PF2100 4-20mA Input Card User Manual v1.2
©2021 Profire Energy Inc
June 2, 2021
Page 17
Appendix A
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Analog 4-20mA Current Loop Background Info
An Analog 4-20mA Current Loop is a signalling scheme commonly used in industrial control and monitoring. It is a fairly old standard circa the 1950s (older than
RS-232). It works by encoding some physical property such as temperature, pressure, pH, or flow rate into a current on a wire in the range of 4 to 20mA.
There are four required components in a typical current loop:
1.
Power Supply
–
Provides the power for the transmitter, typically in the range of 12V to 36V. Must be able to source at least 20mA.
2.
Transmitter
–
Is a type of transducer that converts some physical property (temperature, pressure, pH, flow rate, etc.) to a current in the range of 4mA
to 20mA. It is a current source so it has a high output impedance. May be designed to be either self-powered or to draw its power from the loop.
3.
Receiver
–
A device that receives the current from the transmitter. It has a low input impedance. It may be designed to be either self-powered or to
draw its power from the loop.
4.
Wire
–
Connects the other components together. Only a single wire is required between each of the three components listed above which are arranged
in a loop.
The encoding scale between 4 and 20mA is typically linear (although for rate of flow, it is often the square root of the flow that is encoded). The minimum
current output of 4mA represents 0% and the maximum output of 20mA represents 100%. By encoding the minimum value as 4mA, the following benefits are
realized:
1.
The receiver can detect when there is a wiring fault (the loop is open) since it can differentiate between no current being present (0mA) and an encoded
value of 0% (4mA).
2.
The transmitter can be powered from the loop.
3.
The system has a high degree of immunity to industrial noise which might superimpose voltage offsets onto the loop.
