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Old March 2nd, 2018, 07:20 AM   #1
Mars
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Analog input values S7

Im a little rusty regarding S7 and analog value processing atm so I hope I could get some answers.


What is normally the fault when getting overflow on analog value from a sensor ?

On underrange/underflow its broken wire etc. but on overflow what will the cause most likely be?
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Old March 2nd, 2018, 08:26 AM   #2
BryanG
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If you are talking mA loop then an overflow is too much current. Current depends on Voltage and Resistance, so if the voltage driving the loop is too high for the sensor to control, or if the resistance of the loop falls too low, maybe because of a short circuit. Also for mA systems ground loops can cause both failures, it is where the 0V on the sensor and PLC aren't at the same potential.
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Old March 2nd, 2018, 09:28 AM   #3
boneless
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Some instruments also use overflow to indicate an error.
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Old March 4th, 2018, 06:19 AM   #4
Mars
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Quote:
Originally Posted by BryanG View Post
If you are talking mA loop then an overflow is too much current. Current depends on Voltage and Resistance, so if the voltage driving the loop is too high for the sensor to control, or if the resistance of the loop falls too low, maybe because of a short circuit. Also for mA systems ground loops can cause both failures, it is where the 0V on the sensor and PLC aren't at the same potential.
But ground loops is only relevant if the instruments are powered by the 4-20 mA ? And not by external power supply?
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Old March 4th, 2018, 11:47 AM   #5
BryanG
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Quote:
But ground loops is only relevant if the instruments are powered by the 4-20 mA ? And not by external power supply?
Either one can get ground loops. The instruments with an external power supply by their design have a separate 0V, and unless the 4-20ma output is isolated from that 0V, or the PLC input is isolated from the PLC 0V, you will likely have problems unless the 0V can be tied together.

You can also get ground loops in a 4-20mA loop when a loop powered sensor isn't isolated. Fast sensing thermocouple sensors are often electrically bonded to the case and so need isolated converters to avoid issues. I had problems with a guided radar level sensors, it was fine when one was powered up, but if a second was used on a tank that had the same earth as the first they both gave the reading of the fullest tank.

There is an argument that you can avoid ground loops by not grounding anything, for me it is like saying you shouldn't earth any metalwork on a machine to avoid electric shocks.
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