Battery-operated remote-temperature sensor drives 4- to 20-mA current loop

-December 17, 2004

You can remotely measure temperature using a 4- to 20-mA current loop as long as 4000 feet and a battery-powered, white-light LED driver. You usually configure this equipment to provide a programmable, constant current to an LED from a battery source. The TPS62300 series of ICs, for example, converts a battery voltage of 2.7 to 6.5V into a constant current, which you program using an external resistor and voltage on its ISET pin. The current that normally drives the LED instead powers the loop (Figure 1).

In the sample circuit, which occupies 50 mm2, the LED driver drives the 4- to 20-mA current loop proportionate to a sensed temperature of –10°C at 4 mA and 50°C at 20 mA. The driver applies 0.6V to the ISET pin and monitors current flow from the pin. This current is multiplied by 260 and mirrored to the LED drive output:

Because resistor RISET, which is tied to the ISET pin, is fixed in the example, the output current is proportional to the voltage, VISET, which the output of op amp IC3 determines. Using a 6.49-kΩ resistor for RISET means that VISET needs to be 0.1V to provide 20 mA of loop current and 0.5V to provide 4 mA.

The TMP36 temperature sensor, IC1, provides 750 mV of output at 25°C and varies its output voltage by 10 mV/°C. The output of the TMP36 is 0.4V at –10°C and 1V at 50°C. Because these voltages do not directly match the voltage requirements of VISET, you use a REF2912 voltage reference, IC2, with the OPA374 op amp to scale the output of the TMP36 to the required voltage for the LED driver, IC4. In general terms, the current in the current loop for the circuit is:

Substituting for the component values shown in the figure yields:

The output of the LED driver can drive loops with as much as 180Ω of resistance with battery voltages as low as 2.7V. Therefore, the LED driver can drive more than 1500 feet of 24 AWG or 4000 feet of 20 AWG twisted-pair wire with a 100Ω load resistor at the receiver. You can achieve much longer distances with higher battery voltages. Because this circuit powers the current loop, the battery life for these circuits depends on the measured temperature. For the circuit shown, a loop current of 13.3 mA corresponds to a measured temperature of 25°C. Therefore, using two AA alkaline batteries in series should provide more than 120 hours of remote-temperature monitoring at room temperature. The accuracy for the circuit is about 2.5% of full scale without any calibration. For tighter accuracy, reduce the range of the measured temperature or calibrate the output.

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