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4 to 20 mA Current Loop Output Signal
4-20mA Current loop output is a type of electrical signal that is used in a series circuit to provide a robust measurement signal.
The pressure sensor or instrument generating the signal acts like a current source providing a constant current output signal for a given applied pressure independent of supply voltage or circuit impedence.
Unlike voltage outputs, the current output is the same at any particular point in the circuit and is a relatively high power analogue output which can be used in measurement circuits over long distances.
A typical current output is 4 to 20 mA in a 2 wire configuration, but there are 3 wire configurations which can provide 0 to 20mA or 4 to 20mA current loop output signal with a separate positive supply and output connection.
The current can be converted to voltage output for measurement purposes at any point in the circuit by adding a load resistor in series. The voltage drop across the resistor will then vary proportionally with pressure and current. For example a 250 ohm resistor will produce a 1 to 5 volt output signal with 4-20mA current loop signal.
When connecting up a pressure transmitter (pressure sensor with a current output) to a circuit with a measurement load resistor it is important to ensure that there is sufficient supply voltage available across the pressure transmitter positive and negative supply contacts particularly at full scale.
4-20 milliamps is used extensively throughout the process control industry and is also being used in manufacturing, aerospace and auomotive applications because of the convenience of 2 wire connections, robustness to RFI and the reduction in size of pressure transmitters in recent years.
Site Last Updated: 09/02/12
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