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4-20 Ma Current Loop
Analog Standard
Industrial signaling circuits transmit measurement values across long distances using controlled analog currents. A 4-20 ma current loop uses a minimum current of four milliamperes to represent the zero level, which provides an active sign of life to distinguish a working system from a severed cable. The upper limit of twenty milliamperes corresponds to the full scale of the measured variable.
Circuits of this type are limited by the compliance voltage of the transmitter and the total resistance of the line.
Loop Resistance
Receiver instruments in the circuit add a resistance that converts the current signal into a voltage. For a typical transmitter, the total series resistance in the 4-20 ma current loop must not exceed the value determined by the supply voltage minus the minimum operating voltage divided by twenty milliamperes. Field devices often utilize a two hundred and fifty ohm resistor to produce a standard five volt drop.
This resistance determines the maximum length of the copper wire that can be deployed before the voltage drop becomes too high.
Noise Immunity
Electromagnetic noise is highly prevalent in industrial environments and can distort voltage signals. Because a 4-20 ma current loop relies on current rather than voltage, it is highly resistant to voltage drops caused by long wire runs or electrical noise from nearby motors. The low impedance of the receiver further suppresses induced electrostatic voltages.
Voltage Drop
Electrical barriers in hazardous areas introduce a resistance that reduces the voltage available to the transmitter. When the voltage supplied to the 4-20 ma current loop falls below the minimum required compliance voltage, the transmitter cannot maintain the current at twenty milliamperes. This condition leads to measurement saturation and incorrect readings at the high end of the scale.