Tension Regulator

par Lionel - 16/05/2004
Build a tension regulator yourself.
(for a case fan, or leds...)

Usually, computer fans are loud under heavy load. So it might be interesting to set their rotating speed accurately, to reach a balance between cooling and noise levels. You can find ready-made regulators in a few computer shops, however, building one from scratch is fairly easy and requires very few parts. Such device can also be used with LEDs.

The LM317T

It's a component that can deliver in excess of 1.5A over an output range of 1.2V upto 37V. It only requires two external resistors to set the output voltage. Here are the basics specifications to keep in mind :

- The regulator induces a 3V drop between the input tension (Vin) and the output tension (Vout). For instance, if you need to output 4.5V, you will need to input at least 7.5V to the LM317. Of course, the output voltage cannot be superior to the input voltage.

- The bigger the voltage differencial is, the more heat will the LM317 produce. So, if you want to output 2V, you'd better input the regulator with 5V rather than 12V. Needless to say, consumption is tightly related to this, so if you plan on powering a lot of fans/leds with one regulator, you'd better stick a small heatsink on its back.

- The Vout connector (with the hole) must be carefully and properly insulated, especially if you use a heatsink, to avoid any short-circuits.

- The LM317 can tolerate short-circuits (but don't push it though !)

The schematics

Here are the schematics for the regulator, with the appropriate values for the resistors in case of computer use (5V, 12V, 24V, ...)

The 100nF condensator acts as a noise suppressor, and is only necessary if the the regulator is physically far away from the power source. You can either use an adjustable resistor (can be set with a screwdriver) or a variator (a tad bigger, but adjustable with your fingers). You can directly plug your fans or LEDs to it by using Limiting Resistors. In that case, you can compute the limit needed with the following formula : U=R.I
For instance, say you would like to plug a LED (which draws 10mA) to a standard 12V input :

R= (12-3)/0.01 (Value of U set with a -3 Voltage drop)
R= 900 Ohm (Which corresponds to a standard value of 820Ohm)

Bright LEDs like the white or blue ones usually draw much more power, and thus you will need to use a Limiting Resistor (LR) with a smaller value. The easiest is to experiment with one LED first. Just keep in mind that the regulator cannot be fed too high an input voltage.
Since the LEDs are placed in parallel, intensities add up, so be careful not to go over the LM317's limit of 1.5A. (Sticking to a value of 1A might be a good idea)

To build the device, you can either solder the components directly to each other, or use a tiny PCB.

Below are all the necessary items/components needed to fashion a device that can bee hooked up directly to your Mac's PSU. (including the U shaped heatsink)

Here's what the finished product look like.

For those of you wanting more info

Below are both the LM317 specs and a full scale schematic sheet of the device we just built.

Antoine Bercovici (Kiryu)

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