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