Stepper pulling too much current

I've recently bought a stepper from Sparkfun

http://www.sparkfun.com/datasheets/Robotics/SM-42BYG011-25.pdf

I am attempting to do the MotorKnob tutorial http://www.arduino.cc/en/Tutorial/MotorKnob

The stepper is a bipolar, and I hooked everything up correctly (checked multiple times) and copied the code correctly.

The motor follows the motion of the pot fine, but it vibrates when the pot is not being turned. Also, I have an external power supply (24v). I run that through a 7512 voltage regulator that powers my Arduino as well as the stepper. However, I noticed after about 10 seconds of running, the 7512 is very hot.

I checked the current through the motor wires, and the coils were pulling like 3 amps!! I have no idea why. The datasheet states a coil resistance of about 34 ohms, and with 12 volts I should be right around the rated current of 330 mA.

I even tried a few different resistors on the coil wires (up to 220 ohm!) and it seemed to have very little (if any) effect on the current draw.

What is going on? This is the first stepper I've played with, so hopefully I've missed something simple.

If everything is running correctly, I should be well under the rated current for the 7512, which I believe is around 1 amp.

Thanks in advance for the help.

That does sound wrong - what driver chip are you using? Worth double checking - if you can measure the voltage at the output of the regulator to verify its 12v. Even if the circuit was working normally the motor windings would take 0.66A and this would dissipate 8W in the regulator if the input voltage is 24V - without a heatsink it will rapidly get too hot and hopefully auto-shutdown.

3A is too much - have you measured the winding resistance of the motor to verify its as expected?

My driver chip is a SN754410ne H-Bridge as per the "motor knob" tutorial.

I did measure the voltage out of the regulator - 12.1 volts.

3A is too much - have you measured the winding resistance of the motor to verify its as expected?

Yes, the coils showed 34 ohms each perfect, which didn't make any sense to me how the multimeter was showing the coil was pulling 1.2 amps. I = V/R = 12/34 = 0.353 amps. However, the speed and intensity of the heat the 7512 was putting off made sense.

I noticed the datasheet indicated 46 Mh +/- 20%. Could the inductance of the motor be causing this? I tinkered with this for about 4 hrs...it's got me stumped.

The inductance will further limit the current at high speeds, not increase it - I'm wondering if the multimeter is misbhaving with the AC waveform it is seeing - although the regulator might have been damaged (they are usually supposed to shutdown on over-temperature)?

Have you measured the current through the motor windings individually? Could there be a short circuit somewhere?

the regulator might have been damaged

That's what I thought. Put in another...no change. Put in a different H-bridge...no change.

Have you measured the current through the motor windings individually?

Yes, 1.2 amps per coil.

Could there be a short circuit somewhere?

My first instinct. Checked every connection. Everything seems to be correct.

I would like to think that there could be a problem with the stepper itself, but it just makes no sense. The resistance across the coils measures correctly when de-energized.

I'm going to tear the entire circuit down and rebuild from scratch. Maybe I'll catch something I'm missing. I'm also going to check the datasheet on the H-bridge and see what's exactly going on during the process. Thanks so much for the help.

I did measure the voltage out of the regulator - 12.1 volts.

But did you measure it under load or just before things were connected.

My guess is that is wasn't under load and the 3A you are measuring is due to a short, maybe as a result of miss wiring the H-bridge.

I checked all connections and verified w/ both the example and the datasheet.

I'm going to tear down the circuit and start from scratch. Everything I'm seeing is indicative of a short, but I could never find one.

Thanks everyone for the help.

Just a left-field suggestion - try a different multimeter?

Could be the multimeter but a hot finger is still a hot finger