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How to Hang a Resistor Across a 4-20mA Loop

Distributed control systems (DCS) feature in industrial plants around the globe, keeping key processes within operational limits. A DCS reads, converts and displays the analog values of process variables and issues command signals to pumps, valves and other peripheral equipment. Commonly, engineers use 4-20 milliampere current loops to transmit signals to remote instruments across distances of more than 1,000 feet without significant degradation. Connecting a 4-20 mA signal to a voltage-driven instrument requires the addition of a serial resistor across the loop, selected to develop a specified voltage drop across the input of the remote unit.

Things You'll Need

Voltage-driven instrument
Calculator
One-percent precision resistors

Instructions

- 1
  Note the input voltage range of the instrument to be connected to the 4-20 mA loop. Write down minimum and maximum input voltages. For example, a typical input voltage range is 1 to 5 volts.
- 2
  Divide the minimum input voltage by 0.004 to calculate the value of a resistor that will produce this voltage drop in the loop when a current of 4 mA flows. For example, if the minimum input voltage is 1 volt, the resistor value is 250 ohms.
- 3
  Multiply the resistor value by 0.02 to calculate the voltage drop in the loop when a current of 20 mA flows. Check that this is equal to the maximum input voltage you noted. For example, if the resistor value is 250 ohms, the resulting voltage drop is 5 volts.
- 4
  Check the power dissipation in the resistor when 20 mA flows through it. Multiply the calculated voltage drop at 20 mA by 20 and divide by 1,000. Note your answer, which is the power dissipated in watts. For example, if the voltage drop at 20 mA is 5 volts, the power dissipated is 0.1 watt.
- 5
  Select a one-percent precision resistor with the resistance value that you calculated and that is rated at 10 times the power dissipation that you calculated. For example, if the calculated resistance is 250 ohms and the power dissipation is 0.1 watt, choose a 250-ohm, one-percent, 1-watt resistor.
- 6
  Connect the resistor in series across the output terminals of the loop. Connect the instrument in parallel with the resistor across the output terminals of the loop. Ensure the positive terminal of the instrument is connected to the positive output of the loop and the negative terminal to the loop ground.

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