[LowIQGenius]

Hey there potential helpers! Like the title says, I'm working from scratch and building/programming my own CNC platform that won't work with the driver boards everyone else uses, and I plan on driving the motors with several MOSFETs I have sitting around. I've just purchased two Probotix stepper motors (200oz) which are rated for 24V and 1.5A. I also have their 24V, 6.5A power supply. Phase resistance for the motors only turns out to be 4.5ohm. My question is: is there any way I can wire the motors to the power supply without having to use a pricey 40W 16 ohm resistor? What's the power dissipation for these motors? I thought that most manufacturers design their motors to work without extra resistors...

Thanks!
LowIQGenius (the world's most stupid genius!)

[jschmitt]

You don't need the ballast resistors if you aren't going above 24v since your motors are rated at 24v, but you won't get high rpms. The current formula for volt optimum voltage is (SQ RT of Inductance) x 32. The old formula was 10 to 20 times the rated voltage.

Use a chopper like the one midway down the page here: http://pminmo.com/3axis/3axis.htm if you already have the driver, or design a current sensing circuit with your mosfets. There are many bipolar chopper circuits to learn from here on the zone. Why reinvent the wheel.

[harryn]

I think you can also use the linistepper driver. I have a set of those and they seem fine. (for unipolar mode) They are only about $ 30 / each and you just solder them up and go - pretty easy and the errors are already worked out.

[LowIQGenius]

Hey all, thanks for the help so far. The thing is, I've already developed a program that loads CAD images off of a memory card. The whole thing is gonna be driven directly from my Propeller microcontroller (no computer needed), so I need an alternative driver to the commercially available ones. The concern I have is the amount of current that will pass through the stepper, since it's rated for 1.5A and at 24V will see about 5A.

Thanks again!

[jschmitt]

The concern I have is the amount of current that will pass through the stepper, since it's rated for 1.5A and at 24V will see about 5A.

The motor will only use 1.5A @ 24v. If you raise the voltage then you willl raise the amps, like how a light bulb draws only what it needs at the correct voltage. Ohms law!

[LowIQGenius]

Then am I mistaking the whole resistance/phase thing? I wasn't sure if they were expecting me to fix the circuit to get the right current for the motor...

[jschmitt]

Then am I mistaking the whole resistance/phase thing? I wasn't sure if they were expecting me to fix the circuit to get the right current for the motor...

Voltage is force. Think of a water pipe and what happens to the amount of water you would get out of the pipe when you raise the pressure. Voltage is pressure.

[pminmo]

I don't believe the motor coil coltage is 24V, I suspect your looking at test data that was run at 24V. At 4.5 ohms and 1.5A that would put the coil voltage at 6.75V. So you need to limit the current either via a linear method or switching method (pwm or chopping). If you use a linear method it's going to dissipate around 50W per motor, which is feasable, but.....