55 Temperature sensors

A commonly used device used for sensing temperature is the thermistor. A thermistor utilizes the concept of negative temperature coefficient. Most electrical conductors have a positive temperature coefficient. This means that the hotter the conductor gets the higher is its electrical resistance. This thermistor operates differently; its resistance gets lower as its temperature increases and this is a characteristic of semiconductor materials. There is a well-defined relationship between temperature and resistance. This means that current flow through the thermistor can be used to give an accurate representation of temperature. A typical coolant temperature sensor is shown in Fig. 5.17.

Fig. 5.17 An engine coolant temperature sensor

Figure 5.18 shows the approximate relationship between temperature and resistance. The coolant temperature sensor provides the ECU with information about engine temperature and thus allows the ECU to make alterations to fuelling for cold starts and warm-up enrichment.

The information shown in Fig. 5.18 may be given in tabular form as shown in Table 5.1 (these are approximate figures).

This shows the approximate resistance to be expected between the sensor terminals, for a given temperature. From this it will be seen that it is possible to

0 20 40 60 80 100 Engine coolant temperature (°C)

Fig. 5.18 Temperature vs resistance characteristics (thermistor)

0 20 40 60 80 100 Engine coolant temperature (°C)

Fig. 5.18 Temperature vs resistance characteristics (thermistor)

Table 5.1 Temperature and corresponding resistance for a coolant sensor

Temperature

Resistance

Voltage

(°C)

(ohms)

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