DIN-Rail Signal Isolator

A signal isolator sits between a field instrument and the control system and passes the signal through a magnetic barrier. Its purpose is not to change the value but to break the electrical path: a difference in earth potential between two ends of a plant, or a transient coupled onto a long cable, travels along a conductor, and a signal that is transferred without a conductor cannot carry either one through.

The modules on this page cover current and voltage loops from 4-20 mA and 0-20 mA through 0-5 V, 1-5 V and 0-10 V. They are 13 mm wide and mount on standard 35 mm DIN rail, which allows a group to be built in the depth of a cabinet rather than in the width. Isolation is 2000 Vrms between input, output and supply, transmission accuracy is 0.1%FS at 20°C, and response time is 10 ms or less.

Two versions are offered. The isolator passes a signal from a device that already has its own supply. The distributor supplies a two-wire or three-wire transmitter from the same module and isolates the signal on its return, which is the arrangement used when the transmitter is powered over the loop it measures.

How a signal isolator works

The isolation is magnetic rather than optical on these modules. The input signal is converted to a form that can cross a magnetic barrier, transferred, and reconstructed on the other side as the same proportional signal. Because the two sides share no conductor, a potential difference between them has no path to drive a current through the signal conductor — which is the mechanism by which an earth loop adds an offset to a reading that is otherwise correct.

Suppression of common-mode interference follows from the same arrangement. A voltage that appears on both conductors of the input pair, such as an induced voltage from a cable tray shared with a drive, is a differential of zero at the receiving end. A filter reduces such a signal; an isolation barrier removes the route by which it reached the measurement in the first place, which is why a filtered signal that still drifts when a nearby drive runs is a case for an isolator rather than a bigger filter.

The distributor version extends this to transmitters that take their power from the loop. It supplies 19 to 25 V to the loop at 30 mA or less and returns the signal through the same isolation barrier. Because the supply and the measurement are on the same module, the loop has one reference instead of two and the wiring between them disappears.

How to select the right module

Establish first whether the field device has its own supply. That single question separates the two versions: a two-wire transmitter needs the distributor, and a separately powered transmitter or source needs the isolator. Getting this wrong is not a matter of accuracy, because a two-wire transmitter connected to an isolator is simply unpowered.

Then fix the signal forms. The input and the output have to be chosen together, because the output can only be the proportional form of the input: a 4-20 mA input gives 4-20 mA or 1-5 V, and there is no setting that turns it into 0-10 V. Current and voltage outputs are not interchangeable in the field either; the change requires different hardware, so the form decided at the enquiry is the form that arrives.

Then check the load. A current output drives 400 Ω or less. A voltage output needs its load above a minimum, from 250 kΩ at 0-5 V and 1-5 V to 500 kΩ at 0-10 V, and a load below that takes the output outside its stated accuracy.

Finally settle the channels and the supply. One-in one-out, one-in two-out and two-in two-out forms are available, and the two-out form is what allows one field signal to reach a control system and a recorder without a second transmitter. The modules run from 20 to 32 VDC, and they are best kept off a supply shared with a drive.

Limitations and points to confirm

An isolator transfers a signal; it does not improve it. A measurement that is already inaccurate at the transmitter arrives inaccurate at the other end of the barrier, and the 0.1%FS figure describes what the module adds, not what the loop as a whole achieves. Where a loop has to be reconciled, the transmitter and the receiving card are the places to look first.

These modules are rated for a temperature range of -10 to 50°C and a humidity range of 25 to 85 %RH, without condensation. They are not sealed against washdown and the front face is not described as ingress protected, so they belong in a cabinet rather than on the machine. Vertical mounting helps the internal heat escape, and the modules need a gap when installed side by side.

Three items in the supplied material are incomplete and are published as such. The power consumption figures are assigned to the two versions in opposite ways in the two tables. Two different EMC references are cited for the same product. And one table leaves the response time cell blank. Where an application has to be documented against a particular standard or a particular consumption figure, tell us and we will confirm which reference applies rather than choosing one here.

The supplied document does not state whether an intrinsically safe version exists. A loop that enters a hazardous area is therefore a case to confirm before it is ordered rather than a case this page can answer.