Safest Way To Hook Up Magnet Coil?

#15 magnet wire is about 3.2 ohms per 1000 feet. If your tube was 3" diameter, then you have about 300 or so feet of wire, and about an ohm of resistance. Hooking this straight to 120VAC will suck 120Amps through it, and this is not a Good Thing. You will trip breakers etc, and 120 Amps is very close to the fusing current of #15 wire (meaning the coil will open up when a section turns into a puff of hot molten copper nearly instantly).

I suspect you want to run your coil off of a 12V or 24V power supply. At 12V you need 12 Amps, at 24V you need 24 Amps. This last current will get the wire decently warm but will not get anywhere near melting it.

Do your switching on the low-current AC side of the power supply.

I think the traditional physics-class-demo electromagnet has a current more like 10 or 15 amps go through it when hooked to 120VAC (via a pretty hefty pushbutton usually, sometimes they use a contactor rated at this current and use low voltage DC to control the contactor). You'd need 10 times as many turns to get to this resistance (again depends on your tube diameter), I think these coils are usually wound with 18 or 20AWG wire (you get more resistance per turn and it's not completely dangerous to put 15 amps through these wires for very brief periods.)

Tim.

The safest way probably involves an adjustable voltage, adjustable current limit lab supply, though you may be able to find a big 12 volt supply intended to drive automotive radio equipment that also has a current limit. The current limit will prevent a damaging overload if too low a resistance is connected.

These will also have both a voltage and current meter included, so you know these values during the demonstrations.

At the very least, you need isolation from the mains voltage. A battery charger with a selectable 6 or 12 volt output will probably be the lowest cost way to get a given number of amperes.

Here is an example of a big, capable lab supply, new, on Ebay:

And an adjustable supply with some kind of overload protection, but not adjustable current limit:

What's the coil diamter? Can you measure the resistance?

It sounds like your coil is designed more for a 12V car battery than for 120VAC.

If the coil resistance is too low, the switch will make a brief plasma flame and blow the breakers for your classroom.

You might want to check out the various PIRA physics demo archives for more info. Their electromagnet demonstrations all seem to be battery powered. Very wise. Nobody gets killed, or injured by flying shrapnel.

Hi! I am building an electromagnet with my class, and have found some very basic designs on the internet. The simplest way (on our budget) to show a quick electromagnet I have found is winding a coil (which we have done) of about 420 turns of Formvar #15 heavy magnet wire on a phenolic tube. I am now looking for the safest way to hook this up to wall voltage. Any recomendations on this? I will be the only one operating this, and it will be a momentary switch energizing the coil only for a few seconds (demonstrate Lenz' law, show fields, etc). I realize that heat is a problem, but we will only have this on for a few seconds.

Any ideas?

Thanks.

Terry

Hmmm....the info that I got this from showed it being hooked up to a a DPST relay (Potter-Brumfield, the actual model they do not make any more) with a lamp cord. Why the difference?

Via an isolating transformer, preferably at a safely low voltage.

Watch it, my Dad used to do this (he was a science teacher). On one occasion, when he turned off, the iron bar shot out so fast that it went straight through the partition wall, into the next classroom, and embedded itself in the blackboard just next to the poor guy who was teaching there.

Paul Burke

I played with something like this in my youth. If it is only being used by the person who built it, and if this person is careful and uses an earth leakage breaker (residual current circuit breaker) then just make sure that the connections are mechanically secure and insulated with a couple of layers of insulating tape, heatshrink etc. Obviously don't use it with wet hands etc. etc. standard disclaimers etc. and of course it shouldn't be left unattended like that blah blah don't sue blah. I used just a plastic shrouded terminal block, and that was on 240V. I put tape over all exposed windings to protect the insulation from scratches. I was fairly careful because I already knew what 240V feels like. I guess I was about 10 years old, and I spent weeks playing with that thing and didn't get any shocks, until I stupidly held an old relay coil near it, which formed a secondary winding and in a fraction if a second it burnt right through the skin of my thumb where the relay coil terminals happened to be, i.e. the only thing I got a shock from was not even connected to the mains (line voltage) at all! I certainly learnt about electromagnetic induction and the smell of burning thumb.

Not if it has an iron core - this is AC not DC, and inductance has an effect. Learning about that is a good reason to do this experiment. The coil I used was would on a spool with perhaps 40mm inside diameter and

120mm outside diameter and the length of the coil former was about 120mm. The wire was about 1.7mm diameter enamelled winding wire and the spool was wound full of wire. Inside the former was a loose fitting PVC pipe about 500mm long and the right diameter to fill up the hole in the coil, i.e. around 40mm. The pipe could be inserted and removed from the coil as necessary. I packed the PVC pipe full of soft iron welding rods (for oxy-acetylene welding, not flux coated). I used it on 240VAC for up to about 1 second, it probably drew 10-15 Amps. It could fire a piece of copper pipe with enough force to be painful if stopped by the palm of your hand and dangerous if it hit a person on the head etc.

Probably more interesting was the use of this coil attached to a variac (variable autotransformer) or a 120V stepdown transformer. You could heat metal rings past the melting point of solder and light an automotive light bulb by wrapping a few turns around the iron core. If you tried to pull the core out of the coil, the fuse would blow because the inductance of the coil would be reduced without the core.

I found that the fuse invariably blew before the coil, though slow overheating of the coil was a bigger problem, which necessitated 1/2 hour cooling off periods which I found very tedious.

Really you need at least 1kVA before the coil is much fun to play with, IMHO.

[...]

Yes that's more like it...

But you can get enough inductance easily with a bunch of welding rods.

You probably could benefit from even fatter wire and less turns than I used if you only have puny little wussy 120V power. Anyway have fun and be careful.

Chris

not yet mentioned is what happens when you turn the power off. The inductor would like the power to stay on, and will bite hard to try and make it so. Hard enough to be quite dangerous if you form part of the circuit, and hard enough to break down a fair amount of insulation. So even on low voltage, dont assume its safe by any means.

NT