Transmitter 4–20 mA

A transmitter continuously measures, for example, the level in a tank, a pressure or a temperature, and converts the measured value into a current. The start of the measuring range is 4 mA and the end is 20 mA. A switching sensor only tells whether a limit has been passed. A transmitter gives the value all the time, so the limits can be set in the program.

Two-wire connection

A two-wire (loop-powered) transmitter has two terminals, + and −. It takes its supply from the same current loop that it signals on:

1. +24 V to the + terminal of the transmitter.
2. From the − terminal of the transmitter to the PLC analogue input.
3. From the analogue input to 0 V.

The transmitter sets the loop current, and the analogue input measures it. The current is the same at every point of the loop, so voltage drop in the wires does not change the signal, as long as the supply voltage is high enough. That is why a current signal suits long cables.

Live zero

The measuring range starts at 4 mA, not at zero. A two-wire transmitter takes its own operating current from the loop, so the loop current cannot fall below that. Also, 0 mA is never a real measured value: it means a fault, for example a broken wire. This is called a live zero.

Scaling

The start of the transmitter's measuring range corresponds to 4 mA and the end to 20 mA. In between, the current changes linearly with the measured value:

I = 4 mA + 16 mA × (value − start) / (end − start)

For example, with a 0–4 m level transmitter, 1 m is 4 + 16 × 1 / 4 = 8 mA and 3 m is 16 mA. Working the other way, 12 mA is the middle of the range, which is 2 m.

NAMUR NE 43 fault limits

Under the NAMUR NE 43 recommendation, the measured value is transmitted in the range 3.8–20.5 mA. A current of 3.6 mA or less, or 21 mA or more, means a fault. A transmitter reports its own fault by driving the current into one of the fault ranges, for example to 3.0 mA or 22 mA. A broken wire shows as a current of 0 mA.

The program checks for a fault before it uses the value. Otherwise a broken wire looks like an empty tank or a low pressure, and a pump may keep running.

Analogue input

The analogue inputs of a PLC are AI1, AI2 and so on. The PLC converts the current into a number, 0–1000 on the LOGO!, and the program scales the number into the measured value with a gain and an offset. The analogue inputs of the LOGO! base module measure only voltage, 0–10 V, so a 4–20 mA transmitter is wired to an analogue expansion module whose current input has a resistance below 250 Ω. Use twisted, screened cable for the analogue signal.

In Johdota the analogue input shows the current directly in milliamperes, not as a number 0–1000, and limit values are worked out in milliamperes with the formula above.