Hi, I would like some help figuring out how to build a magnet controller for a 12/24v scrap magnet.

I have read posts from tcmtech and maxheadroom that did have some good pointers but I still need some help.

I am trying to run the magnet from 12v on a small excavator. When just powering it for a short time directly with a boost pack the steel stays stuck to the bottom for a short time then most will drop but there is always some that won’t. So I figured I need a reversible circuit. There is a big spark when disconnecting power, so I need to control the emf as well.

it would be nice if I could control the strength of the magnet as well to sort stuff.

some suggestions on how to go about this would be nice, preferably in language an electronic newbie will be able to understand. Although I do have some experience it is just from tinkering.

 

You probably need to drive the magnet with H-bridge (4 transistors/mosfets or 2 relays). By switching the H-bridge you are able to reverse a current direction.
The Emf is usually clamped with diodes, the mosfets got it internal.

 

So mosfets have the diode built in? Would I still need a flywheel diode in the circuit? And can a mosfet handle the power I will need to run the magnet or do I just use multiples of them?


I was reading up on solid state relays thinking that would be the route to go.

 

To be useful at 12vdc you will require a supply capable of supplying quite a quite a hefty current.
For e.g., scrap yard magnet cranes use 240vdc and even then supply a real heft current, even at that voltage.

 

Do you know how much current the magnet draws?

 

Yes, mosfets have this diode build in.
You don’t need the flyback diode across coil (even not possible).
Todays mosfets can carry >50A at 12V Vcc.

 

ya, I have 3 bigger ones that have different setups, two have a 10-15kw hydraulic driven generator the other is engine driven. They all use 230v rectified to DC. I think my 15kw puts out 80 amps when in use if I remember right. This one will be powered directly from the battery/charging circuit.

From research (mine didn’t come with the controller) I have found that most 12/24v magnets have a 100 amp controller. So if the mosfet can supply 50a then I could double them up to have a big enough supply.

can you give me a part number of one you are talking about? Or a similar one?
Preferably one controlled by 3-5v.

 

So we're talking about an Electro-Magnet. Therefore it's going to have some form of iron core. That core must be able to lose all its magnetic properties when power is disconnected. Otherwise there will be some steel that doesn't want to drop off. I think last time I visited the scrap yard they were using their electro-magnet (or EM for short). I vaguely remember seeing bits of steel continuing to cling to the magnet even after all the other steel has dropped off. It didn't seem to be much of a concern to the yard as they were more concerned with moving bulk materials rather than getting every last bit of scrap.

I also don't think that reversing the polarity will cause anything to fall off from the EM. So I think that idea is out of the question. As for BEMF or Back EMF, at the power levels you're talking about running that's going to have to be ONE HECK of a diode. Something along the line of an industrial welding machine diode, capable of high current. AND the one's I've seen were mounted to a very large aluminum heat sink. These are things I've seen. I'm certain there are plenty of exceptions out there that are no where near what I've seen. But then in the manufacturing environment I've mostly worked with heavy equipment.

 

I also don't think that reversing the polarity will cause anything to fall off from the EM. So I think that idea is out of the question.

That is exactly how they do it in a scrap yard magnet, Just a short reverse-pulse.
I did alot of work on them over the years.

 

That is exactly how they do it in a scrap yard magnet, Just a short reverse-pulse.
I did alot of work on them over the years.

ya, I didn’t realize how important that short burst was until I got this magnet and just used a jumpstart pack to try it.. I had to have a couple hundred pounds hanging from it and some stayed after I shut it off. The magnet has to be clean when you shut it off because when moving around the leftover steel could fall at any time and damage something or hurt someone.

 

So, what is the principal behind reversing the magnetic field for a short time?

Is it based on repelling the little bits due to the residual magnetism in the bits?

 

So, what is the principal behind reversing the magnetic field for a short time?

Is it based on repelling the little bits due to the residual magnetism in the bits?

Basically.


Magnetize junk to attract, then flip the polarity.

 

Like poles repel.
Unlike poles attract.
Learn something new every day.

 

In mid-summer the magnet is prone to getting quite hot, which results in a very large decrease in efficiency, the operators tend to keep a tank of cold water to immerse the magnet in, periodically !

 

That reverse pulse is absolutely required, and the big spark at switch-off is unavoidable. IF a diode were added it would certainly improve the ability to keep the load from dropping. BUT that reverse pulse does need to be quite short. It probably could be provided by a suitable charged capacitor.

 

How would one go about that? Put it from live to negative of the magnet leads?

 

How would one go about that? Put it from live to negative of the magnet leads?

No, It would, in the simplest form, take a double-pole relay to charge a non-polarized capacitor from the magnet supply, and then, when the main contactor releases, connect the reversed polarity cap across the magnet coil to give it the short blast of reversed polarity from the stored charge. The timing will be sort of important, so as to avoid explosions.

 

The wiring shows the use of discharge resistors to quash the induced charge.
Also a 12v-24v version shown

 

The circuit is simple to trace but lacking descriptions of the device functions I am left to guess just how it works. And providing resistors to absorb the power from the collapsing magnetic field does not seem like the fastest method to use. Where is the detailed functional description, MAX???