THERMISTOR
Thermistors are the most common device used for temperature measurement on a motor vehicle. The principle of measurement is that a change in temperature will cause a change in resistance of the thermistor, and hence an electrical signal proportional to the measured can be obtained. Most thermistors
in common use are of the negative temperature coefficient (NTC) type. The actual response of the thermistors can vary but typical values for those used in motor vehicles will vary from several kilohms at 0°C to a few hundred ohms at 100°C. The large change in resistance for a small change in temperature makes the thermistor ideal for most vehicles’ uses. It can also be easily tested with simple equipment.
in common use are of the negative temperature coefficient (NTC) type. The actual response of the thermistors can vary but typical values for those used in motor vehicles will vary from several kilohms at 0°C to a few hundred ohms at 100°C. The large change in resistance for a small change in temperature makes the thermistor ideal for most vehicles’ uses. It can also be easily tested with simple equipment.
Thermistors are constructed of semiconductor materials such as cobalt or nickel oxides. The change in resistance with a change in temperature is due to the electrons being able to break free from the covalent bonds more easily at higher temperatures. A thermistor temperature measuring system can be very sensitive due to large changes in resistance with a relatively small change in temperature. A simple circuit to provide a varying voltage signal proportional to temperature. Note the supply must be constant and the current flowing must not significantly heat the thermistor. These could both be sources of error. The temperature of a typical thermistor will increase by 1°C for each 1.3mW of power dissipated. This highlights the main problem with a thermistor, its non-linear response. Using a suitable bridge circuit, it is possible to produce non-linearity that will partially compensate for the thermistor’s non-linearity.
The optimum linearity is achieved when the mid points of the temperature and the voltage ranges lie on the curve. It is possible to work out suitable values for R1, R2 and R3. This then gives the more linear output. The voltage signal can now be A/D converted if necessary, for further use. The resistance Rt of a thermistor decreases non-linearly with temperature according to the relationship: Rt Ae(B/T) where Rt resistance of the thermistor, T absolute temperature, B characteristic temperature of the thermistor (typical value 3000K), A constant of the thermistor. By choosing suitable resistor values the output of the bridge will be as shown. This is achieved by substituting the known values of Rt at three temperatures and deciding that, for example, Vo 0 at 0°C, Vo 0.5V at 50°C and Vo 1V at 100°C.
THERMOCOUPLE
If two different metals are joined together at two junctions, the thermoelectric effect known as the Seebeck effect takes place. If one junction is at a higher temperature than the other junction, then this will be registered on the meter. This is the basis for the sensor known as the thermocouple. Notice that the thermocouple measures a difference in temperature that is T1 – T2. To make the system of any practical benefit then T1 must be kept at a known temperature. The lower figure shows a practical circuit in which, if the connections to the meter are at the same temperature, the two voltages produced at these junctions will cancel out. Cold junction compensation circuits can be made to compensate for changes in temperature of T1. These often involve the use of a thermistor circuit.
Thermocouples are in general used for measuring high temperatures. A thermocouple combination of 70% platinum and 30% rhodium alloy in a junction with 94% platinum and 6% rhodium alloy, is known as a type B thermocouple and has a useful range of 0–1500°C. Vehicle applications are in areas such as exhaust gas and turbo charger temperature measurement.
