Stepper Driving Problem

[amiller112]

Hi, I have recently purchased a picaxe 18 starter kit, which has the UNL2803A driver chip between the PIC outputs 0->7, I can easily set the pic up to output to these pins, however the problem arises when I attatch a 4-wire bipolar stepper motor to the outputs of this chip.
I get absolutely no response from the stepper motor, though attatching a battery directly to the wires does have an effect.

I have tried individual wires and set the output to be high and low on either side of this, but nothing, I even attached an L293D to the outputs of the chip and still got nothing, I can see a pulsed signal going into the L293d chip, but the outputs are constantly high.

The wires on the stepper motor are: Red, Grey, brown and green

I appreciate any help in this matter, as I find the picaxe manual to be mostly useless

 

[MBrej]

Hey there, you cannot use the 2803 driver for bipolar stepper motors, only unipolar ones. The 2803 is a transistor driver, and the outputs sink current to ground, and cannot source it from Vcc. Bipolar steppers require the current to be reversed thorugh the pairs of coils in the correct order, and to do this you need to source and sink current. The L293D is a dual H-bridge driver, so can source and sink current, and so can drive a bipolar stepper. I believe there is one of these on the 28 starter board..? but the 293 is easily available else where.

Matt

 

[amiller112]

Thanks for that, is it the UNL2803 that is the problem then, would connecting the outputs from this affect the signal reaching the l293d enough to stop it working correctly?

The only way I can think of bypassing the chip is to remove it and wire link through from the input to the output directly, and have the L293D wired up as I already have on a breadboard.

The board I am using is the CHI-030 board 'Pic interface A V2', if it is just the UNL2803 chip that is the problem, I should be fine in bypassing it.

Thanks for the reply

 

[hippy]

If you're driving the L293D through the 2803 you will have to pull those lines up to +V or they will simply float / be pulled to 0V by the darlingtons.

Alternatively, as you suggest, simply wire the L293D to the PICAXE output directly thus bypassing the 2803 entirely. You may likely find that you do not even have to remove the 2803 from the circuit, allowing other control lines not driving the L293D to use the 2803 if required.

 

[amiller112]

Makes sense now, I was using an LED to check for a logic change, but only to +5V so each time it pulled to 0V it would light, time to get another multimeter I think.

Thanks for the helpful replys