(Edit: Corrected first paragraph below.)
Thank you, very much, Perry. I appreciate the pics you sent in your PM to me. And I see that the pictures have now appeared on your original post (but not in their correct places. There are place-holders in the text, and the pictures now appear after the text.)
I have a question: What is the best way to snub a bipolar stepper motor coil (e.g. a Lavet stepper motor, as used in most inexpensive quartz wall-clock movements), when driving such coils from two I/O pins of a microcontroller device? I have seen a few snubber circuits for these (some of which look questionable).
The first one here (which falls into the “very questionable” category) is for a Vetinari type wall clock project:
The original schematic shows D1 and D2 as 1N4001 rectifier diodes.
I think the above circuit is bad, because it looks like there is an almost short circuit across either D1 or D2, whenever the microcontroller pulses a logic high on (respectively) either PB1 or PB0.
Nevertheless, the developer of this circuit has a YouTube video showing that his clock, indeed, works! (In the video, note the red clock on the right.)
I am surprised the video does not show any of “The Magic Smoke”* escaping from either of the diodes or from the microcontroller. Perhaps he added some series resistors into the circuit and did not update the original schematic as published in his blog post. But, even if he did, the voltage that would pulse the coil would only be the voltage drop of the forward-biased diode (about 0.7V ?) instead of the 1.5 V that the motor coil expects. So I am not sure how this clock is working.
BTW, that original schematic also shows a 1.5 V clock module circuit output directly driving the bases of two BJTs (configured as a 2-input NOR gate) WITHOUT any base resistor!. So, again, I wonder that “The Magic Smoke”* has not escaped from the clock module or either transistor, due to a large base current.
(* Of course, we all know that silicon devices, integrated circuits, etc., all work because of The Magic Smoke particles that live inside them. However, if someone were to ever abuse any such device with too much voltage or current, The Magic Smoke would become very unhappy, and escape from the device, which will never work again, because The Magic Smoke is no longer inside to make it work. )
The next snubber circuit (for another Vetinari clock project) looks a lot better:
(Note that, in this case, the original schematic shows the designer using a PIC micro instead of an AVR one, but I wanted to keep this post “Arduino-friendly,” )
I have seen examples of the above type of snubber, using either rectifier diodes (like 1N400X type diodes) or Schottky diodes. But I am wondering if this type of snubber circuit would affect the power supply? Most of these types of projects are battery powered (for use in a wall clock). So, when the magnetic field of the stepper motor coil collapses, would a lot of current flow from the negative battery terminal (connected to ground), and affect the power supply?
The third (and final) circuit that I have seen (I forget the source) looks even better:
In this case, diodes D5 and D6 are 2.4 V, 1 W Zener diodes. my understanding of this circuit is that, when the magnetic field collapses, and the “back EMF” becomes larger than the Zener voltage of (say) D5 plus the forward voltage drop of D6, then both diodes conduct and “short out” the coil.
So, enough (too much?) background, and on to my questions.
Is the second circuit shown above (with either regular rectifier or Schottky diodes, connected to ground) “good enough”, or should the third circuit (with Zener diodes) be used?
Is there a better way (than the ones shown above) to “snub” a bipolar stepper motor coil?
With thanks and regards,