help - electromagnet relay design

12v relays are cheap as dirt. I see no reason to build one. I bought 50 4pdt relays for 60 cents apiece. try i

The magnet current is set by the applied voltage and the resistance of the coil. The strength of the magnet is proportional to the total amperes circling the nail (amperes through the wire times the number of turns of wire). Using finer gauge wire will raise the resistance, lowering the amperes drawn from a given voltage, but the extra turns will somewhat compensate the force for the lower current per turn.

The proportionality factor between ampere turns and force is related to the core structure of the relay. The force is proportional to the rate of change of total flux per increment of armature movement. So the more the total flux varies between released and pulled in positions of the armature, the more force each ampere turn will apply to this movement. In order to get a large change in total flux as the armature moves, you have to provide a flux path that has only one air gap in it. And that is the one between the armature and the pole that attracts it. If there are other air gaps in series with the magnetic flux as it loops out of one magnetic pole, through the armature and back through the other pole, then the armature movement closing one air gap will not have much total effect on the total reluctance (magnetic field resistance) of the whole flux circuit, so will not cause much of a change in the total amount of flux, so will not produce much force. This is why commercial relays have an iron structure that guides the flux all the way round this magnetic circuit, through the hinge on the armature.

Try a tremendously larger number of turns around the nail (and for that matter, a nail is not an ideal core). You probably have something like 20 turns of wire. Try 2000. You can get magnet wire which is insulated with a thin layer of enamel rather than with a plastic sheath. For that matter, you can buy a relay at Radio Shack.

Heck, the OP can buy an entire alarm at Radio Shack.

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Paul Hovnanian     mailto:Paul@Hovnanian.com
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Keep your gnosis out of my business!

How long is a piece of string ?

Graham

Also - for a given packing fraction (round wire) and a given material - say copper, the power needed to generate the same magnetic field is identical, whether you've got 10 turns, or 10000.

But the volt/amp ratio is what needs changing here... he wants to use more volts (of the voltage he is applying) and fewer amps.