Current limiting with resistor divide

mosquito56

I am using a oil temperature sensor that measures about 50 ohms when cold and decreases with temp. If I wire 5v-50ohm-sensor-ground my 5 volt regulator chip is over heating. I am also using a simple variable resistor in the same manner.

Because of the range of the resistors 25-100 ohms, how can I limit the current and still use these resistors? I am sure this is a simple question but I have been stumpted for months now.

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kwinn

The most accurate would be to use a constant current source to provide a fixed current (1-10mA) to the resistor and an op amp to amplify the voltage across the resistor to the desired range. The lower the current the less the sensor will be affected by heating due to the current through the resistor.
A series resistor of 500-5000 ohms would be a simpler current source but would be somewhat less accurate. The op amp would still be required.

Beau Schwabe

mosquito56,

You could use a current mirror such as this one. When powered with a 5V supply, the differential output provides roughly 40mV per Ohm of change from the sensor using about 400 micro amps of current through the sensor.

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Beau Schwabe

IC Layout Engineer
Parallax, Inc.