Instrumentation & Troubleshooting
4–20 mA Loop Load / Voltage Check
Check whether a two-wire loop supply can maintain the selected current through the external series load while preserving the transmitter's required terminal voltage.
Field use
Useful when a loop will not reach 20 mA or HART communication is unstable.
A two-wire transmitter needs enough terminal voltage after the current-driven voltage drop across the rest of the series loop. Excessive receiver, cable, barrier or indicator resistance can consume the available compliance voltage.
Calculation basis
Vtx = Vsupply − Vload
Rmax = (Vsupply − Vtx,min) ÷ I
The calculation uses Ohm's law and treats the external devices as a series load. Product-specific load lines can use a stated maximum current greater than 20 mA; enter that current when checking worst-case design margin.
Do not use this generic calculation in place of the transmitter's published load-limitation graph, intrinsic-safety entity/system requirements, barrier documentation, or approved loop drawing.