Electronic – How to drive a NEMA 17 stepper with a raspberry pi

From the software page I can follow what the python is doing but I don't see where it says that pin 18 on the raspberry GPIO should be enabled as PWM? The docs say that Pin 12 supports PWM ? How does this work?

Not knowing the specifics of the RPI GPIO Library that Adafruit uses, but looking at the page you link to, it enables it with set("mode", "pwm"). As it says, The Python program first sets-up the two GPIO pins to be outputs. It then defines the same convenience function (“set”) that we used in Lesson 8, to write to the PWM Kernel Module. This is then used to set the parameters for PWM. The RPI hardware PWM is enabled through that library.

What if I didn't care about speed? Can I just use an regular GPIO pin for output a high on the enable of the L293D? With the Pi I'm assume that would out a 3.3v? And if I'm looking at the datasheet correct the enable pin can take upto 7v as a high? (What speed would it run on?)

Yes, you can do that. It would run at 100%, because you simply toggled the pin on. The voltage at the pin is not what sets the speed, but simply how fast you turn the pin off or on in a given time period that does. Like flipping a light switch on and off fast enough that it seems like the lights are only half on.

Also again, if I didn't care about speed control, I could hook another GPIO to the second enable and control another motor also right. And I can then control each of they directions?

Yes, but you would actual need three gpio to control both directions of one motor. 1 for the motor enable (on/off), and 2 for the motor 2 direction. Exactly as it is now, except without pwm.

The L293D cannot sustain the 1.5 Amperes per coil that the specified stepper motor needs. While the L293D does specify thermal protection in the datasheet, that is not foolproof. It is a fair bet that the IC has been damaged due to overload.

The other, stronger possibility, is that the back-EMF from the coils each time they are turned off, has damaged the protection diodes of the L293D, subsequently frying the driver circuitry. Using a H-bridge driver for a high current inductive load (the 1.5 Ampere drive coils of the bigger stepper motor) without external protection diodes = silicon suicide - Simply don't do it, no matter what the tutorials say. The internal protection diodes are not designed to dissipate high current transients.

The solution:

• Instead of the L293D, use two SN754410 ICs connected in parallel, with heat sinks on each, to drive the stepper: Each SN754410 is specified for 1 Ampere per driver line, and they work well in parallel. It is good to stay within about 75% of the rated current of these half-H drivers, so with two of them, a current of 1.5 Amperes per coil will work.

• Add protection diodes such as 1n5817 Schottky diodes, in reverse polarity, to each of the output lines, thus:
  

  

^{simulate this circuit – Schematic created using CircuitLab}

• Note also the 10 uF electrolytic capacitor and the 100 nF capacitor shown between each of the Vcc lines and the ground pins. These decoupling capacitors should be added to your circuit, as close as possible to the respective pins of the IC, to prevent significant noise pulses being injected into the supply lines each time the driver switches the stepper coils on or off. Polarity of the electrolytic capacitor is critical: The negative side goes to the Ground line in each case.
• Finally, ensure that all of the ground pins are connected together and to the ground of the supply, preferably with thick traces. Those pins serve as both ground return and heat sink paths; Not connecting them reduces the IC's capability to get rid of heat during operation.

A common hobbyist approach for connecting L293D or SN754410 drivers in parallel is to physically stack them one on top of the other, and solder the corresponding pins together. See this page for a description of this strategy:

The concern with this approach is that the heat dissipation of the lower IC in the stack will be poor, so further derating of power will be called for - or the lower IC will overheat and die, if the stepper motor application requires constant operation at full load. Therefore this is not recommended if parallel connection is an option.