Check my power supply calculations?

[Nealieboyee]

Hi all,
I'm just in the power supply sizing phase of my CNC build.
My specs:
4x DM542 drivers at a max of 50V each.
4x 4.2A/3.8mH/425oz-in Nema 23 steppers.

I've calculated that:
4x 4.2A gives me 16.8A current draw. Make it 70% of that gives me around 12A requirement.
Max voltage of drivers is 50V, so to get an input of around 42V, I'd need a transformer with secondary of about 30V (or 2x15V x 1.414 for regulation and filtering capacitance).

12Ax30V is 480VA, so If I go with 625VA to be safe, would that be ok?

Thanks
Neal

[paulus.v]

12A*30V = 360VA

With toroidal transformer even 60% of the rated current is safe. This is giving you 10A*30V = 300VA.

In my opinion anything above 300VA is ok. Using large smoothing capacitor(s).

[Nealieboyee]

12A*30V = 360VA

With toroidal transformer even 60% of the rated current is safe. This is giving you 10A*30V = 300VA.

In my opinion anything above 300VA is ok. Using large smoothing capacitor(s).

WOW that's embarrassing. I'm putting that mistake down to lack of sleep. Thanks. I'll go for a 500VA then as they aren't too expensive. As for smoothing caps, what is the formula for calculating their size?
I think I read somewhere that its 80000xcurrent/voltage?

So 80000*12A/30V......about 32000uF?

[paulus.v]

I don't know how to calculate. If I remember well, I have put two capacitors with 10000uF @63V each, and I haven't had any problem so far.

[Nealieboyee]

I don't know how to calculate. If I remember well, I have put two capacitors with 10000uF @63V each, and I haven't had any problem so far.

I found a calculator online. It depends how much ripple you are comfortable with. I ended up having 15000uF, so i bought two 10,000uf Caps. I also bought the 625VA transformer as it was the same price as the 500VA.

Thanks very much for your help.

Cheers
Neal

[paulus.v]

I'm glad I could help.

There's one downside with a big toroid. When it's being energised it sucks a lot of power. You could see the lights dimming for a fraction of a second and even your circuit breaker tripping. Large transformers usually have a soft start circuit. If your mains power is strong enough and the circuit breaker is type C, you will be fine though.