Short answer
In a two-wire loop, the same current travels through the supply, transmitter, cable, and input load. The transmitter uses part of the available voltage to operate while controlling the current that represents the measured value.
Quick reference: 24 V DC Current Loop Power Budget & Voltage Drop Breakdown
| Series Component in 4–20 mA Loop | Nominal Resistance (Ω) | Voltage Drop at 4 mA (0%) | Voltage Drop at 20 mA (100%) | Operating Impact |
|---|---|---|---|---|
| Two-Wire Transmitter (Internal Electronics) | Dynamic (equivalent) | 10.50 V – 12.00 V (Fixed Min) | 10.50 V – 12.00 V (Fixed Min) | Minimum compliance voltage required for internal circuitry |
| PLC Analog Input Sampling Shunt | 250 Ω ±0.1% | 1.00 V DC | 5.00 V DC | Translates loop current into 1.0 V to 5.0 V ADC input voltage |
| Intrinsic Safety Zener Barrier | 250 Ω – 350 Ω | 1.00 V – 1.40 V DC | 5.00 V – 7.00 V DC | Essential in hazardous Ex areas; consumes major voltage budget |
| Local Digital Loop Indicator | 50 Ω – 100 Ω | 0.20 V – 0.40 V DC | 1.00 V – 2.00 V DC | Optional field readout; adds series burden |
| 500 ft (150 m) 18 AWG Twisted Pair | ~6.5 Ω (loop path) | 0.03 V DC | 0.13 V DC | Minor voltage loss in typical plant runs |
| 2,500 ft (750 m) 18 AWG Twisted Pair | ~32.5 Ω (loop path) | 0.13 V DC | 0.65 V DC | Significant voltage drop; must be factored into total budget |
What to look for
The loop must leave enough voltage at the transmitter at the highest required current. Cable resistance, barriers, isolators, and input resistance all consume part of the budget.
A loop that works at room temperature but fails at the top of the range may be running out of voltage headroom. Measure the voltage at the transmitter, not only at the power supply.
When the symptom appears in the plant
A pressure transmitter loop included the field device, a local digital indicator, and the PLC input card. The PLC constantly reported a 'Wire Break' error, but the transmitter seemed to function when connected directly to a calibrator. The technician calculated the total loop resistance and found it exceeded the 600-ohm limit of the 24V power supply. The local indicator added too much burden. Upgrading to a higher compliance voltage supply solved the problem.
The Transmitter Bench Supply Calibration Deception
A transmitter calibrated on an instrumentation workshop bench powered by an isolated 24.0 V DC bench power supply with zero external series resistance will behave flawlessly. When that same device is installed in the field—where 250 Ω of PLC resistance, a 300 Ω intrinsic safety barrier, and 1,000 feet of field cable introduce over 580 Ω of total loop burden—the loop voltage drops by 11.6 V at 20 mA. If the cabinet supply sags to 22.0 V, the transmitter receives only 10.4 V, starving its sensing amplifier. The technician will observe the transmitter working normally from 4 mA to 15 mA, and then mysteriously freezing or clipping at exactly 15.2 mA. Always test and verify transmitter calibration with the full operational field loop resistance in circuit.
Five checks before replacement
- Calculate the total loop resistance by summing the resistance of the wiring, the PLC input card (typically 250 ohms), and any other loop devices.
- Determine the maximum allowable loop resistance based on the power supply voltage (e.g., for a 24V supply and 12V transmitter minimum, (24-12)/0.02 = 600 ohms max).
- Measure the voltage across the transmitter terminals while forcing a 20mA signal; it must remain above the manufacturer's specified minimum operating voltage.
- Verify the power supply capacity; it must provide enough current for all loops on the circuit simultaneously (assume 20mA per loop).
- Check for 'hidden' resistance sources, such as corroded terminals, intrinsic safety barriers, or very long cable runs of thin gauge wire.
Always check the burden voltage of passive loop displays or chart recorders; they often drop 3-5 volts from the loop.
A 250-ohm precision resistor is commonly used at the PLC to convert a 4-20mA current loop into a 1-5V voltage signal.
When using intrinsically safe barriers, remember they add significant resistance to the loop.
Bench checklist
- Calculate total loop resistance: R_loop = R_receiver + R_barrier + R_cable_roundtrip.
- Verify transmitter minimum operating voltage (typically 10.5 V to 12.0 V DC) under maximum 20.0 mA loop current.
- Measure power supply voltage under full load to ensure it does not sag below 23.5 V DC.
- Check that no secondary devices in the loop are sharing an unisolated common ground.
- Confirm whether the transmitter is a 2-wire loop-powered (passive) or 4-wire externally-powered (active) device.
Reference families
- ISA-50.1 Standard
- Instrument Loop Design Guidelines
Always verify the current edition and the exact device manual for the installation.
Official references
These links point to standards bodies, industry organizations, or manufacturer documentation. Standards landing pages may require purchase or institutional access.
- IEC 60381-1: Analogue direct-current signals IEC
Official standard landing page for analog DC signals in process-control systems.
- 4 to 20 mA Current Loop Configurations BAPI
Manufacturer application note with two- and three-wire loop-power arrangements.
- What Is a 4-20 mA Current Loop? Fluke
Explains supply, voltage drops, resistance, and loop power budget.
- AN-1312: 4 mA to 20 mA DC Measurements Analog Devices
Application note covering summed voltage drops and loop-powered transducers.