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Luxeon Lights

At last - a low cost way of putting Luxeons in your car

by Julian Edgar

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At a glance...

  • Low cost and efficient power supply for 1-watt Luxeon LEDs
  • Upgrade interior lights, door ajar lights, shiftlights
  • Yep, you pay for the Luxeon - but after that, low cost
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If you’ve been keeping up with battery torch design, you’ll have seen the Luxeon LEDs now being used in the latest and greatest torches. These LEDs are BRIGHT – so bright it’s impossible to look directly at them. Yep, that bright. But they still have the advantages of traditional white LEDs – low power consumption, good colour rendition (ie colours look natural when viewed by their light) and being almost indestructible in use. (In a way it’s a pity that they’re still called LEDs because they’re nothing like ‘em to look at.)

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And that’s all very good – so why aren’t heaps of people using them in car applications? In fact, many concept cars (and one production car) are now appearing with Luxeon headlights and interior lighting, but for the do-it-yourselfer, there’s been a bit of a problem. The LEDs themselves are pretty expensive (1-watters are about AUD$15 each) but then there’s been the added cost of the power supply. Power supply?

Like any old LEDs, you can run a Luxeon with a dropping resistor wired in series. But because of the power being drawn (about 350 milliamps for a 1 watt LED), to use them in a car you need a damn big 5 watt ceramic resistor – one that will get hot! Another disadvantage of this is that the power consumption of the LED/resistor combination is high – so you’ve just lost one of the advantages of using a LED.

The better way of driving a Luxeon is with a dedicated electronic power supply. This regulates the amount of current the Luxeon is fed and in doing so, drops the voltage down from car levels to what is appropriate for the LED. You can buy these power supplies commercially or build them in kit form... at a starting price of about AUD$30.

So, bloody hell, for a 1W Luxeon and its power supply you’re looking at something like forty-five bucks!

Hmmm, no wonder you don’t see much use of them by modifiers....

But, as of now, that just changed. If you’re moderately handy with a soldering iron, you can now modify a phone charger to become a 1W Luxeon power supply for a grand total of about 15 cents! You’ll need a discarded cigarette lighter plug-in phone charger of the sort that gets thrown away whenever anyone buys a new phone. (In fact, have you got one tucked away?) If you haven’t, you’ll find them at garage sales, secondhand stores, out the back of phone retailers – even at the tip.

When you’ve got the Luxeon (they’re available from electronics retailers like Jaycar Electronics) and the phone charger, you’ll need just one more component – a 3.9 ohm, 1 watt resistor. So how do you do it?

Step by Step

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Here’s the starting point – old phone chargers. Don’t worry: you don’t need the five shown here – just one will do.

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The charger pulls apart really easily – just undo the threaded collar at the pointy end of the plug and then separate the two halves. A fuse will fall out – you can put that in your spare parts drawer and the rest of the housing in the bin.

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Here’s what the bare guts look like. Note the red and black wires on the left – they’re the positive and negative (respectively) power outputs. The inputs are on the right – the spring that connected to the tip of the plug is positive and the other wire, negative.

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The first step is to solder on new input and output wires. Solder directly to the Printed Circuit Board (PCB) pads, rather than trying to thread wires through the holes. Keep the correct colour coding – red for positive and black for negative (ground). Remember, the pointy end of the PCB is the input power end.

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The next step is to locate two specific resistors on the PCB. Somewhere on the board there will be two resistors that join to a common track at one end but go to different tracks at their other ends. The track that both resistors join to will also connect to pin 5 of the IC (see diagram below). Given that there will only be three or (at most) four resistors on the board, it isn’t all that hard to find the resistor pair in question. Also, the two resistors are almost always physically located near to one another. (In fact, truth be known, that’s usually all I use as the locating guide!) Using small diagonal cutters, cut off the two resistors and discard them.

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The next step is to locate pin 5 of the IC. This diagram shows the pin layout when the chip is viewed from above. Note the presence of the dot and/or the cut-out to allow you to get your bearings. Locate Pin 5 on top then very carefully turn the PCB over so that you have the same pin identified from underneath.

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As shown here, solder a new wire to Pin 5 of the chip. I used wire with green insulation. Don’t overheat the chip when you’re doing this and remember that you can solder anywhere along the track that connects to pin 5 – it doesn’t have to be actually at the pin itself.

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When you turn the PCB back over, the view should be like this. That pair of resistors is gone and the new green wire (arrowed) connecting to Pin 5 comes out from under the PCB.

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Making sure that you’re working on the output side of the module (that’s the non-pointy end), solder the new 3.9 ohm 1 watt resistor between the green wire (the new one that connects to Pin 5) and the negative output (black). In this view the negative wire got so short it’s lost its black insulation.

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Finally, solder the positive output (red) to the positive terminal of the Luxeon (there’s lots of positive terminals on a Luxeon – just pick one) and the green (Pin 5) wire to one of the negative terminals of the Luxeon.

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Apply power to the input (anything from about 7-24V!) and the Luxeon should come to life. If you have a multimeter, it’s wise to unsolder one wire from the Luxeon and insert the meter to measure the actual current flowing through the LED. It should be about 315 milliamps. Then, when all appears to be working well, run the Luxeon for 15 minutes or so. No component should get really hot (ie too hot to touch) although a number of components may grow quite warm.

Uses

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So, what use do you make of the Luxeon? (Or Luxeons – once you’ve done one power supply conversion, another will take you half the time.)

One really good use is to replace the interior light bulb. The interior will be better illuminated, leaving the interior light on will be much less likely to flatten the battery, and the colour of the light will be improved. Make sure you measure the polarity of the light connections and then solder the Luxeon power supply feed wiring straight to the light bulb terminals. In most interior lights there will be plenty of room for both the circuit board and the LED, perhaps held in place with a few dobs of silicone.

Boot lights can also be upgraded in the same way, you can add an underbonnet light, or (with a red Luxeon) a light on the inside of the door that shows passing drivers that your doors are open. If you want a shiftlight you will NEVER miss, use a Luxeon – you can get them in red, orange, green or white.

Conclusion

Luxeon LED prices will continue to fall as they become more and more popular, and with the near zero price of a modified phone charger power supply, their in-car use becomes quite viable. Sophisticated LED lighting in your car? Easy!

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