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I have a sensor that generates an analog 0-3 V output signal. I would like to connect this sensor to a PLC which has only 0-10 V analog inputs.

My questions are:

  1. Is this recommended to do in general? I guess that I will lose a lot of resolution (70% of it), and this could affect my application requirements.
  2. Is there any other way to add something in between in order to convert the 0-3 V signal to 0-10 V signal?
  3. Related with point #1, how do I calculate the resolution of the 0-3V signal? my sensor specifications only talk about accuracy but not resolution. This is a ph sensor that can measure from 0 to 14 ph, and has an accuracy of 0.1. The PLC has 10 bit ADC for the inputs (Arduino Mega based).

The PLC is being feed by a 24 V DC power supply. This power supply has two outputs and one is not being used right now.

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    \$\begingroup\$ What DC power sources can you use for a converter? \$\endgroup\$
    – Andy aka
    Commented Jan 25, 2023 at 15:48
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    \$\begingroup\$ You need to answer question 1 for yourself. You should be able to determine if the resolution is adequate for your application. If it isn't, then you could amplify the signal using an opamp circuit assuming there is a higher voltage available to you (like 12 to 15 V). \$\endgroup\$
    – Troutdog
    Commented Jan 25, 2023 at 16:28
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    \$\begingroup\$ Is it a DIN mount PLC? If so I would look for a DIN mount analog amplifier with suitable characteristics. \$\endgroup\$
    – George White
    Commented Jan 26, 2023 at 2:44
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    \$\begingroup\$ OPAMP: well, you should be able to find an opamp that will work with 24V. The most common opamp circuit is an inverting amplifier. You will have to decide if that works in your application. For example, the output would be 10V when the input is 0V, and 0V when the input is 3V. If you don't want that, you can use a non-inverting amplifier topology. One thing to make note of: the accuracy will degrade near 0V input due to the nature of opamps. This can be countered if you have a negative rail (like -5V) you can use with the opamp. You can find opamp circuits using a google search. \$\endgroup\$
    – Troutdog
    Commented Jan 26, 2023 at 16:49
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    \$\begingroup\$ In the limit, these OPAMP circuits are just an opamp and a few resistors. You should also add a bypass cap near the power supply pin to reduce noise. Something like LM7301IM-NS would probably work for you. \$\endgroup\$
    – Troutdog
    Commented Jan 26, 2023 at 16:53

1 Answer 1

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  1. No. It's not a good design but if it meets your requirements like this, then there is no need to add a board/module to your setup.
  2. I would suggest checking whether the PLC offers an easy fix (like changing a jumper's position or configuration on the PLC's HMI) to lower its analog input full scale voltage. If not, then you can amplify the signal using an additional external module.
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  • \$\begingroup\$ Regarding answer #1, I understand, but anywas I would like to add something to get the better resolution possible. Regarding your answer #2, I have check before the PLC and it is not possible to scale the voltage. What kind of modules do you recommend to amplify the signal?. \$\endgroup\$
    – bardulia
    Commented Jan 25, 2023 at 21:56
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    \$\begingroup\$ Is this a proper installation or just playing in the lab? Did you check if the PLC manufacturer offers external modules for this purpose? What supply voltages do you have around? How do you intend to mount this module it in your setup? \$\endgroup\$
    – HatimB
    Commented Jan 26, 2023 at 8:49
  • \$\begingroup\$ right now it is just playing in the lab. The manufacturer does not offer external modules but it is an Arduino based PLC. I have a 24V DC power supply available but I could have others if needed. Also the type of mounting is not a constraint. Could you point out a proper type of module for this? \$\endgroup\$
    – bardulia
    Commented Jan 26, 2023 at 9:48
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    \$\begingroup\$ I found a few modules on Google. They're mostly based on the LM358 op amp. They are cheap (3-5$) and the supply voltage can up to 32V or so, you can also change the gain via the fitted potentiometer. Use the "LM358 module" keywords as an example then check all the details of that specific product on your prefered website. EDIT: I don't your PLC's or your target precisions. It's up to you to decide if this combination works. \$\endgroup\$
    – HatimB
    Commented Jan 26, 2023 at 10:17
  • \$\begingroup\$ that is very useful! I will try it and come back here to write the results. \$\endgroup\$
    – bardulia
    Commented Jan 26, 2023 at 10:20

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