Many users get frustrated with the operation of the cooling fan on their laptops and look for a way to boost voltage coming into the system to increase power to the fan. Users might wonder if the voltage coming in through attached USB cables would work. However, because USB cables have a set voltage, and because two of the four wires in the cable are set up for data transmission, not voltage, altering USB voltage is not only virtually impossible, it isn't worth the damage you can cause to the device you're connecting with the USB cable.
Put a resistor on your USB cable. This is the only way to build voltage. However, there is significant risk of damage if the voltage in your device and your computer are not at the same level. For example, if you use a USB cable from your printer to your laptop to increase voltage by adding a resistor, the imbalance in voltage can damage the device with less voltage (in this case, the printer) because you're adding the resistor which sends more voltage to the laptop fan.
Refrain from applying a resistor at the input port where you're plugging in the device. The specifications of your USB cable keep that voltage fixed.
Place a variable resistor on the HDD red LED power cable. You'll find this cable if you open the laptop; it's the cable running from your power source (battery) to the hard drive. For those who are trying to adjust fan speed through USB voltage, this may accomplish the same goal without risking any damage to peripherals. However, if you open your laptop, you will void your warranty, and there's no guarantee your fan's performance will improve by increasing the voltage in this way.