Analog Loops

Internally Powered Analog Loop – Internally powered references the analog signal itself and where the power for the signal comes from. Internally would typically be referenced from the device that the signal comes back to, for example a PLC or a transmitter.

• In this case, we are using a 24 VDC PLC system so our power source for the device is coming through the backplane of the PLC. This means that the signals power source will come from the source device, go out to our end device, and then come back to our source device.
  • In the image below you can see that the signal comes from the PLC, out to the analog sensor and back to the PLC. This would be an internally sourced signal. This is typical of a PLC getting a signal from a transmitter in the field.
    • The transmitter would be connected to the device, let’s say in this case a pH sensor. That pH sensor would relay a signal to the transmitter and in turn the transmitter would give the PLC a signal over an analog loop.

• The image above depicts how this would be wired. The signal from the module to the transmitter and back to the module is powered internally by the module. This does not always have to be the case, but is fairly typical.

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Externally Powered Analog Loop – Externally powered would be a source of voltage that is NOT from your device. That means from an external 24vdc power source in this case.

• We are using the same 24 VDC PLC system as above but our power source for the device is coming from a source outside of our PLC
  • In the image below you can see that the 24vdc power comes from our power supply +, out to the 24vdc + on the transmitter, out the 24vdc – of the transmitter and to the POSITIVE 24vdc + on the module.
    • The important thing to note here is that our transmitter is not given a 24vdc positive and negative. We are not just powering up the transmitter. Our transmitter get’s its 24vdc – signal THROUGH the module. The module has to read the current on the signal so our negative wire is brough from the power supply, to the negative on the module.

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How does it work? – how does the analog signal work

• Every end device has a range. Lets say in this case it’s a pressure transmitter and it’s range is 0-100 PSI. The transmitter will be set up from the manufacturer to output 4ma of current at the lower range value of 0 psi and 20ma of current at the higher range value of 100 psi.
  • If you order a transmitter with a different range, then it will come preset for those new values.
• The transmitter is setup from the factory to take the raw pressure value in this case and transmit that signal over electrical conductors as a 4-20ma signal.
  • This is where you will see that not all transmitters are created equally and that some are more accurate than others. Every manufacturer lists their transmitters with an accuracy range and typically they offer cheaper options that are less accurate and more expensive options that are far more accurate.
    • There are some industries and applications that require extreme accuracy for specific custody transfer applications or weights and measure type of applications. If you are in the market for a transmitter, it’s good to keep that in mind. If you don’t require .001% accuracy, you can find cheaper alternatives out there.

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Calibrations – Analog loops are typically required to be calibrated on an annual basis. What people often confuse is the difference between calibration of the sensor and calibration of the analog loop.

• Specifics
  • Calibration of the sensor – This would require the entire sensor to be removed from the process and calibrated at the factory, or to do a factory calibration in process (typically not possible in most cases)
  • Calibration of the analog loop – This is the common method and involves running the transmitter through it’s range of measure while verifying the 4-20ma signal is calibrated to output correctly at the measured values. In most cases, this is what a calibration will involve.
    • Depending on the quality of the transmitter, the analog signal can have some inherent “drift” requiring calibrations of the analog loop on a more frequent basis. This isn’t as prevalent in newer sensors and transmitters but in the past this was something that could be a common issue to deal with.

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Troubleshooting – If you have an analog loop that’s not performing as desired, here’s a few things that you can check.

• Voltage
  • If you have an internally powered loop, make sure that both devices are set for that. The plc card usually has a setting or in some older cases dip switches that have to be set correctly and so will the transmitter or sensor.
  • You can only have one voltage source on the loop. If two sources are fighting each other there will be problems.
  • There should be 24 VDC + coming out of the power supply
    • If NOT, this would indicate an issue with the power supply or power TO the power supply
  • There should be 24 VDC + coming in to the + terminal of the transmitter
    • If NOT, and the power supply has voltage coming OUT of it, there is a wiring issue between the power supply and the transmitter
  • There should be 24VDC+ coming out of the transmitter on the – terminal. Remember, if this is an analog loop powered device with NO external power, there will not be a 24VDC- connection on the transmitter. There should be 24VDC+ on the – terminal of the device.
    • If there is NOT 24VDC+ on this terminal and you do have 24VDC+ on the in terminal, then the device is typically bad. This is common if the transmitter goes bad.
  • There should be 24VDC+ on the IN + of the module.
    • If there is NOT, and the SI 24VDC+ coming out of the Transmitter, that typically points to a wiring issue between the transmitter and the module.
  • There should be 24VDC- on the IN – terminal of the module. Check for continuity on the IN- terminal to the DC Common of the power supply. This needs to be sourced from the same power supply that the 24VDC+ is connected to.
    • If you have verified all of the above as well as making sure your power to the transmitter and the dc common to the card are from the same power supply, it’s likely that the input is bad.
      • You can try switching to another input to test this theory.
      • Also, make sure that the settings for this module are correct. If there are settings inside the PLC to enable the input, change the input voltage or anything of that kind they need to be set correctly for 4-20ma input.

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