4–20 mA Burden Voltage

Formula

Description

In a 4-20 mA current loop, the receiving device (PLC, DCS, or indicator) converts the current signal to a voltage by passing it through a precision burden resistor. The voltage across this resistor is what the ADC actually measures. A standard 250Ω burden produces 1-5V from the 4-20 mA signal, which is a convenient range for many ADC inputs. The total burden of all devices in the loop must not exceed the transmitter compliance voltage, or the loop current will be unable to reach 20 mA.

Variables

• V — Voltage across the burden resistor (V)
• I — Loop current (A)
• R — Burden resistance (Ω)

Practical Notes

Common burden values: 250Ω for 1-5V output, 500Ω for 2-10V output. The loop power supply must provide enough voltage to overcome all burden voltages plus the transmitter minimum operating voltage (typically 12-15V). For a 24V supply with a 250Ω burden (5V at 20 mA) and 2V wire drop, the transmitter has 17V available, which is adequate for most two-wire transmitters.