Help with an electronics circuit/problem?

Question

I have built a large electromagnet using 3m diameter steel and wound bell wire; the bell wire has an operational voltage of 100v max and a maximum Amp rating of 1.8A. I want to connect this to a mains supply and so have bought the necessary adapter which is 1.25A continuous rating and changeable from 3v-12v, however connecting this results in the power supply overheating or not delivering the power. Is there a circuit i can construct which will allow this to work efficiently i am thinking a capacitor/diode setup? NB: the power supply is AC230-DC3-12v

Answer 1

The resistance of this wire will be very low.

Note that Current = volts/resistance

So even with only 3v you are probably exceeding the current rating of your PSU.  You would probably need quite a hefty current supply.

Using AC instead of DC may help, as the coil will have some reactance.   You will still need a transformer with low voltage, high current output.

The problem is we don't know the resistance of the wire for DC calculations or the reactance of the coil at 50Hz for AC calculations.

Answer 2

1.25 amps is a small current.

I don't understand why you used a 1.8 amp wire, that has to be about a #20, a very small wire, whereas 3 mm steel is a #10 wire. Do you have 2 windings? which one are you energizing?

If the wire is limited to 1.8 amps, all you can do is add series resistance to the 3 volt supply to limit it to 1.8 amps. Just add some small value power resistors, on the order of 1 ohm, until your current drops to 1.5 amps.

But I really need a lot more details before i can give you a definite answer. Measure the resistance of the two windings. Tell me how many turns. You do have a multimeter? they are very cheap.

edit. Why can't you measure the resistance, just clip an ohmmeter on it. The cost $10.

"there no was no measurable current being transferred to the coil" how do you know that?

"9v divided by 1.25A= 7.2ohm" no, when you had 9 volts on the magnet, you has something less than 0.3 amps flowing. which implies the 3 volts supply should work fine, and there would be less than 100 ma of current flowing.

Really need that resistance measurement. Or the length of wire. We don't even know if your voltages are Dc or AC.

We are going around in circles. Experimentation requires a bunch of things, one is measurements. make lots of measurements, and record them all.

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Answer 3

I would say you install a resistance in series. It doesn't have to be too big, it's just to constrain a little bit the current and for your energy not to over heat.

Hope this helps.

Answer 4

Its basically an impedance mis-match The PSU doesn't like the very low impedance of the bell wire coil.  It hehaves like its being shorted out. If you had a transformer that was wound to give you say 230V primary, and perhaps 3V secondary at a high current, there would be no need fo a PSU as such.

I suggest you work out the resitance of the coil you have made. That could be measured directly or computed from a data sheet ohms/metre figure. The PSU impedance needs to be the same or less.

People tend to think PSUs emit all the current they need at any voltage, but its not likely.

You certainly could add some diodes (a bridge rectifier is best) and big caps to smooth the DC.  I'm not sure what you're try to achieve. It sounds like "whatever magnetic field I can get when I put the maximum current through this cheap wire."

3m diameter steel "what"?

I looked up some cheap wire. 2.5mm^2 wire has a resistance of about 0.008 ohms/metre. So if it were 0.5mm^2, then assume 0.040ohms/m.