PC Temperature Controlled Fan Interface

Last Modified 9th Feb 1999

fancircuit.gif (4629 bytes)

This circuit has not been tested.

R1 4K7
R2 100R
R3 1K
R4 100K
R5 2K2
TR1 BD139
D1 1N4001

Component Information

Refer to the index for datasheets.

Product LM35DZ 741 BD139
Datasheet lm35a.pdf ua741.pdf  

Circuit Description

TR1 is a 12.5W 1.5A device which is ample for controlling the motor. It has a typical hfe of 100 (63-160 @150mA) which means for a 400mA fan to be fully turned on 4mA must flow into the base of the transistor. The op-amp must therefore output 9.4V to turn the motor fully on.

The op-amp design is a typical non inverting amplifier arrangement with a slight difference in that instead of R3 being connected directly to ground it is connected to the R1/R2 potential divider combination acting as a crude 0.25V voltage reference.

The LM35DZ is a temperature sensor which outputs 0.01V per degree centigrade. 0.25V is therefore 25C. As the LM35 goes over 0.25V the op-amp amplifies the difference by the ratio (1+R4/R3). Therefore to produce the necessary 9.4V on the output to turn the fan on fully the LM35 needs to output (0.25 + 0.093) 0.343V which it will do at 34C.

Therefore this circuit will vary the speed of a 400mA fan from off to full speed over the temperature range of 25C to 34C.

R4 can be altered to change the operating parameters of the circuit. If the fan comes on at full speed too soon either because it is lower power or because you have the temperature sensor mounted in a warmer part of the case then increase the value of R4.

Replacing R4 with a 47K resistor in series with a 220K preset resistor should enable the circuit to be tuned over a wide range while in use.


The 741 op-amp cannot provide a voltage output which reaches the voltage supply. Running off a 12V supply it's output can only vary between about 1 to 11V. Therefore if the temperature is below 25C then the fan will still be supplied with about 18mA.

If you use a power transistor with a higher hfe and increase R5 accordingly you can make use of this feature to make sure the fan always turns at least at a slow speed. This will keep the fan free of dust incase the computer is located where it does not normally get hot enough for the fan to come on.

Maplin Electronics Order Codes

Below is a parts list and order codes to purchase all the items required to make this circuit from Maplin Electronics.

Quantity Order Code Description Approx cost each (UK Pounds)
1 G4K7 4K7 resistor 0.06
1 G100R 100R resistor 0.06
1 G1K 1K resistor 0.06
1 G100K 100K resistor 0.06
1 G2K2 2K2 resistor 0.06
1 QF07H BD139 transistor 0.92
1 QL73Q 1N4001 rectifier diode 0.08
1 QL22Y LM741 op-amp 0.57
1 UF52G LM35DZ 4.35
1 JP47B Strip board 29 * 39 1.79
1 pack HB22Y 10 self adhesive pads 0.26
    Total Cost 8.27