LM317T Voltage Regulator for Powering an LED

The LM317T provides a simple and effective means of running a Luxeon Star 3 LED. It will work with a range of input voltages while providing a stable current source.

The project requires:

LM317T voltage regulator - Maplins sell them for around 70p

A resistor with at least 1 watt rating - Maplins sell 3 watt wirewound resistors that are ideal. I have found a 2.2 ohm resistor a good choice for bright illumination. If the 2.2 is too bright a 2.7 or 3.3 can be tried . Lower values of resistor give only a marginal increase in illumination but make the LED run much hotter

A capacitor of 0.1uF (100nF) - preferably solid tantalum or disc ceramic rated 20 volts or higher.

A small piece of strip board - this is circiut board with a grid of punched holes and continuous strips of copper on one side. It is convenient for making up simple circuits.

Connecting block - the small size with 5Amp rating is fine though other types of connecter could be chosen or wires can be soldered directly to the board.

A DC power source - just about any voltage regulated DC adapter with a rating of at least 1000mA and a voltage within the range of 4.5 to 12 volts would be suitable. At 12 volts or higher the regulator has to dissipate quite a lot of heat and will require a larger heatsink, though the LM317T is rated for up to 37 volts and 1.5 amps. A 5 or 6 volt adapter would be ideal.

A heat sink - can be purchased or made from a piece of aluminium.

Schematic

This schematic is from OnSemi and shows how an LM317T can be used as a current regulator. It simply requires one resistor of the correct value. The circuit I use also has an input bypass capacitor

This diagramme shows how the LM317T can be mounted on a small piece of stripboard with the components required for a voltage regulator. This layout uses an input bypass capacitor of 0.1uF (100nF) that should be a tantalum or disc ceramic type. The resistor I used is a 3 watt wirewound ceramic.

The LM317T must be mounted on a heatsink - preferably using thermal compound. I made one out of a folded strip of aluminium around 2cm x 10cm. This is fine for an inupt voltage of 5v but for 9v or 12v a larger heatsink would be required. The metal tab on the 317T is connected to the V+out so the heatsink must not be allowed to come into contact with earth (-v).

The 2.2ohm resistor sets the output to 568mW - fine for a Luxeon Star 3. If it is too bright a 2.7ohm gives 463mW or a 3.3ohm 379mW.

The photo shows the regulator assembled. The connecting block is a convenient way to attach the cable from the 5v dc power supply and the + and - connections to the LED. Maplins sell a range of small boxes that it can be mounted inside - though some ventilation holes will be needed for cooling.

The LM317T can accept input voltages in excess of 30volts but higher voltages mean more power dissapated and a much larger heatsink would be required. The forward voltage of a Luxeon Star III is around 3.7v so provided the input voltage to the LM317T slightly exceeds that it should work fine.

Links

http://led.linear1.org/a-cheap-current-regulated-luxeon-star-driver-design/4/ The LED Centre. A simple circuit for regulating up to 1500mA current to an LED using an LM317T voltage regulator IC.

http://www.maplin.co.uk/ Sells a large range of electrical and electronic components. They have a reasonable range of ceramic wirewound resistors, power adapters etc.

http://www.esr.co.uk Sells electrical and electronic components. Good range of 2.5w silicon resistors.

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