Check my power supply calculations?

[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.

[Voicecoil]

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!

[Neale]

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!