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Instrumentation·6 min read·

Loop powered transmitters, and the voltage budget nobody calculates

Two wires carrying power and signal, the compliance voltage arithmetic, and why adding a display stops the loop working.

Short answer

A two wire transmitter draws its power from the same loop it modulates, regulating its own consumption between 4 and 20 mA to represent the measurement. That works only if the supply voltage exceeds the transmitter's minimum operating voltage plus the drop across every resistance in the loop at 20 mA. Adding an indicator or a barrier without redoing that sum is the commonest reason a loop works at low readings and fails at high ones.

4-20 mA scaled to engineering unitsbroken loop4 mA20 mA0 bar10 bar3277 / 553016384 / 27648raw counts: AB / Siemens

A two wire transmitter is a small piece of cleverness: it powers itself from the loop it is modulating, so two wires carry both the power and the signal.

That cleverness has one condition, and the condition is where installations fail.

The arithmetic

The loop supply, usually 24 volts, has to cover two things: the voltage the transmitter needs to operate, and the voltage dropped across every resistance in the loop.

Voltage dropped is current times resistance, and current is at its highest at 20 mA. So the worst case is full scale, which is why these loops fail at the top of the range.

Available voltage = supply minus (total loop resistance times 0.020), and that must exceed the transmitter's minimum operating voltage.

A worked example

24 volt supply. A transmitter needing 12 volts minimum. A PLC analogue input with a 250 ohm burden resistor.

250 ohms times 0.020 amps is 5 volts. 24 minus 5 is 19, which comfortably exceeds 12. The loop works.

Now add a panel indicator, also 250 ohms. Total 500 ohms, 10 volts dropped, 14 volts left. Still works, with less margin.

Add an intrinsic safety barrier at 340 ohms. Total 840 ohms, 16.8 volts dropped, 7.2 volts left, and the transmitter needs 12.

It will work at 4 mA, where the drop is only 3.4 volts, and the loop will die somewhere around half scale.

That is why the symptom is a reading that works and then stops working as the process rises. Nothing in the loop is faulty.

What contributes resistance

Everything, and the ones people forget are the ones that break the budget.

  • The input card's burden resistor, typically 100 to 250 ohms.
  • Any indicator, recorder or second input in series.
  • Intrinsic safety barriers or isolators, which are often the largest single contributor.
  • The cable itself. Usually small, and not on a 500 metre run of thin conductor.

Two wire, three wire, four wire

Worth separating clearly because the terms get used loosely.

Two wire, loop powered. Power and signal on the same pair. Simplest wiring, and the arrangement all of the above applies to.

Three wire. Separate supply positive, shared common, signal. Used where the device needs more power than a loop can provide.

Four wire. Separate power pair and separate signal pair. The device is powered independently and sources the current rather than regulating it, so the voltage budget is not a concern.

Wiring a four wire device as though it were loop powered, or the reverse, is a common and immediately obvious failure. The subtle failures are all in the two wire case.

Sinking, sourcing and who powers the loop

The other recurring confusion. Either the analogue input supplies the loop power, an active input, or it expects the loop to be powered elsewhere, a passive input.

Two active devices in one loop fight each other. Two passive devices with no supply give a reading of zero and no diagnostic.

Read which the card is, read which the transmitter is, and draw the loop before wiring it. It takes two minutes and it is the difference between a working loop and an afternoon.

The diagnostic value of the live zero

The reason the range starts at 4 mA rather than 0 is worth restating here.

Zero milliamps is not a valid reading, so it means a broken wire, a dead transmitter or a lost supply. A 0 to 20 mA range cannot distinguish a broken cable from a genuine zero, which is why 4 to 20 became standard and why NAMUR NE 43 pushes it further, with under 3.6 mA and over 21 mA reserved for fault indication.

Common questions

Why does my 4-20 mA loop work at low readings and fail at high ones?
A voltage budget problem. The drop across the loop resistance rises with current, so at 20 mA there may not be enough left for the transmitter to operate. It works at 4 mA and dies as the reading climbs, which looks like a sensor fault and is an arithmetic one.
What is compliance voltage?
The voltage available to drive the loop, minus everything it has to overcome. The rule is supply voltage minus the sum of all loop resistances times 0.020 amps, which must exceed the transmitter's minimum operating voltage. Every device in the loop contributes resistance, including the input card, the indicator and any barrier.
Can I put two inputs on one 4-20 mA loop?
Yes, in series, provided the voltage budget allows. Current is the same everywhere in a series loop, so each input sees the same signal. What you cannot do is wire them in parallel, which splits the current and gives both a wrong reading.

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