Instrumentation
4-20 mA shunt resistor selection: 250 ohm and 500 ohm examples
Convert a 4-20 mA loop into a voltage signal with Ohm's Law while checking resistor power, input impedance, loop burden, tolerance, and existing receiver hardware.
- Product
- 4-20 mA current loops
- Level
- Field triage
- Read time
- 9 min
- Reviewed
- 2026-07-18
What to establish before troubleshooting
A resistor placed across a voltage input converts loop current to voltage through V = I x R. A 250 ohm resistor produces 1 V at 4 mA and 5 V at 20 mA; a 500 ohm resistor produces 2-10 V.
The resistor is also part of the loop burden. At 20 mA, 250 ohms consumes 5 V of loop compliance and dissipates 0.10 W, while 500 ohms consumes 10 V and dissipates 0.20 W. The loop supply must still cover the transmitter, wiring, barriers, isolators, and every receiver.
Abbreviated worked example
Check a 250 ohm 1-5 V conversion
A 4-20 mA transmitter is connected to a voltage input through a documented 250 ohm precision shunt resistor.
- 1At 4 mA: 0.004 A x 250 ohms = 1.00 V.
- 2At 20 mA: 0.020 A x 250 ohms = 5.00 V; resistor power is I squared x R = 0.10 W.
Result: The nominal converted signal is 1-5 V, provided the loop has at least 5 V of spare compliance for the resistor.
Caution: Select resistance, tolerance, temperature coefficient, and power rating from the installed device requirements. Do not assume a nominal quarter-watt part provides the needed design margin or measurement accuracy.
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