How to send a 4-20 mA current loop to multiple loads

4-20 mA current loopsare commonly used in the process control industry to transmit analog data. By convention, 4 mA corresponds to the minimum expected reading, and 20 mA corresponds to the maximum expected reading. A 4‑20 mA current signal can be transmitted over long distances, limited only by wire resistance, and is immune to voltage noise.

Problems occur when the same 4-20 mA signal from a transmitter needs to be sent to multiple devices, such as meters, recorders and PLCs, as illustrated above. If the devices are in series in the same loop, they cannot share a common ground, but must be electrically floating. This is often not possible. When any device in the loop is removed, fails or if a wiring fault occurs, all other devices in the loop also loose their 4-20 mA signal. The transmitter voltage compliance limit may be exceeded, since the voltage drops across loads in series are additive. Also, the 4-20 mA signal to each device cannot be individually adjusted for calibration purposes.

Model QLS loop splitter / retransmitter by Laurel Electronics, Inc., now solves these problems by sourcing up to four (4) independently adjustable 4-20 mA outputs from a single input, as illustrated above. The outputs can share a common ground. The input and outputs are mutually isolated to ±10V by means of active circuitry to accommodate differences in local grounds with normal industrial grounding practices. If any device in an output loop is removed from a loop or fails, or if a wiring fault occurs, the other loops continue to operate properly. Each loop only drives a single load, avoiding compliance problems. The input to the quad loop splitter can be 4-20 mA, 1-5V, 0-5V or 0-10V, as selected by jumpers. An excitation output is provided on the signal input side to a drive a 2- or 3-wire transmitter at 24 Vdc and up to 30 mA.

Each output loop has two potentiometers for ±10% fine adjustment of zero and span, a yellow LED lamp to indicate loop continuity, and a test point across a 10-ohm series resistor, where 200 mV corresponds to 20 mA. This allows a multimeter to be used to measure the loop current without breaking the loop.

Model QLS is priced at $200 for 95-264 Vac power and $230 for 10-48 Vdc or 12-32 Vac power. For specifications and the product manual, visit