# How should I chose a resistor for a home-made electromagnet?

I have a magnetic wire and I want to use it to make an electromagnet. The electromagnet will be powered by a 5V, 2A power supply.

How do I know what resistor should I use in series in order to avoid damaging the power supply?

I suppose that I can use Ohm's law, which would mean that I would need a 2.5 Ω resistor. Since the resistance of the wire itself is 0.5 Ω, it means that a 2 Ω resistor would be enough.

I also suppose that if I use a higher resistor, such as 4.7 Ω, the electromagnet would be less powerful.

Am I right, or have I missed something?

• That sounds like a good plan. You need to make sure the magnet wire doesn't get too hot, but being prepared to disconnect it quickly is probably sufficient.. – Charles Cowie Mar 29 at 23:34
• The magnet would be more powerful if you could wind more tunes on it until its resistance was 2.5ohms and no resistor. Can you put any more turns on the magnet? – Kevin White Mar 29 at 23:37
• If you are concerned with the electromagnet to be "powerful" with a given power supply, you need to get your magnet (not "magnetic") wire 5 times longer and don't waste any power in external resistors. What size of coils do you plan to have? – Ale..chenski Mar 30 at 2:31
• Two methods you might use are to start with a high resistor and work down until the coil starts to heat up too fast, or to start with one AA battery, test, add one, test, add one. If your current is DC, it's the resistance of the coil that will determine the value it settles on, so you can measure DC coil resistance(just hold the leads on the coil until the resistance value settles out, this takes a while for high inductance coils) and estimate acceptable voltage based on that and the magnet wire's ampacity. A cheap variable voltage regulator would be a good option too. – K H Mar 30 at 3:20
• Bear in mind the plumpness of your core and shortness of magnetic circuit will determine your maximum magnet strength. Having an outer shell of similar-to-core cross section around and at the back of the magnet is idealish. – K H Mar 30 at 3:28