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Topic: Replacing mechanical pot with digital (Read 406 times) previous topic - next topic

akarbarz34

May 22, 2019, 08:36 pm Last Edit: May 22, 2019, 11:22 pm by akarbarz34
Hello, I was wondering if someone could please help me find a digital potentiometer to replace a mechanical one with that way I can control it using an arduino. The circuit surrounding the pot is as follows.



Where V-013 is the current mechanical pot it is the "Contelec PC 262- 10k/k 4AM H2" potentiometer, and P1(the "transformer") is actually a Resistance/potiposition transducer found here https://www.phoenixcontact.com/online/portal/us/?uri=pxc-oc-itemdetail:pid=2902016&library=usen&pcck=&tab=5. I am also attaching the data sheet for that mechanical potentiometer. Turning the pot increases the pressure in a chamber. The current mechanical pot takes in 24VDC, however if I have to step the voltage down into a digital one I can but am not sure if that will have any effect on the control of pressure? Thank you so much.

WattsThat

A digital pot may not be appropriate in several aspects. The first is the voltage applied, the most common parts cannot tolerate more than 5 volts applied to the high end or supply side of the pot. 24vdc is the Phoenix module supply voltage, it's probably not what is actually apply to the pot. Fortunately, it is a simple matter to measure the voltage supplied by the  module. If more than 5 volts is applied to the pot, have a look at the MicroChip MCP41HVX1 parts, they can handle up to 36 volts if you meet the wiper current requirements.

Another issue is the resolution. Digital pots typically have a maximum resolution of 8 bits or 256 distinct steps, some are only 7 bits. Your application may or may not need more resolution than is available and practically speaking, a motorized analog pot (with your cermet element) is considered to have an almost infinite resolution. In reality it cannot be infinite but it's certainly better than 7 or 8 bits. Realistically, anything beyond about 13 bits isn't going to help since the limimiting factor is the output resolution of the MCR-2-POT-UI.

Resolution is the bigger factor of the two and it's the more difficult to quantify especially since we don't know the final application.
Vacuum tube guy in a solid state world

Southpark

Hello, I was wondering if someone could please help me find a digital potentiometer to replace a mechanical one with that way I can control it using an arduino.
At the moment, these digital potentiometers are not like analog classical potentiometers. Sure, there are applications for 'digital potentiometers', but what we often get classical potentiometers to do --- and the way we use them ..... the 'digital potentiometers' cannot do (or cannot use them in the same way --- ie. definitely not a drop in and replace item).

akarbarz34

#3
May 23, 2019, 02:05 pm Last Edit: May 23, 2019, 02:06 pm by akarbarz34
A digital pot may not be appropriate in several aspects. The first is the voltage applied, the most common parts cannot tolerate more than 5 volts applied to the high end or supply side of the pot. 24vdc is the Phoenix module supply voltage, it's probably not what is actually apply to the pot. Fortunately, it is a simple matter to measure the voltage supplied by the  module. If more than 5 volts is applied to the pot, have a look at the MicroChip MCP41HVX1 parts, they can handle up to 36 volts if you meet the wiper current requirements.

Another issue is the resolution. Digital pots typically have a maximum resolution of 8 bits or 256 distinct steps, some are only 7 bits. Your application may or may not need more resolution than is available and practically speaking, a motorized analog pot (with your cermet element) is considered to have an almost infinite resolution. In reality it cannot be infinite but it's certainly better than 7 or 8 bits. Realistically, anything beyond about 13 bits isn't going to help since the limimiting factor is the output resolution of the MCR-2-POT-UI.

Resolution is the bigger factor of the two and it's the more difficult to quantify especially since we don't know the final application.
Wow, thank you! I have definitely been giving the resolution a lot of thought and I believe 256 steps/taps may work, but I cannot be certain since the datasheet of the current mechanical one does not specify its resolution.

The reason I thought this project may be possible is because the Phoenix signal conditioner takes an input range of resistance(I am assuming the wiper pin of the pot outputs a resistance?). And then converts it to a 4-20mA signal to be read by a regulator. So, I was thinking as long as I made sure to output a similar resistance range I would be fine. Anyways, do potentiometers work in this way? Meaning, one of the pins is sending out its resistance,and do digital ones do that as well?

Thanks again! - Alex

septillion

Yes, they send out resistance. But that's controlled by transistors. And most can't tolerate a higher voltage than the supply voltage on the pot pins (both ends and wiper) because that will mess with transistors controlling it.

And even though the Phoenix looks for a resistance, you can't measure that without voltage. And more likely, it does not really care about the resistance but the pot will just act as a variable voltage divider for the Phoenix in order to set a reference.

So like others said, a digipot isn't a drop in replacement for a real pot.
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WattsThat

The Phoenix Contact module most assuredly uses a voltage input from the wiper of the potentiometer which is used a voltage divider. Place a voltage across the pot and the Vout to Vin ratio provides the exact position of the wiper regardless of any long term drift in the applied voltage.

This is referred to as a ratiometric measurement and it is a very accurate and simple way to determine the position of the pot. If you just use resistance, it is referred to as reostat mode where only two wires to the device are used and you have no ratio, only the resistance. It is not the preferred method as it introduces errors from end resistance, temperature coefficient of the element, etc, etc. Reostat mode is used when the pot is used to trim a circuit, not measure wiper position.

Digital pots use semiconductors to emulate resistors and they're not as accurate as pure resistive materials which is why the resolution is usually limited.

Until we know what the chamber is doing and how it interacts with the motorized pot, we're just playing a guessing game that is similar to the x-y problem. If you'd explain the next level of operation, there is most likely a better solution that provides the resolution required and perhaps adds an aspect of automation that actually gives an Arduino something useful to do instead of just replacing an analog pot with a digital one.
Vacuum tube guy in a solid state world

MorganS

Maybe you should focus on replicating the 4-20mA output instead of the pot.
"The problem is in the code you didn't post."

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