This Design Idea describes a two-transistor thermal probe for diagnosing circuit problems, such as hot components and thermal runaway. Although the probe does not provide an accurate temperature measurement, it acts as a quick check for potential thermal problems by simply probing a suspicious component with the sensing diode. The circuit, including the indicating meter, can fit into a small housing, such as an empty 20 g glue-stick tube. An ordinary 1N4148 silicon signal diode serves as the temperature sensor, exhibiting approximately a –2-mV/°C forward-voltage-drop temperature coefficient. The volume-unit meter has a full-scale deflection of approximately 300 μA. You can power the circuit with a 3.6 V rechargeable NiCd battery for portability and easy recharging.
|Figure 1.||In this two-transistor thermal probe for diagnosing circuit problems, such as hot components
and thermal runaway, the trimming potentiometer in series with the meter lets you adjust the
sensitivity of the meter to temperature changes.
The circuit uses transistors Q1 and Q2 with diode biasing of the base-emitter junctions. You adjust the multiturn trimming potentiometer to match the currents through the two collectors when the sensing diode is at room temperature, resulting in a zero meter reading. When the sensing diode becomes hotter than room temperature, the forward-voltage drop across the diode decreases, increasing collector current through Q2 and decreasing collector current through Q1. The collector current imbalance between Q1 and Q2 causes current to flow through the meter, indicating the increased temperature. The trimming potentiometer in series with the meter lets you adjust the sensitivity of the meter to temperature changes (Figure 1).