Current limiting resistor

[pyrojon]

Hi,

I'm in the beginning stages of setting up a CNC router. I'm starting with some simple electronics then moving on to mechanicals and if I get that far I will probably go back and upgrade the electronics.

I have three Minebea steppers (2.3V, 2.3A, 1.8deg) and would like to drive them with the 179 unipolar stepper motor driver. It's a simple full step driver that has internal oscillator, or external control options.

I would like to drive the motors with a higher voltage (12V) but I am unsure how to calculate the current limiting resistor required. I have been looking through the forums but havn't yet been able to find the answer, I am sure it's simple.

Can anyone point me to the proper resource, or do some simple math for me?
It's an 8 wire unipolar motor and I have measured the coil resistances at 1.4 ohms for each coil.


Thanks alot.

[smarbaga]

hello
e/i=r
2.3/2.3=1 ohm
voltage drop =
i x r
2.3 x 1 = 2.3v
12v-2.3v = 9.7v
9.7 volts at 2.3 amps
through your limit resistor
r=e/i
9.7 x 2.9 = 22.3
i would start with a 24 or 25 ohm resistor
and put an amp meter on the power supply
watts = e x i
2.3 x 2.3 = 5.29
so i would start with a 10 watt resistor
just a starting point here
good luck to ya
smarbaga

[scubasteve2365]

smarbaga, your calculations were a little wrong .....

he would have a 9.7V drop across the limiting resistor with 2.3Amps running through ....

the resistance needed would be V/I .... not V*I

therefor 9.7V/2.3A = 4.2ohms resistance

Since you will be having a 9.7V drop accross the resistor and 2.3 Amps running through it then the power would be:

2.3A * 9.7V for power would be 22.3 Watts

4 ohms, and 23 watts ....... I think that may be a difficult resistor to source!!!

If you have to pick the resistor yourself, I would snag a 3V power supply, that way you can use a lower wattage resistor, or you may be able to slightly drive a higher voltage to the motor and get by without one at all ...

[H500]

My suggestion would be to forget about the circuit and go with a chopper drive. The resistors are expensive and even with a 12v supply, the speed will be much slower than what you could do turning it by hand

[smarbaga]

i use the zetex zxct1030 current monitor as a current regulator it is adjustable and i use the outpit on the gate of a hexfet (ex. irfz44) accurate and cheap , under 4 bucks

[pyrojon]

scubasteve2365, thanks for the suggestion, coincidentally that is actually what I decided to do. After coming up with the same values for resistor and figuring that it would be pretty hard to find.

I measured the resistance of the coils and found them to be 1.4 ohm, rather then the calculated 1 ohm. I picked up a PC power supply and figure I can use the 3.3v output (14A max) and the slightly higher current (50mA) shouldn't be a problem.

H500, why will it be slow? I'm sure that you have alot more experiance then me when it comes to CNC but it seems to me that it's only going to be as slow as the clock fed to the driver circuit as it doesn't take a stepper long to move 1.8 degrees. Am I missing something?

I do plan to move to a chopper drive if I bring this project to the point where I'm actually able to cut something. You see I (like most I am sure) have a problem with finishing complex projects that I start and I figure "if it's not going to get done I better make it as cheap as possible."


Thanks
Jon

[smarbaga]

lol ..... never heard that theory before ( as cheap as possible )
now i got that in my head i may just apply it. ( badddd very baaaadddd)

[mannster]

I wanted to say that H500 is right about this type of drive being slow, I have one and it will work fine at slow speeds but if I try to use high slew rates, I experience reduced torque, and the motors do start missing steps. The only thing that helps is raising the voltage, but that increases the current which is not good for your electronics and motors. A chopper is a very good idea, and just requires a little more work (or money). Try looking for controller chip that just needs a few discrete parts to build a simple chopper drive.

In fact, I started a thread regarding this topic Chip Thread

Give it a shot !

[H500]

Because of inductance, the current to your motor would not be able get anywhere near 2.3 amps when stepping at the speed you need.