What is 4-20mA output and how to measure and convert to Volts

The 4-20mA sensor is one of the most popular and widely used outputs for sensors. This Guide demonstrates the benefits to 4-20mA output , how to measure and how to convert into Voltage

Why is 4-20mA used for sensor outputs?

It has become a legacy  standard signal output type for sensors because:

1. Simple wiring . 4-20mA current loops only need 2 wires positive and negative
2. It’s a legacy output type for existing systemsExisting system installations are already designed to accept 4-20mA outputs. Therefore 4-20mA sensor is required otherwise the existing PLC or system will need to be upgraded or replaced to accept different types of output signal , this can be too costly and cause unnecessary down time
3. Wide input range. they typically have a power supply input range from 9 to 32Vdc
4. It has been used for many years,  Therefore it has become trusted with installers and engineers to be one of the most established and reliable ways to measure the sensor output
5. Can be used with long cable runs 50m+ without any significant loss of signal integrity. Voltage output and digital output sensors have voltage drops and signal corruption, the only true competitor for 4-20mA output for long cable runs is RS485
6. Robust against interference in a noisy industrial environment. Industrial environments have inherently electronically noisy equipment such as Motors, Compressorrs and fans , is the sensors are installed near this equipment or they share the same cable run then the electronic magnetic interface can couple into the sensor output cable and distort the output signal. 4-20mA is the most resilient to this type of interference out of all the analog output types

4-20mA output table

Why 4mA at 0% input?

As demonstrated when the pressure or sensor output is at the zero point the output will be 4mA this is good for fault detection and proves the wiring and output are correct even when no pressure or the sensor is at 0% input.

Output formula

The output span is 20mA-4mA =16mA.

 mA current output = 4mA + (16 x ((Sensor Reading– Sensor Low Limit)

How to measure 4-20mA with a meter

• Set Multimer to the mA scale
• Connect positive Sensor connection straight into the PSU
• Connect Negative Sensor connection into the multimeter current connection
• Wire the negative PSU connection into the mulitmeter current connection

How is 4-20mA converted into Voltage output

4-20mA is easily converted into a voltage signa with a single resistor across the 2 output wires.

The resistor converts the current output to a single ended voltage output  that can then be read by an ADC or voltage input measurement.

The resistor value directly affects how the voltage relates to the 4-20mA outputs. Therefore

• Tightest tolerance available e.g. 0.1% should be selected
• Measure the Resistor for the actual value if low tolerance resistors are available

A few examples of the resistor calculation to convert 4-20mA

Converting 4-20mA to 0-10V

Converting 4-20mA to 0-5V

Converting 4-20mA to 0-5V with inaccurate resistor

Measured resistance = 248Ω

Conclusion

4-20mA uses its inherent properties to carry the output of the transducer across long cable runs, resist interference and simple wiring. it is then converted into pressure at the measurement source as appropriate via current or converted into Voltage.