How to build an electromagnet

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metiz
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How to build an electromagnet

Post by metiz » Tue Jan 18, 2011 1:55 pm

I want to make a relay that works with a very specific power (70ma @ 1.3V). I can't find any commercial relays that work with those kinds of voltages.

In highschool I learned how to build an electromagnet using a specific electricity source, but that knowledge is long gone. Can someone help me with the maths, or even better, link me to a company that sells such relays?
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Re: How to build an electromagnet

Post by Mark » Tue Jan 18, 2011 2:31 pm

What are you planning to use it for?
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Re: How to build an electromagnet

Post by metiz » Tue Jan 18, 2011 3:48 pm

Hey Mark

I want to use the relay to hook up to a mobile phone's rumble leads so I can switch devices on or off with a simple call. I already made a device like that for a friend but I used the rumble motor for that as a kind of mechanical relay. It works but it is realy fragile; it needs regular tweaking to make sure the leads make contact. An electrical relay would do the job a lot neater.

There are some other solution, like a light sensitive board that activates once the display on the phone lights up but those things are fairly expensive and elaborate - and not as fun :)
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Re: How to build an electromagnet

Post by metiz » Tue Jan 18, 2011 10:57 pm

Open to suggestions...
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Re: How to build an electromagnet

Post by metiz » Wed Jan 19, 2011 1:37 am

a solenoid will work to btw
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Re: How to build an electromagnet

Post by Washout661 » Wed Jan 19, 2011 12:39 pm

Force = ((N x I)^2 x k x A) / (2 x g^2)

N = the number of turns in the solenoid
I = the current, in amperes (A), running through the solenoid
A = the cross-sectional area, in meters-squared, of the solenoidal magnet
g = the distance, in meters, between the magnet and the piece of metal
k = 4 x pi x 10^-7

Once you have that, find a spring of appropriate springyness.

Point to note is that you may need to derive your current based upon supply voltage & resistance per unit length of wire.

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Re: How to build an electromagnet

Post by metiz » Wed Jan 19, 2011 1:24 pm

Thank you for your help. However, suggesting a solenoid would work too I meant that if someone finds a solenoid that works with the forementioned power source, then that would be fine to. I do not know how to make an effective solenoid, particularly with power levels like that. You seem to know your stuff though: could you give me a similar calculation on how I could make an electromagnet (nr of turns, wire thickness etc)?
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Re: How to build an electromagnet

Post by metiz » Sat Jan 22, 2011 1:06 am

Just to make things clear: I do NOT intend to use this as a remote detonator! not everyone's a terrorist in the making. I used the first version to allow a boot-up of a pc with a simple call. The rotation of the vibrator engine connected two wires connected to the motherboards power-on pins. I want a more robust version. If anyone has alternative ideas not mentioned here before, I'm all ears.

*edit* video
http://www.youtube.com/user/RourkeInsan ... v1NzgyuGQc
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Re: How to build an electromagnet

Post by Washout661 » Tue Jan 25, 2011 12:36 pm

You will be severly limited by the voltage / current available. If the max current available is 70mA, and your mac voltage available is 1.75V, you will be limited to 24 ohms resistance in your system. This doesn't give you much to play with when building your circuit. Have you thought about using a loud speaker coil rather than building one from scratch? may be sensitive enough to give you the movement.
Instead of using a solenoid, what about driving an LED from the phone and using it to active a light sensitive switch, or an 'optocoupler' which only appear to cost a quid or so.

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Re: How to build an electromagnet

Post by metiz » Tue Jan 25, 2011 2:15 pm

hmm a loudspeaker coil might be doable....what's an octocoupler? Also, can I transform the 1.3v/ 70ma into something with a bit more voltage, say 12 volts and a bit less amps?
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Re: How to build an electromagnet

Post by Washout661 » Tue Jan 25, 2011 6:55 pm

optocoupler (might also be called an optoisolator) is like a relay, but is solid state. Uses a light source on one side to control current flow on the other side. Seem to have lower power requirements than relays /solenoids but I've never had the opportunity to use one.

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Re: How to build an electromagnet

Post by tufty » Wed Jan 26, 2011 8:02 am

What you probably want is a darlington driver, optocoupler, or even simple level converter to deliver the powerup pulse rather than some Heath-Robinson electromechanical solution.

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Re: How to build an electromagnet

Post by Anders Troberg » Wed Mar 02, 2011 9:55 am

What you need is not a directly driven relay. You need a switch transistor which can control a larger current, and which is just controlled by the small current. Then you can drive a relay off that.

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Re: How to build an electromagnet

Post by metiz » Wed Mar 02, 2011 2:10 pm

Hey Anders,

I've solved the problem (with a lot of help) using a photomos relay. It works great.
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Re: How to build an electromagnet

Post by pulse jet hobbiest » Sun Oct 02, 2011 8:51 pm

I was board one day while I was camping so I dug a tent stake out of the ground (it was about a half inch wide and about a foot long) went to my friends trailer got some copper wire raped it around the tent stake and touched a AA battery to both ends of the wire and got a magnet with a enough power to lift up a spoon useing a AA battery.

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