If the temperature crosses the preset value, the base emitter voltage of the transistor exceeds the limit and the transistor starts conducting. Now as the heat over the transistor starts rising it base emitter starts rising due to the altering characteristic of the transistor. Alternatively this level may be varied by the preset P1. This voltage level is taken as the reference and the transistor remains switched OF as long as its base emitter voltage remains below this level. The base emitter voltage of the transistor is effectively compared with the reference voltage level produced by the diode at the junction of D1 and R1. In this design the diode is placed at the ambient temperature or at the room temperature which the transistor is placed or attached to the heat source which needs to be measured. The LED is used to get a direct indication of the temperature level sensed by the transistor. Obviously since the diode is placed as the reference, it has to be placed in an environment with relatively consistent temperature conditions, otherwise the diode will also start changing its reference level causing error in the indication process.Ī LED is used here at the collector of the transistor, which directly interprets the transistor conditions and therefore helps to show how much temperature difference is taking place around the transistor. The diode is place as the reference device while the transistor is connected to perform the function of a temperature sensor. Using Diode for Creating a Reference Voltage In the present design a diode and a transistor are configured in the form of a bridge network.Īs both these active parts have identical properties as far as ambient temperature changes are concerned, they both complement each other. Using a Transistor (BJT) as the Temperature Sensor The change with their characteristics with these devices is typically in terms of the passage of voltage through them, which is directly proportional with the magnitude of the temperature difference surrounding them. As we know that all semiconductors have this "bad habit” of changing its basic characteristics in response to ambient temperature changes.Įspecially basic electronic components like transistors and diodes are very much prone to their case temperature variations.
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