Fan controllers are hardware accessories that enable a desktop computer user to control the speed of fans connected to it. There are many reasons why a computer user may want to control the fans, but the two most commonly cited reasons are  reduced power consumption, and  noise reduction. There is enough demand for it that BIOS chip manufacturers have been incorporating fan control into their firmware for a number of years. A number of software-based solutions also exist to accomplish this, which may or may not work depending on whether or not your motherboard supports it. If this is not possible or the software does not give you the level of control you want, you might consider a hardware-based fan controllers. In this case, you probably want to be aware of some of the basic criteria that differentiate controllers:
 Manual vs. automatic. If all you want to do is run the fans at less than full speed, then all you need is a manual fan controller. In the simplest case, all that is needed is something to throttle back the voltage. A simple rheostat spliced into the fan wire may be enough. If you want to regulate the fan speed based on the temperature so the fan runs faster when the computer gets hotter, then you need an automatic controller. You can easily differentiate between manual an automatic controllers because automatic controllers require a means of measuring the temperature inside the computer and therefore include thermal sensors. These sensors typically are cables with one end of the wire stripped away, but with a protective plastic cover over the exposed end of the cable. Automatic fan controllers are typically more expensive than manual ones, and more often than not allow the user to toggle between manual and automatic control.
 Number of channels. Generally, you want to have at least one channel per fan connected to the controller. There are some very simple fan controllers on the market that only have a single channel. Most, however, have at least four channels, with some having as many of eight or even ten. You can connect more than one fan to a single channel by means of Y-connectors, but you will want to make sure the total power consumption on that channel does not exceed the maximum power output for the controller. This brings us to the final criterion:
 Watts per channel. You will want to know the total power output per channel. Most controllers output at least 5 watts per channel, and 10 watts per channel is not uncommon. This should be enough for most users, but it may not be enough, especially if you have large fans or want to connect more than one fan per channel. On the other end of the spectrum are controllers than output 30 watts per channel or more. If you opt for a more powerful controller, you will want to make sure that your power supply unit (PSU) can supply enough power to the unit. Often the manufacturer’s specifications include a recommended minimum wattage for the PSU, but common sense is also a useful guideline. For example, if the controller outputs 30 watts per channel and has six channels, it will output as much as 180 watts. The PSU must provide power for the controller, motherboard, and peripherals, so a 350 watt PSU will likely prove inadequate. A 500 watt PSU might be enough, while a 600 watt PSU should be more than adequate.