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Electrical coil

  1. Mar 11, 2009 #1
    I'm currently making a coil assembly for a customer. The coil has the following specs:

    120 VAC application
    1,600 Turns
    22 AWG magnet wire
    12.5 ohms typically
    coil is in a circuit with a half sine wave rectifier and a solid state VAC controller
    current draw is .6 amps (72 watts)

    Customer has asked if I could make the coil using the same bobbin but for a 240 volt application. I'm not an electrical engineer...more of a manufacturer. I take someones print and build them what they want.

    Anyway if I could come up with the windings needed for 240 VAC I could get some more work.

    So...can anyone give me a suggestion on wire gage and turns to make this coil work at 240 VAC. If you need more info please ask me and I'll try to answer.


  2. jcsd
  3. Mar 11, 2009 #2


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    What's the intended application of these coils? Wire-wound potentiometer/resistor? Power inductor? Transformer? High current choke?

    If you take someone else's drawings and then produce them, you really should be going back to that person to get them to redesign it for 240 V.
  4. Mar 11, 2009 #3
    That is the problem...originally I reverse engineered the 120 VAC coil to see what to make. This coil (120 VAC) works great.

    Now customer is asking if I can make a 240 VAC version of this coil. The application is an electromagnet.

    So to get back to square one, on this project I did not have a drawing and customer does not have electrical engineering in house hence the request that I try to come up with something.

    Based on that can you or anyone else give some suggestions for the windings?


  5. Mar 12, 2009 #4
    Double the number of turns and halve the wire area.
    Go from 1600 turns to 3200 turns.
    Go from 22 AWG (640 circular mills) to 25 AWG (320 circular mills)
    resistance will go from 12.5 ohm to 25 ohm.

    In the practical case, winding area may become slightly larger or smaller, due to incomplete layers. (Assuming you are using film insulation)
    Also the 240 volt may require slightly more insulation to ground.
  6. Mar 12, 2009 #5


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    If you know that your 120VAC coil works, can you put a 10 or 12.5 Ohm, 100W power resistor in series with it? That would reduce the (steady state) coil voltage to around 120VAC. The biggest problem with this would be that you have to find a way to mount this extra resistor.
  7. Mar 16, 2009 #6
    I asked an engineer at a local school and he suggested 3220 turns of 25 ga. wire. I'll be giving that a go as soon as I get some bobbins. Pretty close to what Carl suggested.

  8. Mar 16, 2009 #7


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    Carl I agree with the design, but if you double the turns and halve the area you'll get 4 times the resistance, not 2 times as you state.

    Four times the resistance is good though. It will give you one half the current (because the voltage is also doubled), the same power dissipation as the original coil and the same number of ampere-turns (hence the same magnetic force) as the original coil. That's pretty much exactly what you want.

    BTW. I expect Carl probably knew this and his 2x resistance figure was just a wee slip-up.
  9. Mar 16, 2009 #8
    Forgot to mention that engineer said the resistance would end up around 48 ohms which is 4X what the 120 VAC coil is now ( 12.1 ohms)

    I just received the bobbins...I'll wind the wire on them and get them out to my customer. Should hear something in a week or so.

    Thanks to all again!

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