PWM Fan Controller by LM2902N or LM324N

Description: 

This circuit is a fan controller which is using the pulse width modulation (PWM) method. It is tiny (33.78mm x 54.76mm) and easy to build. Functions of the circuit parts are listed below;

VR1: 10 K Variable resistor adjusts the fan speed.

R9:  This sets the minimum speed. With the 10k pot, a 1k resistor will give 0–100% control which is OK for model motors or lighting, 10k will give around 5v–12v range, more suitable for cooling fans.

C2: This is the timing capacitor, and with the 47k timing resistor R1 and wave amplitude control resistors R2 (22k) & R3 (10k) gives a PWM frequency of around 117Hz according to the formula

Frequency = R2 / (4 x R3 x R1 x C1)

Q1: For load currents up to about 600mA a 2N2222A NPN transistor is recommended. It comes in a TO-18 metal can.

For higher loads go for a darlington power transistor such as the TIP120, 121 or 122, rated to 5A, or a power mosfet. The IRF530 is easy to find, not expensive, and can carry up to 14A. Providing you take the usual precautions for handling CMOS, static electricity is not going to zap it. Most n-channel MOSFETs will do, look for a low RDS(on) and adequate current-handling ability. Both darlingtons and mosfets are in the TO-220 case.

Using the 2N2222A bipolar transistor you might lose 200-400mV from the 12v supply to the fan, double that for one of the darlington types; with the IRF530 I measured the loss at only 40mV with a 200mA fan.

Check the transistor or mosfet pin-outs, base or gate to R9, emitter or source to ground, collector or drain to the fan negative. A heatsink is not necessary at moderate loads. 

D1: The diode prevents back-emf from inductive loads such as brushed motors from damaging the switching transistor. With "brushless" computer fan motors it's not necessary to fit this diode across the load, as they have any needed protection already in-fan.

Click here to download the schematic, PCB and layout files

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Comments (2)

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variable frequency
written by Jeremy Byington, March 12, 2008

Is there a way to alter this circuit to allow for changing the frequencies?

...
written by Komeil Bahmanpour, April 09, 2008

I think the "Description" is mistaken. In C2's description,

1. by R1 you meant R4 (47k timing resistor)
2. by R2 you meant R3 (22k wave amplitude control 1)
3. by R3 you meant R5? (10k wave amplitude control 2)
4. by C1 you meant C2 (in the formula only)

So, the correct formula would be: Frequency = R3 / (4 * R5? * R4 * C2)

using this new formula,

22000/(4*10000*47000*0.0000001)=117.0 Hz (correct, as stated)

using site's formula,

10000/(4*22000*10000*0.0000001)=113.6 Hz

Finally, which exact components needed to be modified to affect the frequency? Would it be R4 and/or C1?