Controlling a stepper motor (particular application)

[Tails]

So, I am currently working on a project which is an arc lamp. It uses two carbon rods that eventually burn away with use, so I need to gap between the rods to be adjusted automatically, by moving the upper rod. After trying almost a dozen of designs, I chose to use a slide mechanism moved by a stepper motor.

Of course the stepper motor needs a driver. That driver must be able to work by itself. The driver will use two sets of solenoids and normally open switches to control the stepper motor. One set is wired in series with the arc (to draw rods apart) and the other in parallel (to bring them closer)

I tried to find a circuit suitable for that particular application, but I didn't find anything. Chances are almost nil I don't have a proper IC (like the 555), so I'd prefer a transistor based circuit. The speed sent to the stepper motor is not extremely important. If the mechanism moves at the speed as a scanner, that's perfect!

So I'm wondering if there's a circuit suitable for the application I intend to use it in?

Thanks.

 

[MikeMl]

Frankly, a DC gear-motor is much better suited for this. I remember working on big movie projectors which had a carbon arc. They used a servo-loop with a current sensor, which varied the speed of the gearmotor to keep the current through the arc constant by driving the motor at a rate which just keeps the gap at the right size. If the gap gets too wide, the current drops, the motor speeds up, making the gap smaller again.

 

[Tails]

The idea of using a standard DC motor came into my mind, but if I need to draw the rods apart? Most DC motors will only spin in one direction (if I'm right), or at least make them spin in both directions would be too much work.

But I sure would make the circuit much simpler, with just a 120V-12V transformer wired in parallel with the arc, and the 12V secondary wired to the motor.

 

[Dr_Doggy]

why not use a current pickup coil around the gap, like a clamp am , and run it to an op amp or something?
then it can wheel in on 0Amps or wheel out when you get over current.