chi035 and a unipolar stepper wiring

[Jeroen]

chi035 and unipolar stepper wiring weirdness - not coding

Hi,

I hooked a 5-wire unipolar stepper motor up to the chi035 board for prototyping. It is exactly as the manual (interfacing circuits) describes.
When all seemed to work fine I started measuring voltages and currents in order to build a final version of the setup not using an experiment board. I found the following: pins low causes a 700mA current draw by the stepper since common and all coils are engaged! Instead of the other way round! So if i program it the other way round it works fine. That is: program pins high to set the mosfet outputs low, program pins low to switch them on. Something is completely reversed?!? I connected the common of the stepper to +12V as instructed by the datasheet of both motor and picaxe parts. The chi035 board is modified (resistor removal) to control 12V motors. I've checked and measured polarity of the wiring many times.
Is this normal? It is definitely not what I expected. The funny thing is that it works well both ways (code pins=%10000000 or pins=%0111000) but when not engaged the motor is acting as a heater in the current setup.

 

[BeanieBots]

To set all outputs low, you must use pins=0 (or pins=%00000000).
pins=%10000000 sets output 7 high and turns on one coil
pins=%01110000 sets outputs 6,5&4 high turning on three coils.

 

[Jeroen]

Let me clarify my problem by using your example:
pins=%10000000 sets output 7 low while 6 5 4 become high where I expected 7 to become high and the rest low. With output I mean output of the mosfets that are on the board and driven by the h-bridge IC.
So when you'd expect te motor to stop, pins=%00000000, all coils become engaged!

Should I perhaps connect the common to ground instead of +12V?
The CHI035 datasheet and the motor datasheet are quite clear on this: +12V.
Is there a chance to damage the mosfets if I try this?

Up till now I had only experience driving bipolar steppers direct from the h-bridge. This worked always out so I assume it is a wiring thing.

 

[BeanieBots]

It's working correctly but your understanding is a little wrong.
pins=%00000000 will indeed switch OFF all the coils.

The FETs effectively invert the PICAXE outputs, however, because the coils are wired common to +12v, this also effectively inverts giving a net non-inversion.
The FET can only sink current, so wiring between 0v and the FET would not work.
So, PICAXE pin high, turns ON FET.
When FET is ON, it pulls the coil down to 0v.
The other end of the coil is at 12v so the coil is ON.

If you measure the voltage at the FET output (with respect to ground) when the PICAXE output is high, you should see close to 0v. However, if you measure the voltage across the coil, you will see close to 12v. ie. it's ON.