5V fan drive circuit composed of single triode
The circuit shown in Figure 3 uses a PNP transistor to control the three speeds of the fan: stop, medium, and high speed. Both Out1 and Out2 go high when the temperature is below the two limit settings. There is no current flowing through the base of Q1, so it turns off and the fan voltage is 0V. When the temperature exceeds the limit value 1, the Out1 drive goes low and the resistor divider R1/R2 sets the voltage of the base of Q1 to 1.8V. Since the base voltage is Vbe, the emitter voltage will be 0.7V higher than Vbe, resulting in a fan voltage of 2.5V (50% of full scale voltage). When Out2 goes low, it pulls the base of Q1 low to ground, and the base current is limited by the maximum absorption capacity of the IC output, typically 6-8mA@Vol=0.4V. Due to the limited base current, the gain of Q1 should be greater than 100 to ensure minimum voltage drop and powerful triode drive capability. The voltage drop between the output device and Q1 limits the maximum fan voltage to 4.1V (82% of full-scale voltage).
Figure 3: A single triode implements a 5V fan drive.
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