Add Fault Protection to 4-20mA Loop Supply
Figure 1. Basic architecture of a 4-20mA current loop.
Many such applications require current limiting or fault protection or both. For example, a short circuit or other 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 size the power supply accurately without over-specifying it.
Figure 2 shows one form of flexible fault protection for the 24VDC power supply of a 4-20mA loop. Also included is circuitry for recovering a digital signal superimposed on that loop. U1 (a high-side current-sense amplifier with comparator and reference) senses the loop current in R1 as an 8-40mV voltage and amplifies it by 100, producing an output-voltage range of 0.8V to 4V. That output (VOUT) can directly drive external meters, strip-chart recorders, and A/D converter inputs.
Figure 2. This circuit provides fault protection and digital-signal recovery for a 4-20mA current loop.
The selected fault-current trip point (for U1's internal comparator #1) is set by the R2-R3 voltage divider at 0.6V. Setting the trip point for a 50mA fault, for instance, establishes the following relationship between R1 and R2:
R2/(R1 + R2) = 0.6V/(R1*100*IFAULT) ==> R1 = 15.67*R2.
When faults occur, the COUT1 output assumes a high impedance and is pulled high by R10. The noninverting cascaded-transistor pair Q2-Q3 provides an interface to the high loop voltage while preserving a proper logic polarity for controlling the gate of Q1. Q1 is held in the OFF state until U1's comparator #1 is reset by the pushbutton PB1 or other reset signal. (To disable this comparator's latched output, tie the RESET# pin to ground.) Zener diode ZD1 protects Q1's gate-source junction from overvoltage.
U2 and associated circuitry can recover any digital information imposed on the 4-20mA loop current by modulation. The HART protocol, for instance, typically uses frequency-shift keying (FSK) between 1200Hz and 2400Hz to modulate the loop current between ±0.5mA levels. (For this circuit, the modulated signal at VOUT (pin 2 of U1) is ±0.1V.) VOUT from U1 is capacitively coupled to U2 and amplified by that device to recover such digital signals.
U1 includes a second comparator with inverting input, which can be used to cancel the inversion in U2's digital-signal output. Though not essential, this comparator output (COUT2) can also (as shown) present the recovered digital signal as a clean rectangular waveform for driving external circuitry.