With the number of fans in our computers ever increasing, many enthusiasts have put a lot of effort into quietening down their systems. A Fanbus is of course a good way to lower the fan noise, whilst having more airflow on demand. Here I will explain how a Linear Voltage Regulator Fanbus works, and show you how to make one for less than the cost of a pint.

The Types Of Fanbus Circuit

There are 3 types of Fanbus circuit in common use, the first is often known as a Rheobus, and is nothing more than a big variable resistor. This has a major disadvantage, in that it cant handle many fans, and reduces the current as well as the voltage, so fans will have a tendency to stall at lower speeds. Another type is the PWM circuit, which basically switches the fan on and off very rapidly, typically 100 times a second or more. This has the advantages of practically no heat output from the circuit, and can potentially drive quite a number of fans. The disadvantage is it is terribly bad for your fans, as the spasmodic current supply means the fan does not have such a smooth rotation, and will be more prone to stalling, and in some cases, even produce growling noises.

Neither of these circuits are adequate, but there is a better solution. A linear voltage regulator. These circuits can be as simple as a single transistor, but you can buy chips that do the job better, adding useful things like over current protection, short circuit protection, thermal cutout, and other helpful things. A linear voltage regulator does not limit the current like a Rheobus, it will produce as much current as the chip is rated for. The chip will produce a perfectly smooth DC supply, which is what your fans are designed for, and this means that you will be able to run your fans very very slowly if you like.

About The Circuit

The circuit I will show you how to make here is one of my own design, using only 4 components. If you can solder wires together, you can build it. The key part is the LM317T chip. It has only 3 legs, and looks like a simple transistor, but it is in fact very complicated inside. Wiring it up is simple. It has an input leg, an output leg, and an adjustment leg. You put the positive 12 volts into the input leg, and put a small current at a voltage set by the potentiometer, into the adjustment leg. This same voltage will come out of the output leg, up to the chips rated current.

It will need a heatsink, because all the power that isn't going into the fans has to go somewhere, and ends up released as heat. This circuit will handle up to 18 watts worth of fans. In fact, my first prototype happily coped with a pair of 120mm 120CFM YStechs, AND a pair of 80CFM 60mm Deltas as well, totaling 25 watts. The 2 potentiometers in the circuit control the speed of the fan. The larger 10k one is the one you mount on your front panel to adjust the fan speed, and the other, smaller one, you use to adjust the lowest speed that the fan is allowed to go.

This controller will give voltages between about 3 volts and 10.6 volts. The reason it wont go all the way to 12 volts is due to the inefficiencies in the design of the chip. Losing 1.4 volts is relatively little for this kind of circuit though. You will need to fit a switch to bypass the circuit if you want to have 12 volts available. The preset potentiometer can be omitted, but you will need to change the 1k resistor for a 4k one, which will give a fixed lowest setting of about 5 volts. If you can then find yourself a heatsink, this makes the parts cost only £1.00.

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