THE 4 – 20 MA CURRENT LOOP
Scaling Formula
Both the Light and Humidity Transducers are configured for a 4 – 20 mA signal. These signals connect to the Analog Input Module of the PLC, channels 0 and 1 respectively. The PLC has an internal 250Ωresistor, resulting in a voltage measurement across the 0V and 0R PLC inputs in the 1 – 5V range.
For troubleshooting purposes and to determine if a transducer is properly calibrated, it is important to understand the signal that should appear on the loop with a given input value.
Value – the Present measured value of the process (130°F is an example of a value)
Range – the low to high values (4 – 20mA is a range)
Lower Value – the smallest value in the range, in the 4 – 20 mA loop that is 4
High Value – the largest value in the range, in the 4 – 20 mA loop that is 20
Span – the high value in the range minus the low value, in the 4 – 20 mA current loop that would be 20 – 4 = 16
Humidity Transducer Measurements
The Humidity Transducer has a range of 0 – 100%. It is presently measuring 82%
Humidity 82%
To determine the current that should be present on the loop and the voltage value present on the Analog Input Module, you can fill in the formula below using A values for the light and B values for current/voltage.
Scaling Formula Explained
Value – the Present measured value of the process (130°F is an example of a value)
Range – the low to high values (4 – 20mA is a range)
Lower Value – the smallest value in the range, in the 4 – 20 mA loop that is 4
High Value – the largest value in the range, in the 4 – 20 mA loop that is 20
Span – the high value in the range minus the low value, in the 4 – 20 mA current loop that would be 20 – 4 = 16
Light Transducer Measurements
The Light Transducer has a range of 0 – 2200 μmol/s. It is presently measuring 800 μmol/s
Light 800 umol
To determine the current that should be present on the loop and the voltage value present on the Analog Input Module, you can fill in the formula below using A values for the light and B values for current/voltage.
Scaling Formula Explained
Value – the Present measured value of the process (130°F is an example of a value)
Range – the low to high values (4 – 20mA is a range)
Lower Value – the smallest value in the range, in the 4 – 20 mA loop that is 4
High Value – the largest value in the range, in the 4 – 20 mA loop that is 20
Span – the high value in the range minus the low value, in the 4 – 20 mA current loop that would be 20 – 4 = 16