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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. 4