Hi
I would like to build an economical circuit to convert 12v DC into 12v AC, to power an AC electromagnet (the application requires an alternating magnetic flux).

The waveform must be pure sine wave, oscillating above and below 0v.
The power output must be 250W.
The input/output voltage doesn't necessarily have to be 12v; it can be higher if need be.

I can't afford to buy an off-the-shelf inverter unfortunately. Any circuit diagrams or advice will be appreciated.

Thanks
Richard

 

How about taking a 120V/240V pure sine wave inverter and using a step down transformer?

The time and effort you spend making an inverter is likely to offset the cost of a professionally engineered one. (That's pretty much the policy of AAC when it comes to inverters.)

 

Sorry, I forgot to add something in my original post: the power supply must be a battery, as mains power will not be available in the location. Otherwise your suggestion would have worked fine.

Surely a simple pure sine wave generator could be feasible?

Thanks for the reply

 

Sorry, I forgot to add something in my original post: the power supply must be a battery, as mains power will not be available in the location. Otherwise your suggestion would have worked fine.

Surely a simple pure sine wave generator could be feasible?

Thanks for the reply

How much current do you need?

You mentioned 250W. At 12V that is 20 amps.

If it were only a few hundred milliamps, you could probably get away with a simple PWM driver and a H-bridge.

But since it isn't, it's going to be difficult. You'd use the same topology, but much larger H-bridges would be needed, and designing the output filter to handle 20A is going to be next to impossible.

Using a 120V/240V inverter connected to a 12V battery and a step-down transformer would probably do the job.

Are you sure you need AC for the electromagnet?

 

What is the application? If it's only the alternation of the field, an H bridge drive should work.

 

Sorry, I forgot to add something in my original post: the power supply must be a battery, as mains power will not be available in the location. Otherwise your suggestion would have worked fine.

Surely a simple pure sine wave generator could be feasible?

Thanks for the reply

120V/240V inverters do work on batteries. Thats the idea of them. You hook them up to a 12V car battery and they output 120V or 240V AC. What tom has suggested is you then take that 120V or 240V and put it through a step down transformer to obtain your 12VAC. If you want 250W at 12V it will have to be one big transformer.

 

The transformer will be a normal, but large-ish one. Current draw for this battery powered inverter will be around 23 amps. 120VAC out at 2 amps. is less than 300 Watts so the inverter would be CHEAP. A unit designed for mounting in a car would work great. Since you are going to step it down with a transformer(think big filter) then a quasi sine inverter will do. One that uses multiple steps in a square wave fashion. The large induction and magnetic coupling will filter most of the steps out of the wave form. An electromagnet doesn't have much of an issue with noisy or malformed current waveforms...so PURE sinewave is a red herring and not a need for a master magnet.

another option is to just bite the bullet on this one and drive the master magnet with square waves, which you could create with some high power triacs driven from a boosted/buffered 555 timer signal. The magnet wouldn't know the difference and the loss of efficiency would be small.

Otherwise, the answer is NO. There is no ECONOMICAL way to do what you ask.

 

Check out some specs for microwave(google microwave oven schematics) They have huge transformers and can be got CHEAP at pawn shops and the like

 

Check out some specs for microwave(google microwave oven schematics) They have huge transformers and can be got CHEAP at pawn shops and the like

They also put out around 2 KV at a half an amp.

 

The application is for testing for cracks in weld. So you reckon I could drive the magnet with a square-wave without too much loss in efficiency?

 

Yes, it shouldn't make much difference to the coils. The main object of the master magnet is to prevent objects from becoming magnetic themselves and it also allows one to pick up objects that aren't iron based, like aluminum and copper scrap.

Junk yard magnet cranes often use these so they can pick up most anything in the junkyard, even stuff without any iron in it, like aluminum scrap, and copper wire.


http://www.rexresearch.com/mrmagnet/mrmagnet.htm has a good description of building one... Dangerous electrical voltages are present if AC mains voltage is used


Kermit

 

You might look at a VNH2SP30-E "Automotive Fully Integrated H-Bridge Motor Driver"

Datasheet: http://www.st.com/stonline/books/pdf/docs/10832.pdf

If you drove that IC with a uC (microcontroller), you could actually synthesize a reasonably good sine wave output. The learning curve for programming the uC would be a bit steep, but you could wind up with a pretty good solution that way.