Confused about power requirements

Hi guys Im making a custom CNC mill and have just recently bought some motors and im a little confused about what kind of power requirements im going to need im really electronics savvy but im just used to working with low voltage and amperage.

Here are the motors I have [www.allelectronics.com]

the specs that i have found:
1.8 Degree step
1.8 Volt
4.2 Amp
0.42 Ohm
127 Oz/in holding tourqe

So now im looking for a DIY/Opensource Controller that will fit my needs... or something i can modify. Im New at stepper motors and dont quite understand them fully yet. So please be gentile

Those steppers are pretty much outside any experience that I have. Maybe Nophead can help you or failing that you might want to post this inquiry over at CNCZone as well.

our stepper motor driver boards are rated for a max of 2A, so those motors you have will be sucking a lot more current than we can provide.

a hack to get around it is to use a resistor to bring that current down. a cheap, low-impedence resistor that people often use is a small lightbulb, such as an automotive light.

Ya I made a post on CNCzone but no one replied yet.

I dont really want to do any hacks i wanna just build a controller that meets my needs.

Im confused about stepper motors and how they use voltage and amps i was reading that your supposed to you like 20 times the voltage or somthing like that and its confusing me, Is 4.2 amps the max that these will pull per motor?

For those low voltage high current motors you need a chopper drive. The higher the voltage is the greater the maximum speed will be. The average current will be reduced by the voltage ratio.

For example, if you use a 12V supply and a chopper drive which gives 4.2A, the theaverage current taken from the supply will be 4.2A * 1.8 / 12 = 0.63A per coil.

The 4.2A rating is the maximum current that you should put through each coil. They would take that current if you put 1.8V across them. If you use a chopper drive, with, for example 12V, it will apply 12V and the current will start to rise from zero due to the inductance of the coil. When it reaches the limit set by the chopper it then switches off and the current starts to fall again. To keep the current at 4.2A with a 12V supply it would be on for 1.8/12, i.e. about 1/6 of the time. That is why the average current is lower.

If you don't need the full torque you can use a lower current and the motor will run cooler. If you halve the current you will halve the torque and reduce the power, hence temperature rise, by a factor of 4.

If you don't need the full torque you could use Zach's chopper drives, and despite what he just said, you should not need series resistors, you just shouldn't set the current higher than 2A.

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[www.hydraraptor.blogspot.com]

What are my options if i want to use full torque ?

There any schematics or projects you could point me to that would work for my application?

Thank you for the help

I used the a power supply calculator here [www.cnczone.com]
is that only for Gecko Drivers?

It says that i should use 39.6V (22 times the voltage) at 2.8 amps does this sound correct?
with that setup i could use any bipolar stepper controller that can do ~40V at ~3A right?

The calculator makes no sense at all to me.

If you run from 40V the current taken from the power supply per motor will be 2 * 4.2 * 1.8 / 40 = 0.38A but you still need drivers capable of providing 4.2A per coil.

BTW, if I follow you link to the motor and look up its part number I find a 3V motor, not 1.8V.

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[www.hydraraptor.blogspot.com]

I never found a data sheet when i look up the part number on google i only saw a few post mentioning it with the specs i posted before.

Did you find the datasheet?

do you happen to know of any DIY 3 axis controllers that would work for me?

Following your link I got the part number from allelectronics: STP-58D4002-03

Googling that I got this: [www.mpja.com]

Same part number but shows 3V 0.42R and twice the price! 3V would give over 7A which does seem a lot for that size of motor.

Sorry I don't know much about the DIY CNC world so I can't recommend a controller.

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[www.hydraraptor.blogspot.com]