Troubleshooting current loop using voltage measurement

If all the components are preinstalled in the control loop there is no chance to connect milliammeter to the loop, as the milliammeter require breaking of loop wire. A technician may still perform useful troubleshooting measurements using nothing but a DC voltmeter.

Take for example this 4-20 mA loop where a controller sends a command signal to an I/P transducer:

There is no standardized resistance value for I/P transducer coils, and so the amount of voltage dropped across the I/P terminals for any given amount of loop current will be unique for every different model of I/P

4-20 mA signals have a normal coil resistance of 176 ohms. Thus, we would expect to see a voltage drop of approximately 0.7 volts at 4 mA and a drop of approximately 3.5 volts at 20 mA across the I/P terminals.

Since the controller output terminals are directly in parallel with the I/P terminals, we would expect to see approximately the same voltage there as well (slightly greater due to wire resistance). The lack of known precision in the I/P coil resistance makes it difficult to tell exactly how much current is in the loop for any given voltage measurement we take with a voltmeter.

4-20 mA transmitter and controller circuit, where the controller supplies DC power for the loop:

Controller circuit with DC power source built its own power supply. The only voltage measurement that directly and accurately corresponds to loop current is the voltage directly across the 250 ohm precision resistor. A loop current of 4 mA will yield a voltage drop of 1 volt, 12 mA will drop 3 volts, 20 mA will drop 5 volts.

The voltage across the transmitter measured will be equal to the difference between the 26 VDC and the volage drop across the 250 ohm resistor.This makes the transmitter terminal voltage inversely proportional to loop current: the transmitter sees approximately 25 volts at 4 mA loop current (0% signal) and approximately 21 volts at 20 mA loop current.

One of the advantages of the loop-powered transmitter circuit is that the source voltage is largely irrelevant, so long as it exceeds the minimum value necessary to ensure adequate power to the transmitter.

If the source voltage drifts for any reason, it will have no impact on the measurement signal at all, because the transmitter is built as a current regulator, regulating current in the loop to whatever value represents the process measurement, regardless of slight changes in loop source voltage, wire resistance, etc.