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Outputs 101: 4-20mA

The 4-20mA output is very popular for its stable performanceA very common output signal for sensors of all kinds, 4-20mA has a few very good reasons why it has been so popular for so long:

Long Cable Runs

The 4-20mA output allows for cable runs over 1000 ft. with almost no signal degradation and without regard to any voltage drops in the wire.

Loop Powered and More

The typical 4-20mA current output is a 2-wire (loop powered) configuration. There is also a 3-wire configuration, which can provide a 4-20mA current loop signal with a separate positive supply and output connection.

Noise Immunity

Current signals are less susceptible to noise than voltage signals. Therefore, the 4-20mA output is inherently noise-resistant.

Signal Conversion

Unlike a voltage output, a 4-20mA current has the same strong signal at any point in the circuit and is a relatively high power analog output.

If necessary the 4-20mA output can be converted to a voltage output for measurement purposes. This can be done at any point in the circuit by adding a load resistor in series. The voltage drop across the resistor will measure proportionally with the current.

Example: A 250-ohm resistor will produce a 1 to 5 volt output with a 4-20mA signal.

Note: Always be sure there is sufficient supply voltage across your sensor’s positive and negative power supply contacts especially at full scale, to be able to obtain a voltage output across the load resistor.

Conclusion

Given its analog nature, the 4-20mA current output is easier to understand, measure and troubleshoot than other outputs or fieldbus communications. However, it can only pass on the current reading – it cannot send a control system any other data. Still, it is well suited for most industrial measurement applications.

As always, let us know if you have any questions.

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