variable electromagnet

You are not making this easy on yourself by starting with a 220 volt AC dimming unit. It is not trivial to link this to a computer, and it involves obviously lethal voltage.

I think you should read up on the concept of a buck switching regulator that is operated in current control mode. This could be run from any DC supply you might have, including an automotive battery charger.

Here is a link to a tutorial on the concept of a buck regulator. The inductor in the regulator would be your lifting electromagnet.

To operate a buck regulator in current control, you need a way to measure the coil current, and a low value resistor in series with the ground end is one way to produce a signal that represents the current. For starters, you can ignore the current and just control the on off duty cycle.

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thankx for your rapid answer. the computer-controlled dimmer is the 'easy' part, as i received one 2nd hand. this equipment is already fully functioning, and thus the reason for the 240v AC constraint. however, i don't know where to buy or how to make such electromagnets, if they can be used for long time spans in safe conditions and whether they are able to attract light metal objects about 10 inches away.

- Andre.

But from the sophistication of your question, I fear for your safety if you use it to start your experiments.

Some generalities:

The reach of an electromagnet follows its size. If you want to attract metal from 10 inches away, you will need to think in tens of a coil with this scale of dimensions. Before you build a big one, scale the whole thing down and learn with a small model.

Coils run with AC are not as useful at attracting things than coils run on DC (they make things hum and make iron cores hot from eddy current), so you will probably need a rectifier between your dimmer and the coil. This may make the dimmer work strangely.

A safer way to use the 240 dimmer would be to drive a step down transformer with it and do all your experiments on the low voltage (say, below 30 volts) side after you tape up all the high voltage terminals. Don't forget to add a fuse on the line side to keep an oops from making the dimmer and/or transformer burst into flames.

An iron core through the coil will concentrate the magnetic field but also retain some field when the current is off.

The OP seems determined to use a computer controlled dimmer.

John, what about a variac? Then, all he needs is a beefy diode ( and the magnet coil, of course ).

"i would like to build a variable electromagnet that is directly connected to a computer controlled (220v AC) dimming unit. i suppose that dimming the electric current will lower the magnetic field and thus its attractive force? the (somewhat creative) goal is to gradually attract light weight suspended metal parts depending on the dimming unit output.

- is this possible? if so, could you give any tips what to do or where to look?

- dangerous? (e.g. to nearby computers or other electric equipment)

- what would be the range of such a electromagnet? the suspended objects will be very light but some object-magnet distance should be kept." ___________________________________ Re; Not sure about the AC dimmer control part; but I would suggest that you use a full wave rectifier to provide DC current to the magnet coil. I would also recommend that you use a soft iron core to minimize magnetic "memory". You can scavenge them from discarded AM radios; they are used in the "antennae coil". They are also found in some large inductors. Assuming that would be meet your needs. Just remember that the magnetic field strength will be proportional to the number of turns of wire you wind around the core and the magnitude of the DC current flowing through it. The more turns the greater the field strength. Thus, even a very small current can create a powerful field with a large number of turns. The "range" of the field will depend upon it's shape and that in turn depends on the geometry of the coil/core assembly. If the ratio of the length to diameter is greater, then the range of the field will be greater. You may also need a power resistor between the rectifier and coil to provide the current control you desire. The characteristics of the AC controller will determine this. Yes, strong DC magnetic fields can do strange things to the electron beam of cathode ray tubes; but they usually recover during the auto degauss at start-up. I hope that helps in some way...

-Dan Akers