Maxim > Design Support > Technical Documents > Application Notes > Circuit Protection > APP 3195
Maxim > Design Support > Technical Documents > Application Notes > Interface Circuits > APP 3195
Keywords: 4-20mA current loop, fault protection, current limiting, current-sense amplifiers
Add Fault Protection to 4-20mA Loop Supply
Jul 08, 2004
Abstract: This circuit provides flexible fault protection for a 4-20mA current loop. In addition, it includes
circuitry for recovering digital signals (such as the HART protocol) imposed on the loop.
4-20mA current loops consist of a power source and current-measuring device at the control end, and a
field transmitter that senses process-variable information (like temperature or pressure) and converts it to
a current (Figure 1). Most 4-20mA industrial loops are powered by 24VDC, but that voltage can range
from 12V to 36V. The loop voltage in older systems can be even higher.
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 currentsense 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.
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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
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.
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Free Samples Page 2 of 3
More Information
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Application Note 3195:
APPLICATION NOTE 3195, AN3195, AN 3195, APP3195, Appnote3195, Appnote 3195
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