Simple Ways to Make Fans Silent

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Multiple speed selection circuit for DC motors using diodes

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My favorite! This method works exactly like the previous one - with the resistors. A number of diodes are connected in series. Each diode has about 0.8 Volts voltage drop across its leads. 10 diodes for example would create 8 volts voltage drop. This method is much more power-efficient than using resistors, as diodes do not generate too much heat as the resistors. Moreover, it is easier than the resistors to calculate the total voltage drop, when you know the drop of each diode. I use for example the 1N4001 that have (according to the datasheet) typical voltage drop around 0.93 Volts. No matter which fan i use, the total voltage drop will be the same.

You should pay attention when connecting the diodes. Unlike the resistors, diodes have polarity! There is a strip near one end of the diode. This strip indicates the cathode of the diode. The diode is properly connected (forward biased), when the anode (the other end) is connected to the positive of the power supply and the cathode is connected to the negative. A reverse biased resistor will not allow any current to flow within, until the break point where the diode is destroyed.

The method with the diodes is by far better than the resistors. With the diodes you can have more steps of speeds and they are more stable and power efficient. Also, the 1N series can handle up to 1A current. For a PC fan, the 1N4001 diode is perfect, and almost have the same price as the resistors. Almost. But it worth its weight.