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Below pictured is the basic circuit im working with, NCT is apparently pretty much a variable pot resistor.

The measuring point ive added. According to the manual for my car, the sensor is provided with 5v by the ECU and it reads the return voltage.

The Sensor decreases resistance with increasing temperature.

However when I measure the voltage at illustrated point, I get lower voltage at higher temp, and higher voltage at lower temp, the inverse of what expected...

What am I missing here about the behavior of this circuit ?

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    \$\begingroup\$ NCT or NTC? \$\endgroup\$ – jippie Sep 16 '13 at 5:07
  • \$\begingroup\$ Yes, NTC sorry... \$\endgroup\$ – Hayden Thring Sep 16 '13 at 5:59
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Most likely you are reading the detected voltage into a computer or MCU. The slope of the voltage graph with respect to temperature should be of little concern because it is so easy to "invert" the detected temperature by subtracting the reading from a constant that is the equivalent of the 5V level in your system.

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  • \$\begingroup\$ yes im using arduino as to be expected, i had already added a invert option to my code after noticing its behaviour, but then after reading the manual again after some time i wondered if i was reading the voltage correctly, but i must be. \$\endgroup\$ – Hayden Thring Sep 16 '13 at 6:02
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According to your diagram, you are reading the difference between the 5 volts supplied by the ECU and the output voltage of the ECT sensor. Thus when the sensor is outputting 0 volts, you will read 5 volts. When the sensor is outputting 5 volts, you will read 0 volts. If you read between the car ground and the sensor output, you would get the reverse because now your reference is ground or 0 volts and your voltmeter would read the sensor output voltage directly.

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  • \$\begingroup\$ I actually get the pretty much the same reading between the sensor output and the car ground as i do between the Measuring points i have marked above. (less about .1 volt in the later). But it sounds like everything is ok about the way im doing it. \$\endgroup\$ – Hayden Thring Sep 16 '13 at 6:25

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