Technical Stuff

You don't need to read this bit to build yourself a Fanbus, but if you want to know how it works, read on. Otherwise, skip to the next page.

Notes On The LM317

To calculate the heat output of the chip, you need to measure the voltage the fans are running at. Take his number away from 12, to give the number of volts being lost inside the chip. Then multiply it by the total rated amperage of the fans, to give the number of watts the chip is putting out. The chips thermal cut out operates at 125 degrees, but I do not recommend you allow it to go over 100. To work out the efficiency of heatsink required, take the deference between your case temperature and the maximum temperature you consider acceptable for the chip, and divide it by the maximum wattage of the circuit, and this will give you the degrees per watt rating. Remember that using thermal goop will increase the heatsinks efficiency, as will any airflow over the sink. I found that using a heatsink pinched out of an old PSU and a little thermal goop made it impossible to overheat the chip, no matter how much load I put on it. The chip is packaged in a standard To220 casing, which you may need to know when looking for heatsinks.

The LM317T is limited to 1.5 amps. This means that when you try to draw more than that out of it, the fans will just slow down even more, and the chip will get even hotter. In practice, this means if you have a reasonable heatsink, there is no way you can break anything. If you require significantly more than 1.5 amps, you can wire up more than one chip in parallel, controlled from the same potentiometers, and outputting to the same wires. This is what I did with my 100watt Fanbus, shown below. It is capable of handling a whopping 50watts per channel.

How The Voltage Divider Works

For those of you that are curious about exactly what those resistors do, here is a brief explanation. They form a simple circuit known as a voltage divider. The output voltage sent to the adjustment leg is a result of the ratio between the values of the resistor on the positive side, and the 2 resistors on the negative side. The 1k resistor is really only there to stop the power supply shorting out if you turn both potentiometers down to nothing. The amount of load drawn from the voltage divider will alter the ratio needed to get a specific voltage. The LM317T draws a steady 50ma on the adjustment line, so this is not a factor you need to consider, unless you are using another chip in parallel, in which case you may need to readjust your presets. For a proper explanation of how voltage dividers work, see this page.

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