Instrumentation · Loop Integrity
Loop Power Supply Calculator
Budget AI card, isolator, IS barrier, cable and other component resistances against your supply voltage to check 4-20 mA transmitter headroom at 20 mA and the 22 mA HART diagnostic peak.
4-20 mA · HART
Worked example
A transmitter loop has a 24 V supply, a 12 V minimum transmitter voltage, and a single 250 Ω DCS analog input card — no isolator, barrier, or extra cable resistance.
Total load = 250 Ω → Max allowable = 600 Ω → V at Tx = 19 V → Headroom = 7.000 V — PASS (41.7% of budget used)
The form above is pre-filled with these exact numbers — press Calculate to reproduce this result, or change the inputs for your own loop.
∑Formula & variable legend
Rtotal = RAI + Riso + Rbarrier + Rcable + Rother
Rmax = (Vsupply − VTx,min) / I
VTx = Vsupply − Rtotal × I
Headroom = VTx − VTx,min
RAI, Riso, Rbarrier, Rcable, Rother — component resistances, Ω, from device datasheets
I — loop current: 0.020 A worst-case, 0.022 A at HART diagnostic peak
Rmax — maximum total load the supply can drive at the given current
Frequently asked questions
What is a loop power supply calculation?
Every component wired in series on a 4-20 mA loop — the analog input card, an isolator, an intrinsic-safety barrier, cable, and any other burden — adds resistance that drops voltage as current flows. This calculator sums those component resistances into a total load, then checks whether the supply voltage still leaves enough headroom for the transmitter to operate at 20 mA and at the 22 mA HART diagnostic peak.
What resistance values go into the AI card, isolator, and barrier fields?
Use the input impedance figures from each device's datasheet: a typical PLC or DCS analog input card is around 250 Ω, signal isolators commonly run 250–300 Ω, and Zener or galvanic IS barriers vary by model but are often in the 300–600 Ω range. Leave a field at 0 if that component isn't in your loop.
Why does the calculator check maximum load at 22 mA as well as 20 mA?
Smart (HART) transmitters can briefly drive up to 22-24 mA during diagnostics or alarm conditions, above the nominal 20 mA full-scale signal. A component budget that just clears headroom at 20 mA can still collapse when the transmitter signals a diagnostic fault at 22 mA — the maximum allowable load at 22 mA is always lower than at 20 mA, so both are checked.
How is this different from the Instrument Loop Calculator?
This calculator works from a component budget — the fixed resistances of an AI card, isolator, barrier and other devices you already know from datasheets. The Instrument Loop Calculator instead derives cable resistance from physical properties — length, cross-sectional area, and temperature — for the cable-run part of the same headroom question.
What's a safe supply voltage margin for an intrinsically safe loop?
Industrial practice is to size the supply at least 1.3 times the transmitter's minimum operating voltage plus the loop's voltage drop at 20 mA, rather than the bare minimum. That margin absorbs supply sag, connector and terminal resistance, and any components added to the loop later without pushing headroom negative.