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Wheatstone Bridge in the Productivity 1000

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  • Wheatstone Bridge in the Productivity 1000

    Hey everyone, first time user on the AD Forums - I have an interesting challenge ahead of me and was looking for some input from those of you who may have some experience....I am working with a combustible gas sensor that has 1/2 of a Wheatstone Bridge circuit in it. The head contains the half of of the circuit which has the known resistance value (8 ohms), therefore the assumption is that the circuit contains 4 ohms of resistance on the either side of the balancing circuit. From the sensor head, I have three wires. One wire is +5VDC ( with known resistance, which I will call Circuit A ) , one is -5VDC ( common to Circuit A and Circuit B ) and the other is the +5VDC with unknown resistance (Circuit B ). I need to connect this sensor to a P1K through a 4-20mA input card and I am doing so by using a P-33 signal conditioner to go from 5VDC to 4-20mA. However - the problem is that I need to determine the amount of resistance required on Circuit B, so that the voltages on A & B match ( change is in millivolts ). I am using a pot to adjust the resistance on B to balance the circuit. So, the questions are -> how do I convey the reading from the pot to Ohms? Additionally, as gas is applied to the sensor, the resistance changes (through the use of a catalytic bead), so I need to actively monitor this change through Circuit B. Are there input cards that can help me with this? I am not opposed to building my own circuits to solve this issue ( currently researching how to build up an Ohm meter that I can use on the 4-20mA input ). Any help is greatly appreciated!
    Last edited by CCarver; 04-07-2018, 12:23 PM.

  • #2
    Im not really sure how you would calculate the ohm value for this, because your input signal is really reading the voltage from the circuit not really the ohms directly right? As you turn the pot, the ohms are changing but because of this the voltage is changing too, and the voltage is going into the converter and coming out as 4-20ma, which gets fed into the P1000.

    So to get your voltage, you would go to your hardware setup and place your analog input card. Double click on the card and set a tag name for the input your are using. In your ladder, use a scale instruction and set the analog card's tag as the input, and make a new tag for the output. Then for the in-min it would be 0, and the in-max would be whatever the maximum bit value of your card is, probably 65535? For the out-min it would be 0, and the out-max would be 5. Make sure the tags are floating point so that you can see all the decimal places of your voltage.
    So that would give you a tag showing your your exact voltage amount, and from there you can probably work with that to do some math and figure out what ohm value that is equal to.

    I dont do a lot of analog stuff, so hopefully someone else here is able to give you some better guidance on this.


    • #3
      Thanks Mike, but that will not solve this issue. I had previously sent both of the 5v signals into a signal conditioner and then into the 4-20mA input card. While I was able to scale out the values, I had to set some averaging and scales, which gave me a piss poor look on things (due to unstable input values). As it runs out, using the pt is only 1/3 of the bigger picture. After some further research, I was able to determine that I need to build my own differential amplifier circuit and couple it to the output wires of the sensor. Why? Because instead of tracking changes in Ohms, I need to track the change in mV, between Circuit A and B. Only after creating the circuit that makes it possible to monitor this difference, can I send it to the P1K. I will continue to post updates on this project, if only to keep a record of my progress. Additionally, I'm using low res 13 bit cards, so the range value is 0 to 8191, just for the record.


      • #4
        Update - I had to design a circuit that implements an instrumentation amplifier and creates a differential circuit. After wiring the sensor to the board I was able to output a reference signal and variable signal. Using the variable signal (mV) I was able to amplify the output and convert to 4-20mA. Through the use of some timer, averaging and scaling inputs on the P1K, I am now able to track, monitor and lock in values for the sensor. Additionally, the entire process is automated. Very cool.


        • #5
          Sounds like a nice solution. Do you have info on how to put together the circuit for the amplifier?