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Help! Electromagnet problem.

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Sparklefly

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Hi everyone. I'm hoping you can help my son and I with a problem that we're having.

We're making an electromagnet for his science fair project and testing how differing voltages make the magnet stronger and weaker. I think our biggest problem is having trouble converting ohms and watts and amps and volts and really, honestly, just not having a damn clue what we're doing.

We're using 8 size D batteries (1.5 volts each), running the current through an on/off switch and into a potentiometer to control the current. We got one and promptly blew it up. So we have just purchased another potentiometer and it says it's a "500K ohm". Are we about to blow this SOB up, too?

Any help would be appreciated.
 
Sparklefly said:
Hi everyone. I'm hoping you can help my son and I with a problem that we're having.

We're making an electromagnet for his science fair project and testing how differing voltages make the magnet stronger and weaker. I think our biggest problem is having trouble converting ohms and watts and amps and volts and really, honestly, just not having a damn clue what we're doing.

We're using 8 size D batteries (1.5 volts each), running the current through an on/off switch and into a potentiometer to control the current. We got one and promptly blew it up. So we have just purchased another potentiometer and it says it's a "500K ohm". Are we about to blow this SOB up, too?

Any help would be appreciated.
P = Watts (Power) I = Amps (Current) E = Volts (Voltage) R = Ohms (Resistance)

P = I * E, E = P / I, I = P / E, E = I * R, I = E / R, R = E / I

EDIT: Sorry, forgot to answer the other questions. I *think* (I can't exactly remember) that if you put voltage at the wrong legs of a pot, since there are typically three legs on a potentiometer that you get at a Radio Shack, it wrecks the potentiometer. Use a multimeter on the potentiometer first to determine where you get resistance, then hook up the arrangement accordingly. I don't know if this would have any significant effect, but you may want to have the positive terminal of the batteries first connected to the potentiometer, THEN to the switch.
 
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It's not the voltage, but the current through the coil that produces the magnetic field. If you want to vary current, use a rheostat rated for about 10 watts, and about 250 ohms value.

A better way would be to use a power transistor to control the current, but that might be a bit much at short notice.
 
Ok. I think I understood what you said, so I'm not going to cry too hard. Yet. ;) I'm going to have him do what you suggested and see if that works.
 
Cry? That's just ohms law =) It's actually pretty simple compared to what you'll end up having to deal with if you get into electronics. There's one big piece of information you haven't mentioned, what are you using for the magnet coil, and do you have a multimeter to use?
 
hi,
As you have 8 of 'D' cells, why dont you just use one cell, get the magnetic values you are looking for,
then use 2 cells, then 3 cell... etc.

This will give you 7 'magnetic values', enough to plot a simple graph.

To do an empirical 'magentic value' test, use the electro magnet to pick up various metallic weights,
use the kitchen scales to get their weights.

If you have a voltmeter/ammeter you could measure and plot the results for each cell added.
If you have an old car 12v lamp, this connected in series with the electromagnet this will limit the current to some lower value,
again plot the results.

A standard potentiometer will just burn out, it cant carry the current.

I would suggest a well plotted graph of voltage against the weight picked up, would be a good answer for a school science project.

Are you winding your own magnet/coils?
 
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Sparklefly - bear with me on my comments and concerns. It's clear that you are a beginner but I don't know how much of this makes any sense to you.

Ohms law will help you determine current if you know the applied voltage and the resistance (ohms) of the electromagnet. The problem I see - metal wire (copper, iron, etc) has a relatively low resistance and it's possible and likely that you are drawing so much current from the cells that you might pull the voltage way down.

What I would do is understand how much current your D cells can deliver - then back off some. I am sure someone here can help with a suggestion on a current limit. You would then need to make the electromagnet so that it's resistance results in a reasonable current flow. The resistance of wire in ohms/ft or ohms/meter is published. The game you'll have to play is determining wire size - the current flow will govern wire size - too small and it will burn out.

If the plan was to apply 12 volts and the current limit was 500 ma (1/2 amp) then you'd need a resistance of 24 ohms - that's an awful lot of wire though it is possible to make a coil. If 30 ga wire were used you'd need about 250 ft. I am not sure if 30 ga can handle 500 ma.

What this suggests to me is that you might want to work with lower voltages - realizing that lifting power will be lower. Just to be clear - the voltage I refer to is the voltage across the electromagnet.
 
Try an LM317 voltage regulator wired for constant current operation, the circuit is on the datasheet which can be found from Google.
 
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