# Thread: Check my power supply calculations?

1. 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

2. 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).

3. Originally Posted by 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).
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?

4. 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.

5. Originally Posted by 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.
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

6. 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.

7. The one advantage of slightly over-sizing the transformer is that it will tend to have better regulation (less difference between on-load and off load voltages) - this means there's less likelyhood of the off-load voltage rising to the point where it's getting close to the maximum rating of the driver modules.

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.
BTW, you can buy off-the-shelf soft start relays/modules if you hunt around, or you can make your own - they're pretty simple, just has a resistor of 10...20 ohms or so in series with the transformer primary to limit switch on current which is bypassed by a relay after a short time when the core remanence has been sorted out and the capacitors are all pretty well charged. Otherwise make sure your mains switch has a decent inrush current rating

And IMHO it's worth putting an LED with suitable series resistor to indicate when those caps are carrying voltage, 20,000uF charged to 45V will make quite a crack if your screwdriver gets too close to the terminals!
Last edited by Voicecoil; 04-12-2018 at 02:10 PM.

8. I've been running a 650VA toroidal supply without any surge protector and haven't had a breaker trip yet, and that's with a standard B curve MCB and a couple of years of use. The other thing is that you don't need massive amounts of capacitor smoothing in this application as the stepper drivers are very tolerant of ripple. I have a nominal 68V supply and when I checked with an oscilloscope (and with the motors working) I see a few volts of ripple and it doesn't make a scrap of difference to performance. Well, theoretically it might but if you are that close to the limit, then you are really pushing your luck anyway!

9. My type B circuit breaker was tripping from the toroid PSU but only in winter at lower temperatures. I have changed it with type C and never had any problem.
Neale, what capacitor value do you have in your PSU?

10. Originally Posted by paulus.v
My type B circuit breaker was tripping from the toroid PSU but only in winter at lower temperatures. I have changed it with type C and never had any problem.
Neale, what capacitor value do you have in your PSU?
Not sure but I'm going out to the workshop shortly and I'll take a look.

Actually, thinking about it, I believe I have a 10A C curve MCB in the control box itself. There is then a 13A fuse in the mains plug (UK standard practice) and then a 32A B curve MCB in the mains distribution box (consumer unit in UK terminology). So I take back what I said about B curve - that's much higher value and upstream of the control box itself.

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