Download Automatic Speed Control

Automatic Speed Control
This project is for those of you who, like me, are too lazy to be bothered to monitor your system
temperature and turn the fan speed up when the temperature goes up. This circuit replaces the
potentiometer with an automatic control circuit. The temperature monitoring is done with a NTC
Thermistor connected to the inverting input of an op-amp. "NTC" means that as the temperature
increases, the resistance decreases. The Thermistor is connected in a voltage divider, so that as
its resistance changes, the voltage at the input of the op-amp changes. The non-inverting input is
connected to a potentiometer to set the reference voltage, which effectively sets the lower limit on
the fan speed. The output voltage follows the difference in voltage at the two inputs. The output is
connected to a small transistor which is Darlington-connected to a power transistor that acts as a
current amplifier, which drives the fans.
This circuit is a difference amplifier with negative feedback. As temperature increases, the
difference in voltage between the op-amp inputs increases. This causes the op-amp output
voltage to increase, causing the transistor output voltage to increase. This makes the fan spin
faster, which hopefully results in the temperature decreasing. As the temperature goes down, the
fan slows down. Use the potentiometer to set the low fan speed to suit your setup. I prefer 6-8V
output at room temperature (i.e. when the computer is cold).
Schematic:
Parts List:
U1 LM741 op-amp
Q1 2N2222A NPN transistor
Q2 TIP41C NPN power transistor
C1 2200uF 16V electrolytic capacitor
RT1 10k NTC Thermistor
R1, R4 1k resistor
R2 10k 1/2W potentiometer
R3 10k resistor
Get the parts all together before starting to build: