Electrically Isolated Precision Temperature Sensor
When creating a precise outdoor temperature sensor, it’s crucial to ensure electrical isolation between the sensor and the signal conditioning circuitry to safeguard it against voltage spikes induced by events like lightning. Opting for digital signal transmission over analog is advantageous; it simplifies circuitry, enhances communication reliability, and streamlines subsequent processing of temperature readings. In this design, both the signal and power for the converter circuit are transmitted using just two wires, optimizing simplicity and efficiency.
Utilizing PT1000 Temperature Sensor and Voltage-to-Frequency Converter
The design incorporates a PT1000 temperature sensor, renowned for its ability to endure temperatures exceeding 130 °C, making it suitable for applications like solar heating systems. The voltage drop across the sensor is utilized as input for an Analog Devices AD654 voltage-to-frequency converter. This converter modulates the power rail with a square wave signal, its frequency varying based on the measured temperature, ensuring accurate temperature readings.
The signal can be carried on a cable over a great distance. At the receiver end an optocoupler provides for electrical isolation. T1 forms a current source that delivers a constant current of 1 mA into the temperature sensor R2. The current can be adjusted for calibration using trimmer potentiometer P1. The voltage across the sensor is taken to the VIN input (pin 4) of the voltage- to-frequency converter IC1. R4 and C1 are chosen so that the conversion factor is 10 kHz per volt.
The temperature is given by the formula
T = (f – 10000) / 38
The temperature (T) in degrees Celsius directly correlates with the frequency (f) in Hertz. Consequently, the frequency spans from 8.8 kHz (at –30 °C) to 15.7 kHz (at +150 °C). This relationship ensures a precise representation of temperature variations.
Demodulation and Current Sensing
In the circuit, the output transistor of IC1 functions with its collector at pin 1 and its emitter at pin 2. Pin 1 links to the positive signal line through resistor R5. While pin 2 connects directly to the negative signal line. T2 is pivotal for demodulation. The value of current sense resistor R6 is meticulously chosen to ensure that when converter IC1 is in its quiescent state, T2 remains inactive.
Modulation and Output Processing
Upon activation of the converter’s output transistor. The additional current flowing through R5 substantially raises the total current consumption. Consequently, the voltage drop across R6 intensifies, activating T2. This leads to a substantial collector current passing through R7, R8, and the LED within optocoupler IC3. As a result, the phototransistor inside the optocoupler also switches on.
Isolated Operation and Signal Availability
To enable single-supply operation, an isolating DC-DC converter is employed. This not only provides the necessary 12 V for the sensor circuit but also ensures up to 1000 V of electrical isolation. The signal, available at connector K4, boasts low impedance, rendering it suitable for seamless further processing and integration into larger systems.