Feature: Sensors


Figure 1: Typical 4-20mA current loop


Figure 2: Loop load resistance vs. loop supply voltage for a typical current loop output sensor


Figure 3 Typical current loop using a three-wire transmitter


Transmitter A sensor or transducer that measures a physical parameter, such as temperature, pressure, position, or fluid flow, is connected to a signal conditioning circuit that converts the measured value to an electrical output signal such as a voltage or current proportional to the measured parameter. If this electrical signal is a 4-20mA DC output connected in a current loop, the hardware and electronics system that regulates current in the loop is called a transmitter. A transmitter may consist of a single device containing a sensing element and some electronics, or it may use a sensor or transducer connected to separate signal conditioning electronics configured as


a 4-20mA current transmitter. Sensors or transducers are usually designed to measure a


range of values of the measured parameter, which is known as the “measurand”. Te measurand value must be converted to current within the measuring device in such a way that the current in the loop will be proportional to the measurand value. Te range of the loop current, 4-20mA, is called the “span of


the transmitter”. Te transmitter is typically configured so that one end point of the measurement value will correspond to 4mA and the other to 20mA.


www.electronicsworld.co.uk April 2021 31


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