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Signals and PLC · Glossary

What is Loop Resistance?

maximum load in a 4 to 20 mA loopSignals and PLC

Loop resistance is the total resistance in a 4 to 20 mA loop, from cable, input resistors and barriers, which must leave enough voltage for the transmitter.

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Loop Resistance Explained

At full current each resistance drops voltage. If the total drop is too large, the transmitter lacks voltage and the signal cannot reach 20 mA.

Check the worst case: lowest supply voltage, highest current including fault current, and longest cable.

Loop Resistance Formula

Rmax = (Vsupply − Vtx,min) ÷ Imax
Vtx,minminimum transmitter terminal voltage
Imaxhighest current, often about 22 mA
Example: 24 V supply, 10.5 V minimum: Rmax = 13.5 ÷ 0.022 ≈ 614 Ω.
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Calculators for Loop Resistance

CalculatorLoop Power Supply CalculatorFree online calculator with formula and worked example.CalculatorCable Loop Resistance CalculatorFree online calculator with formula and worked example.

Free Downloads

Free download4 to 20 mA Loop Calculation Excel SheetFree download in Excel and PDF.Free downloadCable Sizing Excel SheetFree download in Excel and PDF.

Frequently Asked Questions

What is Loop Resistance?

Loop resistance is the total resistance in a 4 to 20 mA loop, from cable, input resistors and barriers, which must leave enough voltage for the transmitter.

Why does HART need at least 250 Ω?

The HART signal is a current variation that needs a resistance to create a readable voltage.

What adds to loop resistance?

Cable, the input resistor, IS barriers, isolators, indicators and fuses.

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