Rectifier stage for electromagnet (help)

[The Guy]

Greetings!

I´m building an electromagnet for charging guitar pickup magnets and I need  little help... quite a noob here

Alright, I want to use a variac for regulating the wall voltage, I already have the electromagnets built from two pieces of iron of 20cm long 3cm height and 8cm width. with a double coil of 30mts insulated 15awg solid wire each. Thing is I need to build a rectification stage for feeding dc voltage to the electromagnets. I´m not sure how to design it for these diodes http://pdf1.alldatasheet.es/datasheet-pdf/view/38121/SANKEN/RM1Z.html I´m thinking I will need to add a resistor after the bridge rectifier to drop the amperage? Am i right?

This is my first time with diodes and electromagnects connected to wall voltage. I´ll appreciate any pointers.

Thank you! 

[ahbushnell]

How much current do you need and what is the resistance in the coils.  How long do you need to run the magnet?

Is this 120 Vac?

[The Guy]

How much current do you need and what is the resistance in the coils.  How long do you need to run the magnet?

Is this 120 Vac?

Hi there, the resistance of the coil is very little, 0.34 ohm each coil. The diodes have a max current rating of 1.2A and can stand a peak surge of 80A, So I need to keep it under 1.2A. Having the electromagnet turned on for around two minutes will do.

This is 220Vac connected to a variac and then to the electromagnet.

Thanks a bunch.

[ahbushnell]

you need to get a step down transformer for between your variac and diodes.  I gather you want to run at 1 amp.  That will make it safer and more energy efficient.  Also get a ammeter    so you you can monitor your current.  For instance a 220 to 6 V transformer for step down. For rectification use a four diodes in a bridge. Use a 3 ohm 5 watt resistor in series with your magnets.  Be sure to put the 220 side of the transformer in a box that is grounded.  I would put a double pole switch on the primary.   

[Benta]

0.34 ohms and 1.2 A give a voltage of 0.4 V. Not really realistic. I think you need to rethink your coil and really do some calculations of how high a B field you really need.

[The Guy]

you need to get a step down transformer for between your variac and diodes.  I gather you want to run at 1 amp.  That will make it safer and more energy efficient.  Also get a ammeter    so you you can monitor your current.  For instance a 220 to 6 V transformer for step down. For rectification use a four diodes in a bridge. Use a 3 ohm 5 watt resistor in series with your magnets.  Be sure to put the 220 side of the transformer in a box that is grounded.  I would put a double pole switch on the primary.   

Thank you! Question: Why not just use the variac to get the voltage down?

[ahbushnell]

You could but the resolution would be bad. Also it makes it much safer because of the low voltage. 

[The Guy]

0.34 ohms and 1.2 A give a voltage of 0.4 V. Not really realistic. I think you need to rethink your coil and really do some calculations of how high a B field you really need.

Im adding a 3.3 ohm five watt resistor in series with the coils. Waddaya think? I get around 4Volts that way...

[The Guy]

ok, tried it but it was way way too weak... i see two choices, get a whole load of wire and wind a heck of a coil or use a car battery. Any ideas?

[Ian.M]

Good quality copper 15AWG wire is supposed to be 0.01 ohms/m.   Therefore the O.P's coils will have a total resistance of about 0.3 ohms each.  In series that's 0.6 ohms.  O.P.'s quoted 0.34 ohms so either its crappy wire or he cant measure low ohms accurately.

With an iron path length of 400mm, and just about any Iron alloy having a relative permeability greater than 1000, the reluctance of the air gap will certainly dominate if its over 0.4mm and may dominate right down to 0.1mm.  Its therefore probable that the only reluctance that needs to be considered is that of the air gap and the pickup magnet, and the only saturation flux that needs to be considered is that of the magnet which will need to be taken well into saturation to remagnetise it.

The core circumference is 0.22m so with a total of 60m of wire its impossible to get more than 273 turns.  If the coils are scramble wound it could be much less.   Is approx 250 Ampere Turns going to be enough to remagnetise the O.P's pickups at a current the coils can carry without burning out

Its fairly obvious that rectifier losses will dominate, and all that series resistor does is get hot and make you need a larger transformer.  Its only use is if the transformer output voltage isfar greater than required to drive the required current through the bridge rectifier and work coils.

Rather than applying continuous DC which will cook the coils due to I²R losses if you want high flux, it may well be preferable to shunt the coils with a really high current Schottky diode, cathode positive to prevent resonant reversal of the current and pulse discharge a large capacitor bank through them.  You could easily put a peak current of hundreds of amps through the coils that way.   If you use a mechanical switch it would need to either be very heavy duty or to have replaceable contacts due to arcing during the contact bounce period trying to weld them together.  A high current SCR would work if the gate drive is hard enough - you woud want to drive the gate with approx 80% of the max gate current rating.  If the SCR blows (shorts), the peak current was too high so either get a beefier one or decrease the voltage you charge the cap bank to.

[ahbushnell]

This transformer should work.  Put the primaries in series and the scondaries in parallel.

https://www.digikey.com/product-detail/en/triad-magnetics/VPP12-800/237-1054-ND/242498

Data Sheet
http://catalog.triadmagnetics.com/Asset/VPP12-800.pdf

[Ian.M]

That transformer isn't gonig to help.  The O.P's already found 1A through the existing coils is "way way too weak"

12V from a car battery is going to drive about 17A through those coils.   A 40A relay with a 555 timer circuit to pulse it on for a second + a 20A fuse would be a good idea.

If that doesn't do it you'll need to either rewind the coils with much longer and somewhat thinner magnet wire to get far more ampere turns so a reasonable current can be used, or build the capacitor discharge rig I suggested earlier.   Dumping a large cap bank charged to a bit over 70V into the coils should get a peak current of over 100A.   You'd have to do the math with detailed dimensions of the magnet, air gaps and rest of the magnetic circuit to see if that's going to be enough to magnetize a particular magnet of known magnetic properties.

[The Guy]

That transformer isn't gonig to help.  The O.P's already found 1A through the existing coils is "way way too weak"

12V from a car battery is going to drive about 17A through those coils.   A 40A relay with a 555 timer circuit to pulse it on for a second + a 20A fuse would be a good idea.

If that doesn't do it you'll need to either rewind the coils with much longer and somewhat thinner magnet wire to get far more ampere turns so a reasonable current can be used, or build the capacitor discharge rig I suggested earlier.   Dumping a large cap bank charged to a bit over 70V into the coils should get a peak current of over 100A.   You'd have to do the math with detailed dimensions of the magnet, air gaps and rest of the magnetic circuit to see if that's going to be enough to magnetize a particular magnet of known magnetic properties.

Thanks Ian, I really appreciate your suggestions. I will try the car battery and if it does not work I am going to buy a pair of rare earth magnets. That's what they use to charge pickups magnets and they cost like 30usd each. I was trying to build a scrap built electromagnet to save a little money and for the fun of it, but since this seems to be getting out of hand I will take the other route probably

Thanks a bunch fellas. 

[Ian.M]

You do need to keep the current pulse time down below 1 second to avoid the risk of melting the coil wires.   You can try it manually with a car battery by tapping two wires together, but don't hold both wires or you risk a big shock from the electromagnet back-EMF when you open the circuit.  Use heavy duty battery clips or proper clamp on battery terminals and *DON'T* make a spark at the terminal post.  Don't skimp on the fusing.   Put the fuse holder inline right next to the positive battery terminal.

Even 1mm air gap is too much - the poles of the magnet to be magnetised must be in flat close contact with the poles of the eectromagnet.