# Temperature measurement using two cables

Posted on Jan 27, 2013

In those cases we want to measure the temperature in an area far from where it is. The imaging unit installed, you should find some way to connect the latter with the necessary sensor. The cables we use most often will have three two conductors for power sensor and one more to carry the signal representing the temperature. But it may have only two, assuming that one will carry the potential of land and the second supply voltage with the signal temperature. This is possible if used as sensors or the LM334 or the LM335. The disadvantage arises but with them is that the temperature display is provided for reference the absolute zero (Kelvin scale), which most often is not as convenient as we would like. If so the issue is a signal proportional to the temperature scale Celsius which in everyday life, you should use a different sensor, such as the LM45.

The sensor is powered alternating voltage, while the signal at the exit temperature is constant. The power sensor is provided by a sine oscillator based on Operational A 1 and A2 (see figure). H, AC supply must him through a two-conductor cable through the coupling capacitor C6. Then the trend was rectified, doubled and smoothed by diodes D 1, L2 and capacitors C1, C2. The resistor R2 isolates the output on the tonnage of cargo and the ballast I1 makes the coupling of the output signal sensor cable. The same device, in combination with C2, protects the sensor output the alternating voltage that exists on the cable. At the other end of the cable netting R3-L2-C4 forming a low pass filter that prevents the alternating voltage overlap continuously enhanced by the output of the sensor. The capacitor C3 is a barrier that prevents the leakage of voltage through R3 to earth. If this happens, the voltage reaches the measuring circuit / display will be reduced, giving incorrect temperatures. The input impedance of the device that measures the thermally dependent voltage should be at least 100 KO. The circuit requires to operate the power of some mA.

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