Instrumentation Signal · Free Calculator
4 to 20 mA to Voltage Resistor Calculator
Size the shunt resistor that converts a 4 to 20 mA loop into a voltage for a PLC voltage input, data logger or microcontroller ADC.
Shunt resistor1 to 5 VADC inputResistor power
V
mA
V
V
Maximum shunt resistance238.1 Ω
Suggested standard value220 Ω (0.1 % precision)
Voltage at 4 mA0.88 V
Voltage at 20 mA4.4 V
Resistor power at max current97 mW
HART (needs ≥ 230 Ω)add series resistance
Loop voltage checkOK
How this result was calculated
- R ≤ Vfull scale ÷ Imax = 5 ÷ 0.021 = 238.1 Ω
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Shunt Resistor Formula
V = I × R · R ≤ Vfull scale ÷ Imax
250 Ω gives the classic 1 to 5 V
250 Ω gives the classic 1 to 5 V
Worked example
For a 0 to 5 V ADC with up to 21 mA, R ≤ 238 Ω; a 220 Ω precision resistor gives 0.88 to 4.4 V for 4 to 20 mA.
Key insight: the shunt tolerance and temperature coefficient become measurement error directly: use a 0.1 % metal film resistor, not a 5 % carbon one.
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Frequently Asked Questions
What resistor converts 4 to 20 mA to 1 to 5 V?
250 Ω.
What resistor for a 0 to 3.3 V ADC?
At most about 157 Ω for 21 mA; 150 Ω gives 0.6 to 3.0 V.
Does the shunt affect the loop?
Yes, it adds burden; check the transmitter still gets its minimum voltage at maximum current.
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