That is, if the control system sees a signal less than 4 mA, it knows a problem occurred. If a transmitter is connected to a control system, it likely has quality alarms on the 4-20 mA input to let operations know when the transmitter has failed. That is not usually a challenge for a 4-wire transmitter, but it can be for a loop-powered transmitter. The total load could then be as high as 525 ohms plus wire resistance. To calculate the total burden on the transmitter, add the isolator load to 275. In other words, the isolator itself looks like a 275 ohm load on the transmitter. To run its electronics, the isolator consumes 5.5 V from the loop. The receiver’s input impedance can be anywhere from 0-250 ohms and it should be a fixed load.Īdditionally, there can be no voltage on the output of the isolator. Because power is limited, the isolator output load is limited to 250 ohms. The transmitter’s 4-20 mA output and its compliance voltage must power the isolator electronics and the isolator’s output. Several rules must be followed if the power source in the loop is from a transmitter. Wiring changes and installation costs are minimal. Just break the loop where convenient and insert the isolator. The beauty of this isolator is its simplicity. However, these are the most misapplied isolators. Its output always has to be some form of 4-20 mA, but signal conversion can still be performed, as can split ranging.Īn isolators that is loop-powered on the input side is a great solution when applied correctly. Loop-powered isolators work just like any other 2-wire DP, pressure or temperature transmitter (Fig. A 4-wire isolator also can be used when the isolator output must be voltage (such as 0-10 V), zero-based (such as 0-20 mA) or bipolar (such as -10 V to +10 V) (Fig. A 4-wire isolator can perform signal conditioning, convert signals, split signals, boost power to the loop and many other functions. Isolators can be 4-wire (line- or mains-powered) transmitters or they can be 2-wire (loop-powered). However, isolators and signal converters can also be used to solve difficult or complex problems. Some engineers think that the need for isolators and signal converters is eroding with the advent of smart instruments, isolated electronics and digital fieldbuses. Isolators can be introduced into the “loop” to solve simple problems such as ground loops or noise. The instrument returns a 4-20 mA signal that typically represents a measurement, such as flow, level, pressure, etc. Isolators and signal converters are most often used on 4-20 mA loops, where a 24 Vdc power source provides power to a field instrument.
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