# Thread: help sizing toroidal PSU to stepper/drives

1. Originally Posted by irving2008
Yes, it's generally true and often much less in practice. But you do need enough bulk smoothing capacitance to cope with the occasional peak demand.
With my maximum of 24A (6 motors at 4A) should I go with 500VA or 630VA and what capacitance do I need for smoothing?

On the webpage of the toroid manufacturer the calculations are slightly different, and I'm a bit confused, probably I should ask them.

Full wave bridge (FWB) With capacitor:
VAC = 0.8 x ( VDC +2 )
IAC = 1.8 x IDC
WATTS = VDC x IDC (output)
VA = 1.4 x ( WATTS + 2 IDC* )

2. Originally Posted by paulus.v
With my maximum of 24A (6 motors at 4A) should I go with 500VA or 630VA and what capacitance do I need for smoothing?

On the webpage of the toroid manufacturer the calculations are slightly different, and I'm a bit confused, probably I should ask them.

Full wave bridge (FWB) With capacitor:
VAC = 0.8 x ( VDC +2 )
IAC = 1.8 x IDC
WATTS = VDC x IDC (output)
VA = 1.4 x ( WATTS + 2 IDC* )
Those calcs are for steady state load and make assumptions about power factor etc. to limit core heating and losses. If you go that route you'll end up with a massively overspec'd, physically large and overpriced transformer.

Steppers are nothing like that, the duty cycle is nothing like steady state. I don't know why you have 6 motors but I'd be surprised if you need more than 60% of the total load and most of the peakiness is handled by the capacitors.

So use:
VAC = (VDC + 1.5)*0.7
IAC = IDC
VA = IAC * VAC, then chose next largest

for 2 parallel secondary windings each is rated at IAC/2

The 1.5 is voltage drop across rectifier, can be 1, 1.5 or 2 depending on product.

So 24A @ 60% = 15A approx. = IAC (or 2 windings @ 7.5A each)
VAC = (42+1.5)*0.7=30V
VA = 30*15=450 so use 500 or 625VA

3. Thank you a lot for your help.

I willl go with 500VA.

Can you recommend the capacitors specs that I will be needing?

Originally Posted by irving2008
Steppers are nothing like that, the duty cycle is nothing like steady state. I don't know why you have 6 motors but I'd be surprised if you need more than 60% of the total load and most of the peakiness is handled by the capacitors.
I intend to build a lightweight 2 axis head for 3D foam milling and a rotary axis for wood.

4. Can anyone recommend the needed capacitance for smoothing a 30V 500VA AC --> 42V 16.5A DC toroid?

If someone could point them on this site I would be very grateful. I found this ones but not sure if I need them to be DC.

Thanks

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