Typical Component Resistance in a 4–20 mA Loop
Every device placed in series with a current loop consumes a portion of the supply voltage according to Ohm's Law (V = I × R). When planning a loop or troubleshooting transmitter dropout, sum the resistances of all series elements:
| Loop Component | Typical Resistance (Ω) | Voltage Drop at 4 mA | Voltage Drop at 20 mA | Notes |
|---|---|---|---|---|
| PLC Analog Input Shunt | 250 Ω | 1.00 V | 5.00 V | Standard 1–5 V internal ADC conversion resistance |
| Intrinsic Safety Barrier (Zener) | 150 Ω – 300 Ω | 0.60 V – 1.20 V | 3.00 V – 6.00 V | Consumes significant loop voltage in hazardous areas |
| Galvanic Loop Isolator | 50 Ω – 100 Ω | 0.20 V – 0.40 V | 1.00 V – 2.00 V | Active repeaters reduce total series burden |
| Field Digital Loop Indicator | 25 Ω – 75 Ω | 0.10 V – 0.30 V | 0.50 V – 1.50 V | Loop-powered display mounted near the transmitter |
| HART Modem / Communicator | 250 Ω | 1.00 V | 5.00 V | Required minimum loop resistance for FSK frequency modulation |
| 18 AWG Shielded Cable (500 ft / 150 m) | ~6.5 Ω (loop total) | 0.03 V | 0.13 V | Two conductors (outbound + return) in series |
| 18 AWG Shielded Cable (2000 ft / 600 m) | ~26.0 Ω (loop total) | 0.10 V | 0.52 V | Long-run field wiring across industrial plant |
Why Loops Fail at 20 mA but Seem Normal at 4 mA
The most confusing failure mode in industrial instrumentation is a transmitter that calibrates perfectly at zero (4 mA) but freezes or resets when the process rises above 14 mA. At 4.00 mA, a 600 Ω loop burden drops only 2.4 V, leaving 21.6 V across the transmitter. But at 20.00 mA, that same 600 Ω burden drops 12.0 V, leaving only 12.0 V at the transmitter. If the transmitter requires a minimum of 12.5 V to power its sensor excitation circuitry and microprocessor, it starves of power and clips the output at 14 mA. Always calculate and measure loop voltage headroom under maximum 20.00 mA load conditions.
Minimum Headroom Formula
To ensure guaranteed stable operation across the full operating temperature and measurement span, the 24 V DC power supply must satisfy the fundamental inequality:
V_supply ≥ V_transmitter_min + (0.020 A × R_total) + V_safety_margin Where V_safety_margin is typically recommended to be at least 1.5 V to 2.0 V to account for power supply line drift, temperature-induced copper resistance increases, and terminal contact corrosion over years of service.
Related Field Guides & Power Calculations
How the supply, transmitter, and receiving card share one current path.
Prevent elusive control faults by calculating wire and terminal resistance.
Convert process values and loop current with live zero checks.