View Full Version : Help Please!!!! Unipolar motor drive question

06-04-2012, 11:20 AM
Hi There,

I have three unipolar 8 wire motors - rated 3V/3A - my question is this.....
Will the following transformer drive three motors - from a discete unipolar drive?

PSU ENC I=220VAC O=5 @ 26A
INPUT: 88-264VAC OUTPUT: 5VDC @ 26A 130W

To my mind the motors should work connected to the PSU and driven by unipolar drives - but I will need 0.66ohm - 6Watt current limiting resistors - in front of each motor.

A friend of mine - that has built a couple of routers - using Gecko Bipolar drives - recogns that the voltage of the transformer is way too low - Could it be because he uses the Gecko's to power the bipolar motors - and that the motors are not connected directly to the PSU - to draw the rated motor voltage and current?

I guess the bottom line is - will the PSU work for my application?

Thanks in Advance for your time in reading this post :)

Kind Regards.


06-04-2012, 11:39 AM
I guess the bottom line is - will the PSU work for my application?

What is your application?
They will probably only run at a few rpm at 5V. If you want a few hundred rpm or more, you'll need to run them at 24V minimum.

06-04-2012, 11:43 AM
Hi Gerry,

Thanks for getting back to me so promtly :)

I am building a cnc router.

Why will they run slow - if they get the rated current - is it safe to supply more voltage that is specified by the supplier?

Kind Regards.


06-04-2012, 06:59 PM
There is a long way to go from three motors and a power supply to a CNC router.

-The 3V are not rated voltage but used for reference to calculate the resistor.
-Typical voltage is 24 Volts, but you can go as high as the motor driver allows.
-The current limiting resistors are part of the motor driver.
-The Gecko driver, your friend is using would be a good choice.
-Which driver did you had in mind to run the motors?
-The 8 motor wires can be connected unipolar or bi-polar, serial or parallel.
-150 Watts for the power supply will likely not do it. Calculate roughly with 3A per phase, that is 6A per motor X 3 motors X 24V = 432 W

06-05-2012, 10:56 AM
Hi Guys,

Thanks to Gerry and Stewi - I'm ignorant but I'm learning everyday.

I have managed to swop the 5vdc PSU for a 24Vdc/6.5A, and purchased another 24Vdc /14.6A PSU.

I'm planning to use Tachus discrete unipolar driver boards - two off on the 14.6A PSU and one on the 6.5A PSU. The motors have 8 leads - of which I will connect the coils to form a six wire unipolar setup.

The motors are 3A/6A - 3V. I'm planning to use 35Watt/1ohm current limiting resistors infont of each motor - will this be sufficient?

Thanks again for your help in this regard.

Kind Regards.


06-05-2012, 11:20 AM
The current limiting resistors are already on the driver board.
The circuit measures the voltage drop over this resistor and limits the current.

06-05-2012, 11:29 AM
Hi Stevi,

Is that on the new board with the 4081 IC onboard? - As I have built the drives previously on the older plans - and the motors got extremely hot?

Thanks for your help!!!

Kind Regards.


06-05-2012, 11:53 AM
The schematics show a 0.33 Ohm resistor for 3.5 Amp.
Else, I have no experience with this driver board.
You can use only one power supply.