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4–20 mA Loop Transmitter

Calculate 4–20 mA current loop voltage budget, sensor value from current, and maximum loop resistance for industrial sensor transmitters.

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Formula

I = 4 + 16 × (X − X_min)/(X_max − X_min) mA

I— Loop current (mA)
X— Process variable (eng units)

How It Works

The 4–20 mA current loop is the dominant standard for industrial process control sensor transmission. A transmitter converts a process variable (pressure, temperature, flow, level) into a proportional current in the range 4–20 mA. The current is constant anywhere in the series loop, making it immune to voltage drops in long cable runs (unlike voltage signals). The live zero (4 mA at zero input, not 0 mA) allows detection of a broken wire (0 mA) versus a genuine zero reading. Mapping is linear: I = 4 + 16 × (X − X_min)/(X_max − X_min) mA, where X is the process variable. The receiver converts current to voltage across a burden resistor (typically 250 Ω, giving 1–5 V) for ADC input. The voltage budget requires: V_supply ≥ V_transmitter_min + I × R_loop_total. With 24 V supply and 12 V minimum transmitter voltage, maximum loop resistance is (24 − 12)/0.02 = 600 Ω, enough for ~300 m of 24 AWG cable. HART (Highway Addressable Remote Transducer) protocol superimposes a ±0.5 mA FSK signal on the 4–20 mA current without disturbing the process signal, enabling digital configuration and diagnostics.

Worked Example

Problem
A pressure transmitter is calibrated for 0–500 kPa (4–20 mA). The PLC input uses a 250 Ω burden resistor. The 24 V supply feeds 120 m of 22 AWG cable (0.054 Ω/m × 2 = 10.8 Ω). What is the transmitter voltage at 200 kPa?
Solution
1. Loop current at 200 kPa: I = 4 + 16 × (200/500) = 4 + 6.4 = 10.4 mA 2. Loop resistance: R_loop = 250 (burden) + 10.8 (cable) = 260.8 Ω 3. Voltage across loop: V_loop = 10.4×10⁻³ × 260.8 = 2.71 V 4. Transmitter voltage: V_tx = 24 − 2.71 = 21.29 V (well above 12 V minimum ✓) 5. ADC input: V_adc = 10.4×10⁻³ × 250 = 2.60 V → 200 kPa Result: 10.4 mA at 200 kPa, 2.60 V at ADC, 21.3 V available for transmitter.

Practical Tips

  • ✓Use 24 AWG or heavier twisted-pair cable with overall shield for 4–20 mA runs; ground the shield at one end only to avoid ground-loop currents.
  • ✓Add a transient protection diode (e.g., 1N4007) in parallel with the burden resistor to clamp inductive spikes from cable disconnection.
  • ✓For HART-enabled transmitters, the minimum load for HART communication is 230 Ω; always use at least 250 Ω burden to ensure HART FSK signals are detectable.

Common Mistakes

  • ✗Connecting multiple receivers in series and forgetting to sum their burden resistances — if two 250 Ω inputs are in series, total burden is 500 Ω, halving the maximum cable resistance allowance.
  • ✗Using the 4 mA current for 'zero' rather than 'fault' detection — 4 mA represents zero process input; a true wire-break outputs 0–1 mA, which is the fault indication.
  • ✗Measuring 4–20 mA with a voltmeter across the loop — you must measure across a known burden resistor; measuring the loop open-circuit voltage gives the supply voltage, not the signal.

Frequently Asked Questions

4 mA live zero serves three purposes: it powers two-wire transmitters (the transmitter draws 4 mA minimum from the loop to operate), it distinguishes a genuine zero reading from a wiring fault (0–1 mA indicates open circuit or shorted transmitter), and it allows transmitters on a single 24 V supply without separate power wiring.
It depends on cable resistance and required transmitter voltage. With a 24 V supply, 250 Ω burden, and 12 V minimum transmitter voltage: maximum external resistance = (24 − 12)/0.02 = 600 Ω. Subtracting the 250 Ω burden leaves 350 Ω for cable. At 0.11 Ω/m for 22 AWG, this allows 350/0.11 = 3180 m of cable. Practical limit is typically 1000–1500 m with good cable.
HART modulates a ±0.5 mA FSK signal at 1200 bps onto the 4–20 mA DC current using Bell 202 modulation (mark = 2200 Hz, space = 1200 Hz). The average value of the AC component is zero, so the process variable reading is unaffected. The burden resistor of at least 230 Ω converts the current modulation to a voltage that the HART modem can detect and demodulate.

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