How passive temperature sensors work ?? 3

[OzgurALAGOZ]

Hi everybody! Can someone inform me how does passive temperature sensors work (without a supply voltage). It has a resistor in it and the resistance changes according to the temperature but where is the voltage source ? (If I'm not wrong the pull-up voltage in the ECU is only to determine the fault situation of the curcuit - so it cant be the supply v).
Or any websites that gives the fundemantal principles thanks for the info...

 

[circuit1952]

The pull-up resistor limits the current flow through the sensor to limit the self-heating effect. The fixed resistor in series with the sensor forms a voltage divider.

http://en.wikipedia.org/wiki/Thermistor

http://www.thermometrics.com/htmldocs/whatis.htm

http://www.omega.com/temperature/Z/pdf/z036-040.pdf

 

[OzgurALAGOZ]

Thanks for the info circuit1952. But I am still confused I have examined the pages, they all talk about the change in resistance as the temp changes. My sensor has only two cables. One is the ECU return and one for the signal. without a supply voltage the change in the resistor means nothing to me

 

[Ron07663]

OzgurALAGOZ, everything you are saying is consistent with circuit1952's explanation.
The sensor is a two terminal device with a resistance that is a function of temperature. It usually has one terminal connected to ground (return) and the other terminal connected to the ECU. In the ECU, this terminal is connected to a fixed resistor. The fixed resistor's other terminal is connected to a voltage supply. The circuit formed is a voltage divider. So, for example if the sensor's resistance, at a certain temperature, is equal to the fixed resistor's value, the voltage measured at their junction is exactly half the supply. In the ECU, the junction voltage is measured with an A to D converter.
For fault detection, if the sensor or cable is open, the junction voltage goes to full supply voltage. If the sensor or cable is shorted to ground, the junction voltage goes to zero. Since either voltage extremes are illegal, the ECU can declare a fault.
How this helps.

 

[MacGyverS2000]

Batt+
  |
  Rf
  |
  ------ECU
  |
  Rv
  |
Ground

The circuit looks like the above, where Rv is your temperature-based variable resistor and Rf is a fixed value resistor inside the ECU.

Operation:
Current flows from Batt+ to Ground through both resistors. As Rv changes with temp, the voltage seen at the ECU node changes. This voltage is read by the ECU and translates it to a temperature.

Dan - Owner

http://www.Hi-TecDesigns.com