Introduction
This page describes 4-20mA current loops.
4-20mA current loops are a commonly used method to transmit an analogue signal value such as pressure or temperature.
Commonly the 0% sensor output is transmitted as 4mA, and 100% as 20mA (why this is the case will be covered later).
For example, a 4-20mA pressure sensor that reads from 0 to 100kPa could transmit (or set) the current within a 4-20mA loop to be 4mA at 0kPa and 20mA at 100kPa. A receiving instrument would be able to measure the current in the loop and hence calculate the pressure measured by the pressure sensor.
History
According to National Instruments, current loops have been used to transmit data since the 1950s.
The exact details of how the 4-20mA range was decided upon as the 'standard' is not entirely clear to this author, however before electrical current loops air pressure was commonly used in a similar fashion with 3-15psi commonly used as the operating range. Apparently an operating range of 5 times the live 'zero' value enabled pneumatic instruments of the day to operate in their most linear range.
i.e.
operating range : zero,
15psi - 3psi gives an operating range of 12psi,
12psi : 3psi gives a ratio of 5 : 1
It would seem that the ratio of operating range : zero remained at 5 : 1 when 4-20mA analogue current loops were adopted.
In regards to 4-20mA being chosen as opposed to say 5-25mA or 10-50mA (or another arbitrary range), it is worth nothing that in the beginning 10-50mA was specified in the standard ANSI/ISA-50.1 as an alternative to 4-20mA. However it is the authors understanding that 4-20mA was chosen as at 24VDC & 20mA, intrinsic safety was still possible and 4mA was still able to provide enough power for (most) instruments to be loop powered.
While the exact history and reasons for the choice of 4-20mA may not be thoroughly understood by this author, it is helpful to have some understanding of the thoughts and background of the selection.
Current to Voltage Conversion
Internal to most / all PLC and DCS systems the 4-20mA current is converted to a voltage, as a voltage is significantly easier to convert to the digital signal that is used inside almost all electronics.
A common way to convert the 4-20mA current signal to a voltage signal is via a high precision resistor.
A voltage range of 1-5V DC is common due to the fact that most early Microcontrollers ran from a 5V DC supply (TTL level).
The voltage conversion from 4-20mA to 1-5V can be achieved via a 250Ω precision resistor and measuring the voltage across the resistor as shown in the below image.
4-20mA Wiring Arrangements
There are two main / common 4-20mA wiring arrangements. These are detailed below.
Loop Powered (2 Wire)
In a "Loop Powered" arrangement the sensor is usually a 2 wire device. The two wires are used to both power the sensor and also receive the signal (loop current) from the sensor.
The common arrangement is for the receiver to contain a power supply (commonly 24V) and a precision internal resistor as shown in the below image.
The sensor 'looks' like a variable resistor to the receiver (Note: the actual construction or implementation may vary), and assuming no loop / cable resistance and a loop power supply voltage of 24V the resistance of the sensor is between 6000Ω (4mA) and 1200Ω (20mA).
Field Powered (4 Wire)
In a "Field Powered" arrangement the sensor is usually a 4 wire device. Two of the wires are used to power the sensor, with the remaining two wires being used to transmit the signal (loop current) from the sensor to the receiver.
In this arrangement the sensor should (ideally) look like a current source. A current source can be represented as a constant voltage source with a variable resistor to represent the system in a similar fashion as to the above Loop Powered example (Note: the actual construction or implementation may vary).
The receiver will normally contain a precision internal resistor to convert the loop current to a voltage as shown in the below image.
The sensor power circuit and associated power wires have been excluded from the below image for clarity however they are present / usually required in real world field powered system.
4-20mA Advantages
4-20mA current loops have a number of advantages. Such as,
Loop Validation
As the sensor 0% value is conventionally transmitted as 4mA, open circuits in the loop can be detected. This arrangement is often refereed to as a 'live zero'.
Going back to the original example of a 4-20mA pressure sensor that reads from 0 to 100kPa, if the receiving instrument measures a value of less than 4mA then the receiving instrument is able to determine there is an issue with the loop (likely an open, or high resistance loop circuit).
If the receiving instrument measures a value of more than 20mA then the receiving instrument is able to determine there is an issue with the loop (likely a short circuit).
Loop Resistance
One significant advantage of 4-20mA current loops is their ability to function over long cable distances or high cable / join resistances. Compared to voltage driven signals, loop current signals are not effected by the voltage drop of the loop, as long as the supply voltage is able to provide the required loop current.
For example, given a loop supply voltage of 24VDC, the maximum total loop resistances can be calculated using Ohms law.
V = IR,
R = V / R,
R = 24V / 20mA
R = 1200Ω
The maximum total loop resistance of a 24VDC powered current loop at 20mA is 1200Ω.
V = IR,
R = V / R,
R = 24V / 4mA
R = 6000Ω
The maximum total loop resistance of a 24VDC powered current loop at 4mA is 6000Ω.
It can be seen in the below diagram that the loop resistance (i.e. cable and termination resistances) are accounted for by the sensor.
Noise Immunity
Due to the relatively low impedance of the current loop in a 4-20mA system the effect of noise is significantly less than that of a voltage driven system which would usually have a higher loop impedance and hence be more affected when subject to an equivalent noise source.
Scaling
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References
http://en.wikipedia.org/wiki/Current_loop
http://zone.ni.com/devzone/cda/tut/p/id/6940
http://www.bapihvac.com/CatalogPDFs/I_App_Notes/Understanding_Current_Loops.pdf
http://www.digikey.com/en/articles/techzone/2013/apr/smart-sensors-in-4-to-20-ma-control-loops
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