Help me to choose transformer

[Doddy]

It's certainly true that onboard capacitors are probably smaller than those that someone building their own PSU would use. However, we are talking stepper drivers here, not audio amplifiers, and a bit of PSU mains frequency ripple really makes no practical difference. I've measured around 10V ripple on my own PSU with the machine running a test toolpath and it really doesn't seem to worry. Maybe you'll lose a tiny bit of the torque theoretically available, but that's about all. The stepper driver itself is chopping the motor output current at a much higher frequency, and it will adjust dynamically to whatever the input voltage happens to be. After all, even with the ripple, the supply voltage still stays within the nominal working range.

Completely agree with the sentiment - and perhaps I can be a bit too precious about vagaries around electronic design. If you're talking about reduced torque due to ripple - again, I'd agree, and this would be an instantaneous effect (i.e. not related to holding torque - which *could* result in a bit of wobble on a stepper under load, particularly on a micro step). A bigger capacitor can help help to absorb energy from back-EMF from the motor.