Electronic – understanding how a motor is controlled with an L293D and raspberry pi

Based on your (further) info, if there is no control wire, I think the reason it is not moving properly is that what you have now is basically a DC motor. To control a standard DC motor with PWM, you simply use a duty cycle between 0% (off) and 100% (full speed)

The current needed will be a lot higher than a servo control wire, as rather than controlling the feedback, you are controlling the actual power to the motor. This is okay as you have the L293D though, which will supply more current than a uC pin can, and exactly what they were designed for.

I suspect that the 2ms is too low to provide enough power to start the motor turning - if you have a 20ms period, 2/20 = 10% of full "power" (roughly - note actual power is I squared R, so at 10% will actually be 1/100th of power at 100% - RMS voltage would perhaps be a better term here) Try a higher value and see where the motor starts reliably. Let us know the results.

Here is a page that explains the differences quite well:

Dc vs Servo

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.