Use a transistor as a heater
It is common to use transistors for driving resistive heating elements. However, you can use the heat that a power transistor dissipates to advantage in several situations, eliminating the need for a separate heating element because most transistors can safely operate at temperatures as high as 100°C. A typical example is in a biological laboratory, in which the need for maintaining the temperature of samples in microliter-sized cuvettes is a common requirement. The space/geometry constraint and the less-than-100°C upper-temperature limit are the basic factors of the idea.
You can use an N-channel IRF540 MOSFET to directly heat and control the temperature of a biological sample from ambient to 45°C. Figure 1 shows a simple on/off-type control circuit in which an LM35, IC1, is the temperature sensor, whose output a DPM (digital panel meter) can display. IC2 compares the voltage that VR1 sets with the output of the LM35 to turn on Q2 accordingly, with the positive feedback through R9 providing a small amount of hysteresis. S1 switches the DPM between a set value and the actual temperature readout. You derive the reference voltage from a TL431 shunt regulator (not shown). The LED lights up when Q2 is on.
IC1 and Q2 thermally mount on the metal block that forms the sample holder; use thermal grease on both components for maximum heat transfer. Note that the mounting tab of the TO-220 package electrically connects to the drain, and you may need to insulate it from the cuvette with a thermal pad. Setting bias control VR3 for a Q2 current of 270 mA is sufficient to hold the cuvette at 45°C.
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