# How to size the capacitor on the dc power supply for stepper motor?

• posted on April 11, 2006, 6:00 pm
Hello,
I have a stepper motor with 48Vdc and 3 Amp. I would like to design a non linear dc power supply to powe this stepper. Could somebody point it to me how can I size the capacitor on the dc power supply? I am
planning to use a regular transfomer to conver regular 208Vac to 48Vdc. For the calculation, the RMS value on the transformer's secondary side is 34Vdc (48Vdc/1.41) and the voltage across the load capacitor is 68Vdc. My question is that how do I size the proper capacitor to power my stepper motor?
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<%-name%>
• posted on April 11, 2006, 10:13 pm
snipped-for-privacy@gmail.com wrote:

You should be able to find info on power supply design on the web. Offhand, I'd say you're in trouble on this, and you might be better off springing \$100 or so to buy a regulated supply off the shelf.
Roughly speaking, you want to keep the ripple down to 1%, which is probably still high, and you'll need roughly 50,000 uF.
At 68v, your 48v stepper will pull something like (68/48) * 3A = 4.2 Amp. If you use full-wave rectification, then the half period is about 0.008 sec. For a 1% drop, the dv will be about linear, so you can estimate the capacitance from I = C dv/dt, or C = I * dt/dv, giving
C = 4.2A * 0.008 sec / (0.01 * 68V) = 49,000 uF
First guess.
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<%-name%>
• posted on April 11, 2006, 11:08 pm
Thanks. In this case, I would probably to spare \$100 to buy a regularated power supply and at the same time I would build one power supply to try out.
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• posted on April 12, 2006, 4:02 pm
snipped-for-privacy@gmail.com wrote:

You could probably forget the transformer all together and rectify the AC to 200+ volts into a smallish capacitor, and then switch down about 150 watts into an L/C filter. That's what most power supplies do these days,