Motorised Variable Resistor part selection [closed]

Question

Background:

I am building a bench power supply from scratch using a Meanwell power supply I’ve got laying around. It’s a 24VDC with 14.6Amp output which is pretty decent for my use. In addition to standard DC power supply function, I want to add some digital capabilities such as controlling CC/CV from computer remotely. I’ve procured 300Watt DC-DC switching regulator which has two 100K trim pots which controls Constant Current and Constant Voltage outputs. Initially I was planning to use 100K Digital Potential resistor to replace the two trim pots, add an Arduino to control them via I2C / SPI and then send commands to Arduino via USB. So far so good…

Problem:

After some research I realized that the Digi key has a range of digital potentiometers which come in 8-bit (256 steps) and 10 bit (1024 steps). My problem is that even with 1024 steps, I get a voltage resolution of 24/1024 = 23mV which is okay for a hack-job but not even comparable to some of the mid ranged bench power supplies that have an accuracy of 10mV.

Question:

I am therefore wondering if there’s a product out there which uses analog potentiometer but with high precision digital control that’ll give me 10mV accuracy?

Answer 1

The more I read your question and try to understand the problem, the slightly more problematic it seems to become to me:

I think your situation is that you want to modify an existing DC/DC converter, which is based on an IC that does all the regulation for you.

So, the good news is that precision potentiometers are a thing, and so is motorizing potentiometers. The bad news is that this solution will get likely exceed the cost of your DC/DC converter by a multiple and still need you to regularly calibrate.

I wonder where your assumption you get a better-than-1/256 of the whole range resolution out of about any DC/DC regulator comes from; that is not a feat that the usual DC/DC architectures allow for, as the internal references are usually not precise enough.

Now, you say you build this from scratch, but that would mean you get to choose your reference. That would have instantly solved the issue: instead of changing the division of the feedback voltage with a potentiometer, keep that a fixed voltage divider out of precision resistors. Use a precision comparator - and an adjustable reference voltage!

Generating a fixed, stable, precise voltage is easier than dividing an unknown, varying, noisy, sometimes overshooting output voltage.

It can be done with a DAC with a sufficiently precise voltage reference.

So, none of the potentiometer business, just adjust the potential to which you compare.

Answer 2

You can gang digital pots. Use one as a voltage divider, then use that output voltage to drive another digital pot voltage divider.