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Electromagnet Run From Powr Supply

  1. Dec 15, 2007 #1
    Hello All,

    I am trying to run an electromagnet from my new power supply (TekPower HY3010E - 0-30V @ 0-10A). My electromagnet is a very simple air-core solenoid - 3-layers wound with 8 turns per layer from a 20-ft-long #12-gauge wire. The coil form is not big at all - a simple cardboard section of a toilet paper roll, and as my turns are few, a fair length of the wire trails the end of the winding uncoiled. I was running this off a 6V alkaline lantern battery, and now I want to adapt it to being powered by the power supply in a variable fashion.

    Obviously, this is not the ideal electromagnet, but I am engaged in doing this as a learning exercise. I am after the flexibility/durability of power supplies as opposed to batteries, and I want to make sure I understand things correctly from the safety and workability perspectives, thinking on the right scale for such equipment. I anticipated that hooking up the coil as wound to the supply would trigger its short-protection mechanisms, and it has - the setup simply does not work. I know I need RESISTANCE, as my total resistance as calculated is .0318 ohms given the gauge of wire and its length. So here's my question:

    What kind of resistance is best to add to my circuit such that I have maximum variability of operation (in terms of strength) of the electromagnet, either by varying the supply or by adjusting the resistor itself? How many ohms, rated at how many watts?

    My initial attempts to work this out on my own are as follows:

    (current state of affairs)

    R is fixed at .0318 ohms.

    I = V / R, so if I tried to apply even .32V to my circuit, I would go over 10A, already exceeding the limit of the supply.

    Looking at it the other way around, if I applied 10A to my circuit, all I would get is a measley .32V. The supply likely has very high sensitivity to dead shorts, thus I can't even fine-adjust into this from 0. Fair assumption or not?

    (where I want to be)

    R = V / I, so if I want to be able to adjust my electromagnet to the max of my power supply's capabilities, I need an R = 30V / 10A= 3 ohms resistance in the circuit? And since P = I * V, I need it rated at 300W? (3 Ohm, 300 Watt resistor from Milwaukee Resistor, Part # 061813603.00E)? This all assumes a fixed-resistance solution, of course. I have not done any figuring regarding the fixed-supply-setting, variable resistor approach.

    So am I on the right track here, or am I missing something?


  2. jcsd
  3. Dec 16, 2007 #2


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    Science Advisor
    Gold Member

    The strenght of the magnet will ONLY depend on the current; the voltage is irrelevant.
    However, you need to be carefull when you choose the resistor and the wire since the power dissipation is R*I^2, i.e. even with only 1 Ohm resistance you end up dissipating 100W if the current is 10A. Also, make sure the wire you are using is really rated at 10A, otherwise it will melt.
    Hence, I would use a small resistor in series with the coil, something like a 0.1 or 0.2 Ohm resistor rated at 50W should work (but it will get hot!). There is no point is using a potentiometer since you can just vary the current from the supply.

    Just make sure you ramp up the current slowly.
  4. Dec 17, 2007 #3
    I understand now, thanks very much for the explanation!
  5. Dec 17, 2007 #4
    Reflecting on things from an economic standpoint for a moment, what the above tells me is that, given how hard acquisition of such small-value resistors is, it would be far more economical to simply increase the length of my wire by a bit over three times. This would give me .1+ ohms of resistance for my 12-gauge wire.
  6. Dec 17, 2007 #5


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    Looking at the power supply specs it seems that it has a constant current mode, and it also seems that this is simultaneous with constant voltage, so I think it should work. Does the meter show current flowing, and what have you set the current limit to?
  7. Dec 17, 2007 #6
    I'm pretty sure you can just set the current limit on the power supply. If I understand what you're trying to do, you don't even need to add any resistance - just set the current limit to the current that you want to run through the electromagnet.
  8. Dec 18, 2007 #7
    Hi A power supply is a stable source of current ie.. in your case 10A. Another cheap way to solve the resistance problem and what I did is to use a tail light from a car. The bulb from a car is .9 ohms and also is a good indicator to remind you that current is flowing.
    I used a controller from an old electric train set as a to make mine work. Just keep being creative and look around you and use what you have
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