very slightly warm. Hardly
    noticeable but I think it was warm. I think I'm going to run with it.


  7. Guest

    This is actually trivial. First, a 100 VAC relay coil is not going to burn
    out quickly if it "sees" 120 VAC. It will happily take an overvoltage for
    quite some time before it dies. A simple power resistor will work
    fine. There
    is NO need to worry about inrush current harming the relay. You DO want
    to reduce the voltage to reduce the heat in the coil, not because it
    will burn
    out quickly, but to make the thing last for a long time. Next, your 100
    relay will probably pull in as low as 80 volts, maybe lower, so precise
    computation of the resistor's value is not needed.

    Try and test, as you mentioned, will work fine. Put the resistor in series
    with the coil and measure the voltage at the coil. You want it under 100

    You DO need to use power resistors, say 10 watts or higher. And once
    you have determined the resistance, you want to make sure the resistor
    won't burn out. In this case, use the formula 400/R (where R is the
    resistance in ohms) and double that figure to come up with the wattage.
    Go to the next higher standard wattage value if the figure computed is not
    Example: Say a 100 ohm resistor is selected. 400/R would be 4, double
    that would be 8, and the next higher standard wattage would be 10 watts.

    By the way, you could buy 4 40 ohm, 10 watt resistors for $2.00 from
    All electronics. In series, or series/parallel or parallel you could get
    the following useable combinations: 40 ohms at 40 watts, 60 ohms at
    30 watts, 80 ohms at 20 watts, 120 ohms at 30 watts and 160 ohms
    at 40 watts.
  8. Louis Bybee

    Louis Bybee Guest

    In spite of my concerns regarding your "w/o the overload protection" I'll
    suggest that you consider the following site (and many others using "buck
    boost" as a search string in google) as a possible solution for
    experimentation purposes. Using a small control transformer of the proper
    current rating with a ratio of 120/16 or 120/24 connected in a "buck"
    arrangement could address your "over voltage" situation.

    It is very important that you at least consider the reasons/necessities for
    overload protection, and many other safety issues before pursuing your
    quest. The energy available in the circuit you are working with is well
    beyond lethal. If you lack experience in this area you should at least have
    a competent electrician/engineer review your intentions before the actual

    Watch out for word wrap.

  9. John Gilmer

    John Gilmer Guest



  10. Robin S.

    Robin S. Guest

    I can assure you I would as well in your position.
    My relay was about $4.00 (Canadian dollars).
    That's true. I was originally going to wire the motor such that it could be
    be reversed. Unfortunately, the starter coil is not wired independently of
    the running coil (there are only three wires instead of four). Not that I
    would know, but it looks like that wiring does not allow reversal. Anyway, I
    thought I'd wire up a box to put it all in. Call it a learning experience.


  11. Bob Peterson

    Bob Peterson Guest

    its about the standard price paid by those who buy them from distributors at
    oem level discounts. does not matter much what brand. often the oem price
    is 1/4 the list price, sometimes even less.
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