Stepper motor vibrate instead of turning?

philba said:

forced air will certainly help. I'm glad you are happy with it. Just watch out for high speed stalls, you are running the chip over spec.

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stall => fast deacceleration => big Back EMF => power voltage going high...

allegro" said:

35 V - This must not be exceeded under any circumstances.

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power supply have BIG capacitors (40000uf)
plus
overvoltage protection => power zener (Q1,D1,D2)

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important thing: "Vref should not exceed 4V" (=> proper R14)

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now I'm tuning Ct/Rt (R8/C1 & R9/C4).... to proper work with external diode (D3..D10)
.. some faulty steps will accure
decoupling is good...