Electromagnet Switch Timer

A repeat-cycle timer with SPDT contacts can also do this. They are economically available in octal socket form.

ATTENTION: The magnetic force is proportional to the current but independent of the direction it has in the solenoid. If you change the direction you still get same attractive force.

Unless you have two opposed DC solenoids (or a solenoid and a permanent magnet) with current in one solenoid being reversed.

Please make a sketch how a reversed curent changes the magnetic force in a solenoid.

May I mention that no PM was in the question so that I only gave an answer related to the solenoid magnetic field problem.

thank you

Google something like "hand rule for magnetic polarity in a coil" for an answer to that question.

If one coil was connected to maintain the same polarity at all times but the other was to reverse for alternate operations, then attraction and repulsion would occur.

As to the original question, depending on the size of the device and hence the current required, Mosfets, IGBTs or even normal transistors could be used with the correct circuitry. As has also been said, it could be done with an electro mechanical switch.

Personally, I would be looking for a simpler approach using some form of spring return solenoid actuated unit that didn't require the reversal, and then a simple pulse circuit to turn it on or off.

Right hand solenoid rule:

I do not contest this aspect but please look at how an ELECTROMAGNET for industrial usage is build.

Please note that I used the term "solenoid" rather than "electromagnet".

Please note that the question concerned :

"Im trying to design a clamp on a hinge that will clamp shut with two (or one) electromagnets attracting for a set interval of time"

and not free bobbins.

It doesn't change the magnitude of the force but it does change the direction. Imagine two identically wound solenoids with the same number of turns wound in the same direction, with the start wire marked plus and the end wire marked minus. If both start wires are wired to the positive supply then both have the same magnetic field direction, if the connections are reversed on one of them then that one has the direction of its field reversed.

As far as the plunger is concerned, it makes no difference because it is usually made from "soft" iron that does not retain any magnetic field once the source is removed, it only reacts to the magnitude, not the direction of the magnetic field from the coil.

In general an electromagnet applies a force on a soft magnetic piece as in above sketch.The magnetic force is proportional to B^2 thus independent of curent flowing direction. Which ever the connected polarity to the coil will be the attraction force will always be an attractive force and only depending on curent value.The answers given are very far from the way a magnetic actuator is build.The problem is NOT the physics of magnetism but how it is used.A positive aspect of our community would be not to consider that the other is stupid but to try to understand why he wrote something which apparently does not correspond to own opinion.The inverted curent proposal could have been correct ONLY if the armature (clamp) would have been equipped with a PM but the OP considered 2 electromagnets and I very much doubt that at least 1 coil was assumed on the moving part.

I was once confronted with same problem. It was necessary to damp as fast as possible oscillations of a scale. Although I mentioned the fact that force is independent of curent polarity and the best would be to have two actuators in opposition the chief of the project who was an electronic engineer (very gifted by the way) neglected my counsel (I was an external consultant) and the project did not work as expected.

I re-read the posts leading to this one and nowhere could I find the word "stupid" there, or anywhere in this entire thread. This is an engineering forum, not a political science forum, our egos should play no role in determining the "rightness" or "wrongness" of anyone's posting. In engineering answers, only engineering facts should matter.

555 timer and a power transistor or Mosfet.

What an amazing coincidence--a verbatim repetition of post 3!

It's deja vu, all over again.

Can anyone say and mean "ditto" only once.

Is there an echo in here or is that just my ears ringing in ASCII.

Instantly reversing the current of a DC solenoid is going to give one hell of a reverse current.

I think you mean an immense reverse voltage.