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Add fault protection to a 4- to 20-mA loop supply

-January 22, 2004

A 4- to 20-mA current loop consists of a power source and a current-measuring device at the control end and a field transmitter that senses process-variable information, such as temperature or pressure, and converts it to a current (Figure 1). Most such industrial current loops are powered by 24V dc, but that voltage can range from 12 to 36V. The loop voltage in older systems can be even higher. Many such applications require current limiting, fault protection, or both. For example, a short circuit or another high-current fault in one of several loops powered by a single source can produce a power-supply failure that disables all transmitters powered by that source. Intrinsically safe loops, on the other hand, include a barrier module that limits current and voltage to the transmitter. Fault-protected sources can add another level of system safety. Setting a current limit on each loop lets you accurately size the power supply without overspecifying it. Figure 2 shows one form of flexible fault protection for the 24V power supply of a 4- to 20-mA loop. It also includes circuitry for recovering a digital signal superimposed on that loop. IC1, a high-side current-sense amplifier with comparator and reference, senses the loop current in R1 as an 8- to 40-mV voltage and amplifies it by 100, producing an output-voltage range of 0.8 to 4V. That output, VOUT, can directly drive external meters, strip-chart recorders, and A/D-converter inputs.

The R2-R3 voltage divider sets the selected fault-current trip point for IC1's first internal comparator at 0.6V. Setting the trip point for a 50-mA fault, for instance, establishes the following relationship between R1 and R2: R2/(R1+R2)=0.6V/(R1×100×IFAULT), so R1=15.67×R2. When faults occur, the COUT1 output assumes a high-impedance state and is pulled high by R3. The noninverting cascaded-transistor pair Q2-Q3 provides an interface to the high loop voltage and preserves a proper logic polarity for controlling the gate of Q1. Q1 is held in the off state until pushbutton PB1 or another reset signal resets IC1's first comparator. (To disable this comparator's latched output, tie the Reset# pin to ground.) Zener diode ZD1 protects Q1's gate-source junction from overvoltage.

IC2 and its associated circuitry can recover any digital information imposed on the 4- to 20-mA loop current by modulation. The Highway-Addressable Remote Transducer Protocol, for instance, typically uses FSK (frequency-shift keying) of 1200 to 2400 Hz to modulate the loop current between the ±0.5-mA levels. (For this circuit, the modulated signal at VOUT (Pin 2 of IC1) is ±0.1V.) VOUT from IC1 is capacitively coupled to IC2 and amplified by that device to recover such digital signals. IC1 includes a second comparator with inverting input, which you can use to cancel the inversion in IC2's digital-signal output. Though not essential, this comparator output (COUT2) can also present the recovered digital signal as a clean rectangular waveform for driving external circuitry.

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