Feature: Sensors


Advantages of a current loop But, a question arises: Why use this particular loop to transmit the analogue data from a sensor? Te answer is that a 4-20mA current loop offers several benefits


for sensor data transmission: • A major reason is that the loop current does not vary with long field wiring, as long as the voltage developed in the loop can sustain the maximum loop current.


• Another benefit is that the current loop has a low impedance and is not particularly susceptible to noise or EMI at large.


• A third advantage is the live-zero feature of the loop (the 4mA low limit), which makes the loop self-diagnostic if there is a break or bad connection or a loop power failure.


• A current loop permits other current-operated devices such as a remote readout or a recorder to be put in series with the loop, within the constraints permitted by the loop’s compliance voltage.


Understanding current loop output sensors By John Matlack, VP of Sales and


Marketing, Alliance Sensors


power supply, through the transmitter using field wiring connected to a loop load resistor in the receiver or controller, and then back to the loop supply, with all elements connected in series. All current-loop-based measuring systems use at least four elements.


A 30 April 2021 www.electronicsworld.co.uk


4-20mA current loop has become the standard for signal transmission and electronic control in most analogue control systems; see Figure 1. For analogue sensor data transmission, this type loop is a very common method to convey the sensor data acquired.


In a 4-20mA current loop, current flows from a DC loop


• Te low maximum loop current (20mA) allows the use of relatively simple safety barriers to limit loop current to an intrinsically safe level that prevents ignition in a hazardous location.


Loop power supply and compliance voltage When current is transmitted in the loop, there are voltage drops due to the field wiring conductors and the connected devices. However, these voltage drops do not affect the current in the loop as long as the total loop voltage is sufficient to maintain the maximum loop current. Te element responsible for maintaining a stable current in


the loop (as shown in Figure 1) is the loop DC power supply. Te range of voltage over which the loop will function is its “compliance voltage”. Common values for 4-20mA loop supplies are 24 or 36VDC. Te voltage chosen by a designer depends on the number of elements connected in series with the loop, because the power supply voltage must always be higher than the sum of all the voltage drops in the circuit, including the field wiring voltage drop. Te sum of all these voltage drops is known as the loop’s minimum compliance voltage. Tere are certain requirements that the compliance voltage must


be able to fulfill, the two most important of which are: • Te power supply voltage must be able to power all devices in the loop, including the field wiring voltage drop, when the current is at its maximum value, normally 20mA.


• The loop power supply maximum voltage must be equal to or lower than the maximum voltage rating of any device in the loop.


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