Sensors - Self Heating Phenomena
Temperature sensors can exhibit a phenomenon known as self-heating.
When a temperature sensor is self heated, the sensor itself contributes
heat to raise its' own temperature. In this short note, we want to
examine that possibility. We'll look at it in the worst case scenario,
one with a thermistor. Here's a voltage divider with a thermistor.
Let's examine what happens here.
In this circuit, the voltage source, Vin, acts to produce
a current in the resistor, Ra, and the thermistor temperature
sensor, represented by Rt. The current that flows
in the circuit (through every device) is given by:
Notice that the resistance
of the thermistor comes into play when you compute the current through
the series combination of resistors. Now imagine the following sequence
The only real question here is whether this ever stops. If the voltage
is low enough the process should stop. However, even if the process
stops, the thermistor is at a temperature higher than its' surroundings.
That means that the temperature it measures is not the surrounding temperature
(which is what you want it to be), but one that is higher. When this
happens the measured temperature is higher than the temperature you wanted
to measure. This phenomenon is called self
The voltage source is
turned on, producing a current through the series combination of resistors,
Rt and Ra.
The current flowing through
the thermistor generates some heat because the thermistor dissipates electrical
The heat causes a temperature
rise in the thermistor.
The temperature rise in
the thermistor causes the resistance of the thermistor to decrease.
The decrease in resistance
causes and increase in current through the thermistor.
The increased current
through the thermistor generates more heat.
The additional heat raises
the temperature even higher.
Go back to step 4.
Self heating is not the worst thing that can happen. If the voltage
is high enough, and if the series resistor, Ra, is low
enough, the entire process can accelerate with the result being thermal
runaway. When you have thermal runaway,
the thermistor just keeps getting warmer at a faster rate. The end
result is a very hot, damaged, and unusable thermistor. It's not
going to give you a good temperature reading. Not now, and not in
When using thermistor
circuits, you want to minimize self-heating. You do that by minimizing
the current through the thermistor. Given a choice, choose the situation
where the thermistor has the smallest amount of current flowing through