How much maximum voltage can I use?

Hi guys,
I am building a driver for a step motor from the scematic below (CNC mill). The gentleman who created this project says: "use 12v or more" What does more mean exactly? It is a driver for a 2 phase stepper motor rated 3v and 3a/phase. As you can see, the author also has a light bulb in lieu of a resistor and I am using a 18w bulb.. In other words, I need enough current for the 18w and the motor too, which is a nema 23 with quite a lot of torque. I have a power transformer laying around from an old printer that takes 120AC down to 28V at 2.85a... Is this too much... Will I blow my circuit..or my motor? Remember that when the whole driver is done, it will drive 3 of those motors and 3 of those 18w light bulbs.

Thank you very much all.
MAX

You'd have to evaluate all areas of the circuit to know the answer. A quick inspection of the zener/R1 combination suggests that heat dissipation of that combination has a limit. The bulb will limit current to the motor but as you increase the available voltage you'll reach a point where the bulb will fail.

Stepper motors work best when the current in the phases can change quickly. That is usually done by increasing the supply voltage, and adding some current limiting device.

The circuit design assumes that the bulb is rated for the supply voltage, and the bulb current is equal to the required stepper motor current. If more speed is required from the stepper motor, the supply voltage and the bulb voltage can be increased, but at the expense of power consumption.

If you want to run 3 stepper motors from 28 V at 3 amps you will need over 200 W of lightbulbs.

The current can also be controlled with a PWM switching circuit, which would use a lot less power.