Sunday, May 1, 2011

Not so classic electromagnet for kids

Promised my nephew we'd make an electromagnet. So I cobbled up one using the entire coil (6.5 meters) of enamel wire from the inductor of a burnt out compact fluorescent lamp, a 1/4" x 2" zinc-plated bolt, and a couple of 15ohm 3watt resistors from some old busted I don't remember what appliance. Juice provided by a power supply.

I measured the enamel wire's resistance at 2.5ohms. I could've used a single dry cell (say, a D battery) to power the EM but at 600mA current draw it would run out pretty quickly. So I decided on using a power supply. Unfortunately the PS's minimum output is several volts which meant I had to add some resistance to decrease the current flow or the EM would get too hot to hold!

To build the EM I just coiled the wire around the bolt, going back and forth its length a couple of times till the wire ran out, leaving the two ends of the wire sticking out to make electrical connection with the resistors and power supply. The two 15ohms were paralleled (to decrease resistance and increase power dissipation capacity) and the pair connected in series with the enamel wire. With wire resistance of 2.5ohms and the paralleled resistor value of 7.5 ohms, the total resistance was a nice round figure of 10ohms.

I soldered the resistors on a piece of stripboard and added two PCB terminal blocks on both ends to connect wires to so that when this electromagnet circuit is scrapped I'll still be able to use those porcelain-encased resistors. The resistors are paralleled with jumper wires on the terminal blocks.

I initially powered the circuit using 3.3VDC from a regulated supply. That's gives 330mA of current. Magnetic flux was fine but not at all strong. Once nothing blew up or got fried I increased the supply to 5VDC. With half an amp the resistors were dissipating almost a watt each [(0.5A/2)^2*15ohms] and the electromagnet (the enamel wire), 625mW.

Increasing the current through the coil would increase the magnetic flux but will proportionally increase the heat dissipation of the circuit as well.

Optimally of course I shouldn't have to resort to adding resistors. As the above computations show I was throwing away 2W as heat while less than a watt was doing useful work. Well I do have more dead CFLs whose circuit boards still have to be salvaged from the lamps. If the 5-year old brat doesn't get bored with the EM after just a few minutes and ends up wanting to play with it some more in the coming days I'll crack open (literally) those lamps and splice more enamel wire to the EM to bring the total wire resistance to 10 or more ohms. May have to use a larger bolt. And have to check that it doesn't become too warm to handle comfortably!

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