View Full Version : NEW MEMBER: Hello

25-03-2010, 07:57 PM
Firstly can i say hello to everyone, i have been reading through the wealth information here , i have been attempting to make a
small mill i will only be working with delrin (acetal) on it, i have the frame and rails x,y z all done its only about 300mm x 300mm in size but still have to attatch the stepper
motors yet, this is the hard stage for me now, i have some (3) stepper motors st57sth76-3008b-1 , 8 wire , on the body these are marked 1.8 and 3.0A , i will be using it as unipolar , current 2.8A , resistance 1.0 , inductance 1.6 , holding torque 1.4 ,
i have some power supply's, 2 x 24 v 15A 1 x 36v 9.7A , will these be strong enough or will i be needing more volts ?
my driver board is a Chrystian j unipolar stepper driver model cj-c12 8A per axis max .
i have tried to get the motors going and they are but slowly but stall if i try to make them move faster than 30mm per minute so what do i need to do to make these move at a faster speed
I have x on the 36v 9.7 amp supply with the power straight to the motor by passing the stepper driver board and i have a 50W 8 ohm resistor on each phase as well , the other axis are the same except for the 24v 15A power where do i find out how big the power supply's should be and what size power resistors i need for each phase of each motor

Thank you for any help


Robin Hewitt
25-03-2010, 08:33 PM
i have tried to get the motors going and they are but slowly but stall if i try to make them move faster than 30mm per minute so what do i need to do to make these move at a faster speed

Hi Keister

What would 30mm/minute be in rpm? Seems amazingly slow.
And welcome to the forums :beer:


25-03-2010, 08:43 PM
Thank you for your reply , It is probably near 30 rpm as well LOL , i know the set-up is wrong not enough power and the power resistors are not right either but i was hoping i might find out how it is worked out, ihave been looking but my brain cannot take in all the numbers involved i was wondering is there any where you can type in your motor size and power supply and it gives you the resistor size and the proper power supply needed, i would like to see them working properly before i fit them to the machine,

Robin Hewitt
25-03-2010, 09:57 PM
Why do you need resistors? Does your driver not limit the current? That would be a problem.

I put 220 volts on to 3 Ohm coils, the only reason it doesn't pull 73 Amps is because the drivers limit the current. The massive overvoltage overcomes the coil inductance and the only speed limitation is how brave I feel :heehee:

25-03-2010, 10:40 PM


Ok. I'm assuming you know little about stepper motor characteristics so forgive me if this is a bit basic. A stepper motor has two 'modes' of operation. Below the 'corner speed' - which is governed by a complex equation using the motor inductance and the motor supply voltage - the motor torque is essentially flat. Above the corner speed the torque drops linearly as the revs increases.

With your driver you need a 7.5ohm resistor at 24v (to keep the motor current at 2.8A - 8ohm is fine) and 12ohm at 36v (do not run the 36v supply with an 8ohm resistor, you will overheat the motor and could demagnetise it), but changing the supply voltage (and therefore the resistor) will make no difference to the speed of the motor. The problem here is you are running a non-current-limiting unipolar driver - therefore the supply voltage has no effect, the only voltage the motor sees is that driven by the potential divider effect of the motor resistance and the external resistor - effectively your motor is running at 2.8v and has a corner speed of about 110rpm. I am surprised you are only getting 30mm/min (is that really correct - how did you measure this if you have not got them attached to the mechanics? and what screw pitch are you using?).

The power supplies you have are fine - its the driver that the problem. A unipolar driver is incredibly inefficient - its fine for tiny stepper motors but completely useless for big ones like this except at very low speeds - pointing an astronomical telescope for instance where the short term power dissipation during the few steps a second allows for a simple driver.

Change the driver for a decent bipolar one and run those steppers in bipolar parallel mode and it'll be fine at 24v, the corner speed would be around 1000rpm. You don't say what screws you are using but a 3mm pitch trapezoidal screw will achieve 900mm/min (a reasonable cutting speed on a small 300mm unit) at 300rpm and thats easily within the capabilities of these motors. Even a 1mm pitch threaded rod should give you 500mm/min without too much trouble on a decent driver.

I hope you didn't pay $95 for that driver as shown on the CJ website, thats a real con for its incapabilities compared to whats available elsewhere...

25-03-2010, 10:56 PM
the board is here http://www.chrystianj.com/cj-c12-driver-board-for-unipolar-stepper-motor-store.html , it needs current limit resistors , the size depends on the size if your motors and the power supply

Hello irving2008 im afraid i did pay the $95 and have been thinking of changing it but i do not know enough about them, i had been looking at some on ebay item 390162374628

High performance 3.5A 40V Stepping motor driver

Bipolar ( A+, A-, B+, B-) stepping driver,

Capable to drive up to 40V @ 3.5A Per phase these are alot more expensive but i would save up if they where worth it , i have a 10mm lead screw one turn = 2mm travel
would you mind recommending a board please , and thank you for your detailed reply