From the course: Input Sensors with PLC: 4-20 mA Current Loop

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Load resistor in a 4-20 mA current loop

Load resistor in a 4-20 mA current loop

- [Instructor] Sometimes when constructing a four to 20 milligrams current loop circuit, the receiving device, like the digital display, for example, expects and reads voltage signals rather than current signals. In this case, a load resistor is used to convert the current values back to voltage values so that they can be measured by the receiver device. Also, for some PLCs, the input module or card does not read current values directly, but instead measure the current by measuring the resulting voltage drop across a resistor. So we add a load resistor in the 4 to 20 milligrams current loop to be able to read the values. Most commonly, a 250 ohn resistor is used. Here is an example from Allen-Bradley showing you need to attach a 250 ohm resistor across the channel, the input pins. This is another example from Schneider Electric. Sometimes the analog input cards or modules have a built-in resistor to convert the 4 to 20…

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