What is the function and potential drawback of a burden resistor in a 4-20 mA current loop?

What is the function and potential drawback of a burden resistor in a 4-20 mA current loop?

• A. The burden converts current to voltage for measurement; too small a burden reduces transmitter headroom and accuracy, too large a burden reduces signal-to-noise ratio.
• B. The burden acts as a fuse to protect the transmitter from surges.
• C. The burden provides digital signaling to the controller.
• D. The burden stores calibration data for the loop.

Answer: (B)

In a 4-20 mA loop the burden resistor is the element that turns the loop current into a voltage that a measurement device can read. Since the current is 4 to 20 mA, the voltage you get across the resistor is V = I × R, so choosing the right resistance lets the receiver monitor the current as a meaningful voltage level. The best explanation of its function and drawback is that the burden provides the measurable signal by converting current to voltage, but its value must be balanced. If the burden is too small, the voltage is tiny and can fall below the measurement system’s resolution or accuracy, making the current readout noisy or imprecise. If the burden is too large, the voltage drop eats into the loop’s available supply voltage—the transmitter’s headroom to regulate current decreases, which can cause the current to fail to be maintained properly, especially under load changes or supply variations. Larger resistance also increases power dissipation in the resistor and can introduce more noise in the signal chain. Other options don’t fit: the burden isn’t a fuse, it doesn’t provide digital signaling, and it doesn’t store calibration data.

In a 4-20 mA loop the burden resistor is the element that turns the loop current into a voltage that a measurement device can read. Since the current is 4 to 20 mA, the voltage you get across the resistor is V = I × R, so choosing the right resistance lets the receiver monitor the current as a meaningful voltage level.

The best explanation of its function and drawback is that the burden provides the measurable signal by converting current to voltage, but its value must be balanced. If the burden is too small, the voltage is tiny and can fall below the measurement system’s resolution or accuracy, making the current readout noisy or imprecise. If the burden is too large, the voltage drop eats into the loop’s available supply voltage—the transmitter’s headroom to regulate current decreases, which can cause the current to fail to be maintained properly, especially under load changes or supply variations. Larger resistance also increases power dissipation in the resistor and can introduce more noise in the signal chain.

Other options don’t fit: the burden isn’t a fuse, it doesn’t provide digital signaling, and it doesn’t store calibration data.