AN-300| Application Note
Simple Circuit Detects Loss of 4-20 mA Signal
Simple Circuit Detects Loss of 4-20 mA Signal
Four-to-twenty milliampere current loops are commonly used in the process control industry. They take advantage of the fact that a remote amplifier can be powered by the same 4-20 mA current that it controls as its output signal, thus using a single pair of wires for signal and power. Circuits for making 4-20 mA transmitters are found in the LM10, LM163, and LH0045 data sheets. In general, an expensive isolation amplifier would be required to detect the case of a 4 mA signal falling out of spec (e.g., 3.7 mA) without degrading the isolation of the 4-20 mA current loop. But this new circuit (Figure 1) can detect a loss or degradation of signal below 4 mA, with simplicity and low cost. The LM10 contains a stable reference at pins 1 and 8, 200 mV positive referred to pin 4. As long as the loop current is larger than 4 mA, the I x R drop across the 47.6 resistor, R4, is sufficient to pull the LM10's amplifier input (pin 2) below pin 3 and keep its output (pin 6) turned OFF. The 4-20 mA current will flow through
the LED in the optoiso-
National Semiconductor Application Note 300 Robert A. Pease May 1982
lator and provide a LOW output at pin 5 of the optoisolator. When the current loop falls below 3.7 mA, the LM10's input at pin 2 will rise and cause the pin 6 output to fall and steal all the current away from the LED in the optoisolator. Pin 5 of the 4N28 will rise to signify a fault condition. This fault flag will fly for any loop current between 3.7 mA and 0.0 mA (and also in case of reversal or open-circuit). R1 is used to trim the threshold point to the desired value. CR2 is added in series with the LED to make sure it will turn OFF when the LM10's output goes LOW. (While the LM10 is guaranteed to saturate to 1.2V, the forward drop of the LED in the 4N28 may be as low as 1.0V, so a diode is added in series with the LED, to insure that it can be shut off.) Note that most operational amplifiers will not respond in a reasonable way if the output pin (6) is connected to the positive supply pin (7), but the LM10 was specifically designed and is specified to perform accurately in this "shunt" mode. (Refer to
AN-211 application note, TP-14 technical paper, and the LM10 data sheet.)
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FIGURE 1. Current Loop Fault Detector
AN-300
While you could manually adjust R1 while observing the status of V3 output, this would be a coarse and awkward trim
procedure. Figure 2 shows an improved test circuit which servos the current through the detector circuit, forcing it to be
2002 National Semiconductor Corporation
AN005640
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