Help me to choose transformer

[Gytis]

Hi and thank you for your input Doddy

but those drivers are AC not DC..,
or I don't understand here something?

[Doddy]

Good point well made, but the argument still stands.

Another point - the drivers typically have their own onboard rectifiers (they are DC-driven devices even with a AC supply), if you look at the physical size of the drivers vs a PSU supply capacitor then you can expect that there is a compromise with the onboard rectification (that needs to be balanced against the losses introduced by the additional rectifier). But, yes, you can still opt to use the AC supply straight from the toroidal. In that case I'd chose to wire each driver straight back to the transformer, and not to daisy-chain them.

[Gytis]

thank you for clarification,

I was more worrying about Current, if I understand it right then
4 steppers requiring 16 amps, 1200 VA transformer can provide 16,7 amp but I guess it overkill a litle bit or no?

[routercnc]

I am running 5 NEMA 23 motors from a 750VA toroid and there is a fair bit of inrush when it switches on (lights dim momentarily). I needed a 10A type D MCB in the end to avoid it tripping out.
1000 or 1200 VA is going to be interesting.

Do you need that much as the steppers do not take anything like the rated current added together

[Gytis]

Thank you for you're input,
I will take your advice and begin with a 750VA toroid, I can always upgrade if it will be necessary.

btw. you have an impressive machine and great vidoes!

[routercnc]

Thank you. Had fun making the machine and videos and the best thing was there was great feedback and it has helped give ideas to other builders.

On the toroid sizing question- I’m only repeating what others have said on here in the past and the Gecko website but the guide is 1/3 x number of motors x maximum current. So 1/3 x 8A x 4 =9.6 A. I think I would start somewhere there.

Sometimes people use a larger toroid for future proofing, depends if that needs to be considered. I suspect pulling small currents from very large toroids is not very efficient.

[Neale]

Good point well made, but the argument still stands.

Another point - the drivers typically have their own onboard rectifiers (they are DC-driven devices even with a AC supply), if you look at the physical size of the drivers vs a PSU supply capacitor then you can expect that there is a compromise with the onboard rectification (that needs to be balanced against the losses introduced by the additional rectifier). But, yes, you can still opt to use the AC supply straight from the toroidal. In that case I'd chose to wire each driver straight back to the transformer, and not to daisy-chain them.

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.

As Doddy says, though, try not to daisy-chain them and wire each driver back to the transformer independently. That makes sure that any voltage drop in the wires to one driver does not affect the others.

[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.