Using an Electromagnet with a switch?

[ashishvinayak]

I'm making an electromagnet for a contraption. As far as I know all the electromagnets demonstrated on the web are working permanently with supply of battery (Direct current). I need to know how i could make an electromagnet which works on an AC supply with a switch ? Please provide some diagram if possible... Thanks

 

[Bob S]

Electromagnets running on ac make very poor electromagnets if they have iron cores. They also make very poor inductances, unless they have laminated cores. Do you have specifics on the magnet; air gap, number of wire turns, wire gauge, resistance, required magnetic field and stability (ripple, etc.)? We may need to design or specify a dc power supply. What is the application? Please review the attached drawing and equations for a simple electromagnet geometry. See
https://www.physicsforums.com/attachment.php?attachmentid=21781&d=1258084057

Bob S

 

[ashishvinayak]

I haven't started making the e-magnet as yet due to the fact that I want to use it with an AC supply. The purpose of the magnet upon being activated (with AC supply) is to pull an iron block over a "Seesaw" to make one side heavy (the side where the iron block is pulled). As the side becomes heavy the other side moves up to do the intended task.

I don't mind using the DC supply. But, will the magnet be strong enough to pull the iron block of about 50-75 grams.

My main purpose is to make an electromagnet which is strong and which can probably be controlled with a switch. (to work well in my contraption.)

Also it will work with an iron core as i am presuming it would make a stronger magnet.

 

[Bob S]

An iron core will make the magnet much stronger, but an iron core will need a dc current, (i.e., an ac-to-dc rectifier) and maybe a filter and regulator (depending on your stability requirements). To determine the sizes of magnet to give 50 to 75 grams of pull, we need to know the air-gap separation and a little bit about your geometry. The magnetic force depends inversely on air gap..

Bob S

 

[sophiecentaur]

Why not put a nice fat diode in series with the coil to give it current in only one direction? It won't matter if the DC is 'a bit rough'. The electromagnet may buzz a bit alarmingly. If you don't want that, then use a bridge rectifier (loads of diagrams on the web) to make it better. If there is no actual contact between magnetic parts then the buzz level should not be too much. Also, if you iron core is laminated (like a transformer - or even use transformer bits; old transformers are a handy, free, source of iron) it won't get hot due to eddy currents.

 

[ashishvinayak]

@bob s - I'll let you know about the geometry in a day or two. What i still didn't get is if i use an iron core where does the air gap come into play? Also is it possible to use a car battery or something similar as supply to the magnet? I know that it will act as DC but I think it would make it possible to make my magnet stronger instead of using simple batteries.
*Anyway I'll let you know the specifics on everything in a day or so. Thank you*

 

[ashishvinayak]

@sophiecentaur The problem is that I don't have much knowledge about diode and the other things you've mentioned. I might be able to connect the entire connection in series as you mentioned but i still didn't understand what you've stated fully.

 

[Bob S]

If you have a seesaw with an iron block moving up and down, and a stationary electromagnet, then you have an air gap between the electromagnet and the iron block. The magnetic force will get stronger as the air gap gets smaller. This is positive feedback, and will lead to unstable operation.

Bob S

 

[sophiecentaur]

@sophiecentaur The problem is that I don't have much knowledge about diode and the other things you've mentioned. I might be able to connect the entire connection in series as you mentioned but i still didn't understand what you've stated fully.

Fair enough. - not everyone's cup of tea!
It all depends whether you want to use this for an extended time. A car battery would give you plenty of amps for several hours and would save you having a mains supply.
The force you are after is less than 1N (I assume you mean the weight of 80g) but how much of a gap do you need it to act over? The actual layout is quite relevant if you want to know what force an electromagnet will give you.
'Suck it and see' is quite a good method, though. But I don't think you would necessarily need a big 12V source for a light weight apparatus - you can very easily produce red hot wires with a car battery!

 

[ashishvinayak]

I posted some crude pics of what i actually want. The air gap should be around 5 cm or so.

http://picasaweb.google.com/ashish.vinayak/ElectromagnetSketch#