Skip to content
RFrftools.io
Sensor

4–20 mA Loop Transmitter

Calculate 4-20 mA current loop voltage budget, sensor value from loop current, and maximum loop resistance for industrial transmitter design.

Loading calculator...

Formula

I=4+16×(X−Xmin)/(Xmax−Xmin)mAI = 4 + 16 × (X − X_min)/(X_max − X_min) mA
I— Loop current (mA)
X— Process variable (eng units)

How It Works

This calculator designs 4-20 mA current loop transmitter and receiver circuits, essential for process control engineers, industrial automation integrators, and instrumentation technicians. The 4-20 mA current loop is the dominant standard for analog sensor transmission in industrial environments per ISA-50.00.01 and IEC 60381-1. A transmitter converts a process variable (pressure, temperature, flow, level) to proportional current: I = 4 + 16 * (X - Xmin)/(Xmax - Xmin) mA. The live zero (4 mA at zero input, not 0 mA) enables broken-wire detection (0-1 mA = fault) and powers 2-wire transmitters. Current is constant throughout the series loop, immune to voltage drops over cable runs up to 3000 m with 24V supply. The receiver converts current to voltage across a burden resistor (typically 250 Ohm for 1-5V output). Maximum loop resistance = (Vsupply - Vtx_min) / 20 mA; with 24V supply and 12V minimum transmitter voltage, R_max = 600 Ohm. HART protocol (IEC 62591) superimposes +/-0.5 mA FSK at 1200 bps for digital communication without disturbing the analog signal.

Worked Example

Problem

Design signal conditioning for a Honeywell STD800 pressure transmitter (0-1000 kPa, 4-20 mA output) in a refinery. Supply is 24V DC, cable is 500 m of 18 AWG, PLC input has 250 Ohm burden.

Solution
  1. Cable resistance: 18 AWG = 20.9 Ohm/km 0.5 km 2 (round trip) = 20.9 Ohm
  2. Total loop resistance: R_loop = 250 (burden) + 20.9 (cable) = 270.9 Ohm
  3. Voltage at 20 mA: V_loop = 0.020 * 270.9 = 5.42V
  4. Transmitter voltage: V_tx = 24 - 5.42 = 18.58V (>12V minimum, OK)
  5. At 600 kPa: I = 4 + 16 * (600/1000) = 13.6 mA
  6. ADC input: V_adc = 13.6 mA * 250 Ohm = 3.40V
  7. PLC scaling: 4 mA = 0 kPa = 1.0V; 20 mA = 1000 kPa = 5.0V
  8. Resolution with 12-bit ADC (0-5V): 5V/4096 / 4V * 1000 kPa = 0.31 kPa/LSB
Result: 13.6 mA at 600 kPa produces 3.40V at PLC input. System has 18.58V available for transmitter, well above 12V minimum.

Practical Tips

  • ✓Use 18-24 AWG twisted-pair cable with overall shield for 4-20 mA runs; ground the shield at the control room end only to avoid ground-loop currents per ISA-RP12.06.01 installation practices
  • ✓Add transient protection (TVS diode or gas discharge tube) at both ends of long cable runs; lightning-induced surges can exceed 1000V and damage transmitters and PLC inputs per IEC 61643-21
  • ✓For HART-enabled transmitters, ensure minimum 230 Ohm load for communication; if burden is <230 Ohm, add external 250 Ohm resistor in parallel with HART modem per HART Foundation specification

Common Mistakes

  • ✗Connecting multiple receivers in series without summing burden resistances: two 250 Ohm inputs in series = 500 Ohm burden, halving maximum cable resistance allowance; verify total loop resistance stays below (Vsupply - Vtx_min)/20 mA
  • ✗Interpreting 4 mA as 'fault' instead of 'zero': 4 mA represents zero process input per ISA-50.00.01; fault condition is <3.6 mA (NAMUR NE43 defines 3.6-3.8 mA as under-range, <3.6 mA as sensor failure)
  • ✗Measuring 4-20 mA with voltmeter across open loop: inserting a high-impedance voltmeter breaks the current path; measure voltage across known burden resistor (V = I * R_burden) or use clamp-on mA meter

Frequently Asked Questions

The 4 mA live zero serves three critical functions per ISA-50.00.01: (1) powers 2-wire transmitters - the transmitter operates on the loop current it regulates, requiring minimum 3.5-4 mA to function, (2) enables broken-wire detection - a reading of 0-3.6 mA indicates open circuit or failed transmitter versus genuine zero process value, (3) allows transmitters on single 24V supply without separate power wiring. NAMUR NE43 defines: <3.6 mA = sensor failure, 3.6-3.8 mA = under-range, 20.5-21 mA = over-range, >21 mA = output saturated.
Maximum length depends on cable resistance and transmitter voltage requirement. With 24V supply, 250 Ohm burden, 12V minimum transmitter voltage: R_cable_max = (24 - 12 - 250*0.020)/0.020 = 350 Ohm. For 18 AWG (20.9 Ohm/km), max length = 350/20.9/2 = 8.4 km round-trip = 4.2 km one-way. Practical limit with margin for connectors and temperature variation is typically 2-3 km. For longer runs, use 4-20 mA to digital converters (HART, Modbus) at the field end per ISA-50.00.01.
HART (Highway Addressable Remote Transducer per IEC 62591) modulates a +/-0.5 mA peak FSK signal onto the DC current using Bell 202 frequencies (1200 Hz = '1', 2200 Hz = '0') at 1200 bps. The AC component averages to zero over each bit, so it does not affect the 4-20 mA process reading. The burden resistor (minimum 230 Ohm per HART spec) converts current modulation to voltage that a HART modem can detect. HART enables remote configuration, diagnostics, and secondary variables while maintaining backward compatibility with analog-only receivers.

Related Calculators

Sensor

Current Shunt

Calculate shunt resistor voltage drop, power dissipation, amplifier output, and ADC resolution for current sensing circuits. Free, instant results.

Sensor

Sensor Accuracy

Calculate total sensor error using RSS and worst-case methods. Analyze offset, gain, nonlinearity, resolution, and temperature drift contributions.

Sensor

Load Cell Amp

Calculate load cell output voltage, required amplifier gain, and mV/V sensitivity for Wheatstone bridge weigh scale and force sensor designs.

Sensor

NTC Thermistor

Calculate temperature from NTC thermistor resistance using the Steinhart-Hart beta equation. Get Kelvin and Celsius outputs for sensor circuit design.