Yokogawa and Emerson Loops: Mastering 4-20 mA Voltage Drop and HART Headroom
A two-wire transmitter can work at low loop current yet lose voltage margin near the top of its range. A loop voltage budget helps explain the symptom before replacing a Yokogawa or Emerson instrument.
Why can a loop fail near 20 mA but work at 4 mA?
The voltage dropped across series resistance grows with current: V = I × R. If the supply is 24 V and the effective series resistance is 300 Ω, the resistive drop at 20 mA is 6 V, leaving 18 V for the transmitter before accounting for other voltage drops. At 4 mA the same resistance drops 1.2 V. The example demonstrates the method, not the minimum operating voltage of a particular device.
Do Yokogawa EJX and Rosemount 3144 have the same voltage requirements?
No single figure should be applied across brands, model revisions, communication modes, indicators, and hazardous-area options. Read the exact transmitter manual and its load-versus-supply specification. HART communication also needs a compatible loop impedance and sufficient terminal voltage; an analog reading that survives does not prove digital communication margin. Do not assume 18.1 V is the HART requirement of every Rosemount 3144 or that all Yokogawa EJX variants share a 10.5 V minimum.
How do you build a voltage budget?
Measure supply voltage under operating load. List cable and terminal resistance, analog input burden, indicators, and any isolator or safety barrier. Some components have a specified voltage drop rather than a fixed resistance, so use their published data instead of putting every item into I × R. Calculate the worst relevant loop current, including configured fault current where applicable. Subtract all series drops and compare the remaining transmitter terminal voltage with the exact model's requirement, then add margin appropriate to the site design. Record the assumptions on the loop sheet.
What about the HART resistor and analog input card?
A 250 Ω load is commonly used to support HART communication, but some input cards already provide the required impedance or have built-in HART circuitry. Removing a resistor or swapping to a low-burden card without checking the communication requirements may make HART less reliable, not more. Products such as the Yokogawa AAI143-S03 HART analog input module and Emerson Ovation HART analog input module illustrate receiving-side hardware; confirm each installed module's burden, HART support, and wiring before changing the circuit.
How should a fading loop be tested?
Under an approved maintenance procedure, measure voltage directly at the transmitter terminals while observing the loop current and device status at several operating points. Compare the measured voltage at the highest relevant current with the transmitter manual. If voltage is low, measure individual drops across the input, barrier or isolator, cable, and terminals to locate the loss. Never drive a live process or bypass a protective loop just to create a 20 mA test condition; use a safe simulation or planned outage when required.
What belongs in the commissioning record?
Keep the supply measurement, current and voltage readings, input and barrier specifications, cable length, transmitter model and options, HART communication check, and the final voltage budget. Recalculate after any device or input-card change. This prevents an apparently harmless retrofit from consuming the margin needed at full-scale current.
Further reading: InstrumentationTools.com, “Loop-powered 4-20 mA Transmitter Circuit Voltage Drop.” Use the installed manufacturer documentation for actual voltage and load limits.
Author: Xu Jianguo is an industrial automation engineer with over 10 years of experience in PLC, DCS, and control systems.
