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White LED Driver

Efficient circuit runs off 12V and drives up to 30 white LEDs. It can even switch on automatically when darkness falls.

By Stephen David

While we have now published quite a few LED driver circuits, to date we have not published a design to drive a bunch of high-brightness white LEDs. Such a circuit is now quite desirable as the price of white LEDs has fallen and you can have a handful for not a lot of dollars.

However, white LEDs do present a problem because they need a higher drive voltage than monochromatic types such as red, green, orange etc. Instead of around 1.8V to 2V or thereabouts, they normally require more than 3V to produce their rated brightness. In fact, if you are driving a bunch of them you need to drive them all at constant current otherwise their individual brightness tends to vary markedly.

However, if you only have a 12V supply available, you can only put two or maybe three LEDs in series together with a constant current source and this leads to poor efficiency.

The approach in this circuit is to boost the 12V supply to something around 21V and this means that we can have groups of five LEDs, each in series with their own current source transistors.

The result is a single PC board with the drive circuitry and 30 white LEDs. It can be used for lighting in caravans and recreational vehicles, emergency lighting or whatever application you can think of. Current drain is around 190mA at 12V.

Where To Buy A Kit
Click for larger image
The optional swivel-mount unit is fitted directly to the bottom of the PC board and allows the “LED Lamp” to be adjusted to a convenient angle.

This kit has been designed by Oatley Electronics who own the copyright. The kit comes in two parts:

(1) K202 which includes the PC board, the driver circuitry, 10 high brightness white LEDs and two current source transistors; and
(2) K202A which provides 10 high brightness white LEDs and two current source transistors, so with K202 and two K202A kits you get the full complement of 30 white LEDs.

Pricing is $17 (including GST) for K202 and $8 for K202A. The optional swivel mounting bracket is $1.00 while postage and packing is $6 within Australia. Kits may be obtained direct from Oatley Electronics, PO Box 89, Oatley NSW 2223. Phone (02) 9584 3563; Fax (02) 9584 3561. Website: www.oatleye.com

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