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I currently am rebuilding a machine that uses 3 large 24V stepper motors, running at 1A per phase.

The machines' original transformer has a 10V and 28V output, and I have tried limiting the 28V output to 24V, with varying degrees of success.

Below is the current design for powering the motors (the 6A load is the maximum power draw)

Ninja edit - The IGNOREME resistor changes the regulator to a 24V regulator (originally 15V) Circuit

Simulating the circuit, at 1A load gives a ripple of about 3V, but at 6A it gives up to 14V ripple. (With or without the regulator part)

I recently did read on a different website that large stepper motors can handle up to 35V before you need to worry about power considerations, but we don't have replacement motors, but the forum didn't have many replies, and a few of them contradicted the main post. (https://forum.arduino.cc/index.php?topic=462832.0)

I am asking for ideas of reducing the ripple further, and if the regulator circuit is unneeded.

Thanks for your time.

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2 Answers 2

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There is no need to over complicate your supply, and putting inductors in will seriously cause problems where the current drawn (by the stepper motors is pulse in nature).

You simply need to increase your filter capacitor and ensure your diodes have a peak current capability at least twice your load current (so about 12A rating).

To calculate your filter capacitor size is simply:

C(F)= (I(Load) * (1/2F)) / V(ripplemax) //2F because you have a full wave bridge rectifier

 (6A * 8.3 mS) / 3V  --> 16000 uF

I selected 3 V ripple here, but you can select whatever you want.

The output peak DC voltage is about (1.4 * Vrms) - (2 * diode Vf)

The minimum is V(outputpeak) - V(ripple)

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Use a larger electrolytic filter capacitor to reduce ripple. A series filter choke would both reduce ripple and probably avoid the need for a voltage regulator. N.B. The filter choke must have low DC resistance or it will drop too much voltage at 6 A. For example, 1 Ω would give a 6 V drop. Supercapacitors are likely not a good option, because at least a dozen in series would be needed (each unit has ~2.75 V maximum voltage limit, and some margin must be left because not all have exactly equal capacitance).

There are online tools to calculate the ripple reduction. Using a 20,000 µF capacitor with a choke of 0.1 H results in ~60 dB (a reduction by a factor of ~1,000) in ripple.

BTW, your schematic shows 28 VDC input, apparently varying 0 - 28 V; should that be VAC?

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  • \$\begingroup\$ The 28VDC text is because I originally wasn't simulating the AC-DC rectifier, then expanded the schematic to fully test the whole circuit. Thanks for the answer. \$\endgroup\$ Jul 13, 2018 at 9:35

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