# Power Supply refuses to Power Coil

Discussion in 'Power Electronics' started by roonyroo, Jul 13, 2012.

1. ### roonyroo

13
0
Jul 13, 2012
Im trying to make a Electromagnet, but my bench power supply thinks its a closed circuit & refuses to power it

Im basically wrapping some copper wire around a steel rod ...

Can i add a resistor or something to make it power the coil?

thnx

2. ### CDRIVEHauling 10' pipe on a Trek Shift3

4,960
651
May 8, 2012
No, the power supply needs to see a resistive load, which is what it sees after initial settling time after power is applied to your load. Wind enough wire on there to produce a DC current not greater than your supply can handle. If it's a light duty supply you're going to need small gauge wire or a whole lot of larger gauge wire. Copper wire is rated in Ω/Ft. for each AWG.

3. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,490
2,832
Jan 21, 2010
If you find the number of ohms per foot (it will be a very long number) then you can calculate the length of wire you require.

Here is a web site that has some figures. Google will reveal many more if this doesn't cover the type of wire you're using.

First determine the voltage and maximum current from your power supply. (Let's say it's 12V at 5A). For safety, we'll try to limit the current to 4A (about 80% of the rated current).

You then use ohms law to determine the total resistance required. R = V/I. In this case R = 12/4 = 3 ohms.

Now we go back to the wire.

We check that the wire can carry the current. Look here. Use the value in the second-last column, and it MUST be greater than the current you wish to draw (or you need to recalculate for a lower current). Also you want to have some leeway in these figures. Your turns of wire can't get rid of heat very well, so pick something that can handle twice the current you require. In this example, I would want to use 22 gauge wire or thicker.

Let's assume I was planning to use 20 gauge wire. The table tells me that it has a resistance of 10.15 ohms per 1000 ft. The other web site I gave you agrees, and gives me the resistance per foot (which may be easier to use). It is 0.01015 ohms per foot.

So now I know I need a total resistance of 3 ohms and the wire has a resistance of 0.01015 ohms per foot. I need at least 3/0.01015 feet. That is 295 feet.

Easy!

If that sounds too much, then you might try a finer wire. Let's say you pick 28 gauge. It can safely carry 1.4A, so lets go for 1A.

From the calculations above, you need 12 ohms. The resistance is 0.0649 ohms per foot, so you need at least 184 feet. This doesn't sound a lot less, but the wire is much finer and there is much more on a reel. Also when wrapped around your steel rod, it will be much more compact.

Also note that it is important that the wire be insulated. Often fine wire like this has enamel insulation. Bare copper wire will short out and it won't matter how much length you use.

4. ### roonyroo

13
0
Jul 13, 2012
Thanks for the reply guys, is there anything else i can do apart from buying more copper wire?

This is a simple test & i really dont want to buy more copper

I was thinking of running the coil through a circuit with a variable resistor or something

thnx

5. ### davennModerator

13,812
1,945
Sep 5, 2009
no it wont work it will just cook the resistor and you will most likely see smoke

you MUST use more wire, many many turns of reasonably fine wire
you mite get get away with several 100 turns of ~ 20 gauge but will probably need much more or finer wire like 24 or 26 gauge

you cannot use just a few turns 10, 20, 50 turns etc, it will just present a total short circuit to the power supply

I havent read the links steve provided you but I'm willing to bet it will probably back up my comments

cheers
Dave

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,490
2,832
Jan 21, 2010
If you have a relatively few turns, the best protection is a light globe rated at the voltage of your power supply (let's say 12V) and a wattage low enough not to overload the power supply. If it's a 12V power supply, get something with a wattage about 10 times the maximum current (so if it's 5A, get a 12 volt globe rated at 50W or less).

Place this in series with your electromagnet and the lamp will prevent the current from rising so high as to damage the power supply.

Note that you won't be getting the most from your electromagnet. If you try this and then wonder "How can I make the electromagnet stronger?" you'll have to go to my post above and follow the instructions.