Introduction to Two-Wire Transmitters and 4-20mA Current Loops – 8500904
The Basics of 4-20mA Current Loops
In two-wire 4-20mA control loops, we use 2-wire transmitters to convert various process signals representing flow, speed, position, level, temperature, pressure, strain, pH, etc., to 4-20mA DC for the purpose of transmitting the signal over some distance with little or no loss of signal. This paper reviews the operation of this transmission standard and its advantages; in particular as it relates to two-wire transmitters and the associated 4-20mA current loop.
What makes 4-20mA signal transmission so attractive?
Probably the greatest advantage of using a current loop for signal transmission is the current loop’s low sensitivity to electrical noise. This is very important for long distance transmission in harsh industrial environments. As a generally low impedance system, it is much less sensitive to induced noise, than perhaps the high impedance input of a voltage amplifier. The currents injected by typical noise sources are generally no more than a few hundred microamps, usually insignificant to the 16mA span. The use of a “Live Zero” also improves the signal to noise ratio at low levels, allowing us to accurately discern low signal levels without added noise or interference.
Another advantage to the 4-20mA current loop is that it is essentially lossless with respect to the transmission media (wire) and the interconnections (connectors). That is, the accuracy of the signal is not affected by the voltage drop in the interconnecting wiring. This allows the signal transmission to occur over long distances, with varying conductors. Compare this to voltage signals, which will always have an associated signal loss related to the length of the wires—the 4-20mA signal current does not exhibit any signal losses under this same scenario. Kirchoff’s Current Law teaches us that the current in a loop is equivalent at any point in the loop. That is, if you happen to be reading 12mA at your receiver input, you can be certain that 12mA is passing through your transmitter.
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