# limiting current to an electromagnet

Hi-

I’ve got a project where I am switching the polarity of an electromagnet through an h-bridge, controlled from Arduino. With my bench variable power supply I have determined that I need 9-12V @ 1A to drive the load but the coil will draw more (I wound the coil myself so there are no specs). Using an online inductance calculator it looks like its inductance is 10.9 microhenries.

Photo and small Flash video here: flutter

It’s working (yay!) but I need to drive a bunch (64) of these and am wondering about what sort of power supply I need. They won’t all be on at the same time but many will.

I have a couple of 13V, 20-40A power supplies that I’ve used for other projects- but those will fry my coils (mmm…smoke

My question: my bench supply limits the current - what do I need to add to my circuit to to keep the current <= 1A. I can’t just slap a massive resistor in the circuit (can I??).

Any tips/nudges much appreciated!

–Roy

Ahh, en electro mechanic pixel :-)

You cant get resistors that can handle large currents, but they are not cheap.

The one's i have seen, look like little boxes made of some ceramic material like this:

http://www.electronixexpress.com/cmp_3255.htm

You would have to use Ohms law to find the value need to limit the current to 1A

Edit: You should also be prepared to get rid of a lot of heat :-)

Can you rewind the coils using finer wire so they end up with 12 ohms resistance? That way you'll automatically limit the steady state current to 1 amp. This could be cheaper than 64 power resistors.

You can't calculate the rating of your power resistor untill you know what the coil's resistance is.

I don't believe your inductance is ten microhenries. A coil with 10 or 20 turns would be more than 10 microhenries. How did you calculate this?

Can you rewind the coils using finer wire so they end up with 12 ohms resistance? That way you'll automatically limit the steady state current to 1 amp. This could be cheaper than 64 power resistors.

You can't calculate the rating of your power resistor untill you know what the coil's resistance is.

I don't believe your inductance is ten microhenries. A coil with 10 or 20 turns would be more than 10 microhenries. How did you calculate this?

I used this online inductance calculator: http://www.66pacific.com/calculators/coil_calc.aspx

Turns: @ 170 Dia: .3" Length: .75 Depth: 4 (multilayer, multirow)

It came up with 10.9 microhenries

I'm using 28 gauge wire - that was the finest gauge that my local electronics store had (yes! a real brick and mortar store that isn't Radio Shack). I guess I can do a test to try to bulk up the coils (of course I already wound 64 of the little suckers ;-)

I'll keep tinkering

--Roy

Greetings Roy,

These http://www.parts-express.com/pe/showdetl.cfm?partnumber=004-8 8ohm non-inductive dummy load resistors used for audio volume control circuits aren't too expensive at \$0.98 in 4+ quantity and should work fine unless you are keeping the electro magnets on for 100% duty cycle.

The advantage of limiting current with a resistor instead of lots more coils on your electromagnet is a lower LR which causes a higher-peak current and helps your magnet pull harder for the first few milliseconds and then drop to a lower holding strength.

Regards, David

Greetings Roy,

The flash video looks like the electro-magnet is wound around a metal bolt --- if this is so the inductance is much higher than your calculation for an air core.

[EDIT] BTW, this kind of application only needs a very small holding current, so your duty cycle is probably extremely small. I would suggest 2 H-bridges for each magnet: one to switch at 1A, and one to hold at 10 to 100 milli-amps, so so. There might even be enough residual magnetism in the bolt to allow you to switch off current and maintain position unless there's a strong breeze. [/EDIT]

Regards, David

Greetings Roy,

The flash video looks like the electro-magnet is wound around a metal bolt — if this is so the inductance is much higher than your calculation for an air core.

[EDIT] BTW, this kind of application only needs a very small holding current, so your duty cycle is probably extremely small. I would suggest 2 H-bridges for each magnet: one to switch at 1A, and one to hold at 10 to 100 milli-amps, so so. There might even be enough residual magnetism in the bolt to allow you to switch off current and maintain position unless there’s a strong breeze. [/EDIT]

Regards,
David

Thanks David (and all),

Air core - doh! I didn’t track the part @ that calculator assuming an air core. Thanks for tipping me to that.

I’ve been building this through trial, error and a little learning. Somewhat more of the former than the latter, alas!

The bolt is helpful in a couple of ways: it bumps up the magnetic charge, retains some of it and also provides an attractor for the magnet on the rotating shaft- so the dot doesn’t have to be powered to hold its position. Also it provides a way to attach & secure the coil. The coil is small but there is still a kick when it is initially powered.

So… stumbling up the learning curve here… my options seem to be:

a. get some of the big resistors
b. try winding with a heavier gauge wire with enough turns to provide sufficient resistance
c. mmm…

Oh - @ duty cycles: I’ll have to see what the limits are/how hot things get and then throttle back. I have a bunch of ideas about the programming aspects but will have to tailor them to my practical “refresh rate” (duty cycle and how fast the dots can flip) once I get the power issues sorted out.

–Roy

Hi Roy,

We're all learning at some point or another. ;D

Depending on your goal, it might be possible to drive all these coils with a matrix approach using 8 half-bridges on one side and 8 more half-bridges on the other side. This would only require 16 high-powered bridges and 8 high-power resistors, but it would limit your animation to 1 row of 8 at a time --- but if they were flipped really fast it might be good enough.

Regards, David