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Typical Current Loop
Loop powered indicators are commonly used to display process information
that is being transmitted to a control room or control system. The display is
either analog with pointer and scale or a digital LCD. Their purpose is to
provide easy-to-read process values for maintenance or troubleshooting the
control system.
Yokogawa loop indicators are typically powered by the output of a
transmitter to display flow, level, pressure, temperature or other parameters.
Signal sources such as 1-5mA, 4-20mA or 10-50mA are common in the process
industry. For a two-wire transmitter the power supply is usually located in the
control room. Since the current loop rides on the power supply it is important
that voltage and current outputs are adequate to power up the number of
instruments in the loop. Therefore, loop impedance must be considered to
prevent overloading the power supply. A current loop is illustrated below.
Most loop indicators do not contribute to very much voltage drop in the
series circuit. However, some recorders and other receivers require 1-5VDC
input, which means the 4-20mA signal must be shunted through a 250 Ohm
resistor. Care must be taken to ensure the system voltage does not drop
below the minimum operating range of the instruments in the loop. This
example only produces a voltage drop of 5.13V and will not affect the
integrity of the loop. Also, wiring adds a small amount of voltage drop to the
loop. Twisted pair-20 or 22 gauge- or shielded wire is recommended. During
installation or maintenance, any opening of the circuit will cause the entire
loop to open.