Fabrication: Car interior lighting
By Ian Mander 18 December 2007, updated 8 November 2010.
Every so often I get sufficiently annoyed with the feeble lighting inside my car that I do something about improving it. I've now changed the boot light and the cabin light.
Modification installed 4 May 2007
I went camping last month and the light my boot light provided was quite pathetic. It just wasn't enough to see stuff in the boot. Time for a change, using a modern Cree XR-E high powered LED. At 350mA these things can produce 70 lumens/watt – considerably better than the original wedge bulb does.
There's not much space in there, so I'm limited to using only one LED. The simplest way of doing it was to wire the LED in series with a big resistor – or actually two resistors in parallel, I think because I just happened to have them and it doubles the power handling. It's not the most efficient way of doing it, but it does the job quite nicely, since there are quite a few volts to drop.
The resistor is attached to the back of the light fitting, which means this modification isn't a straight swap. It requires a screwdriver to remove/replace the fitting, but it can still be done in just a minute or so.
Exposure setting on the before & after shots are 15 seconds, f8, and I think ISO 50. The auto white balance accidently used makes the incandescent light look less brown that it really is, although our eyes compensate anyway. I should have turned the cabin light off, too.
Well, it lasted almost two years before the LED went low-output on my latest camping trip last month. I've finally got around to constructing and installing a new one, taking out the high power resistors in the process. The new unit uses a buck driver with an EQB8L chip, and with a rectifier built in. It has pins so it fits into the original W5W socket quite nicely, although I shortened the pins a bit so it wouldn't protrude so much.
That 91.6% is particularly impressive bearing in mind that there's a rectifier in there too. The second line is what the driver efficiency is after the rectifier, using info from the diode datasheet for how many volts are dropped across two diodes in the rectifier.
The interval between changes is decreasing. I've swapped the EQB8L driver for a similar one with a PT4115 and also with a rectifier. Because the EQB8L is reasonably efficient at low voltages and low currents I figured I'd use it for something else. However I've since worked out that for technical reasons I can't use it how I wanted, so that driver is not being used for now.
While changing the driver I noticed that the goo under the LED lens had started to leak out – a bit strange, hadn't seen that before.
Modification installed 16 December 2007
This uses two Cree XR-E high powered LEDs, of the now inexpensive P4 brightness/efficiency bin. The LEDs are mounted on a small rectangle of aluminium using thermal adhesive and wired in series with a regulator board supplying a constant 750mA. The board is an efficient buck regulator, so only draws 500mA from the car. This means it uses less power that the original bulb did, but as you can see, produces a lot more light.
Unlike the boot light, this one is an easy retrofit, not requiring a screwdriver or any other tools.
Photos above are each 50:50 blends of individual photos taken at 3.2 seconds and 10 seconds, all at f2.8, ISO 50. (One set looked a little too dim, the other a little too bright.)
Update 8 June 2008
My car interior light lasted six months before one of the Crees apparently overheated on Friday, and for some reason went into ultra-low-light mode, affecting both LEDs. In hindsight I guess driving two LEDs at 750 mA was too much heat in too small a space – that of a 31mm festoon bulb.
Shorting out the bad LED made the other one work properly, but it is nice with the two of them, so I swapped the dud for a new one yesterday and it's all go again. The white ink on the old and new Cree stars mounted next to each other are different shades – I think they were being toasted, poor things – so I also put a bit more aluminium on the back to help wick heat away. It looks even rougher now, and takes up almost all the room in the light fitting.
I took some more measurements of voltage and current in and out of the driver board, and confirmed that the driver is very efficient. It's a pity it's no longer available.
The old 31mm festoon bulb is rated at 8 W, and I tested it as well.
This means the double LED light at 5.88 W is using just 62% the power of the standard bulb.
Update 7 September 2009
After being left on for a while yesterday (while investigating options for an always-on lighter socket) I noticed my dome light wasn't working any more. A bit of investigation showed the driver has an intermittent fault. Sometimes it works fine, most of the time it doesn't. I couldn't track down the problem, although at times I could consistently make it turn on and off with a small rotation of the unit!
So I've replaced the driver with a new one using an EQB8L chip (this one, but the driver chip has since changed), with a rectifier on the board. This means that – like the boot light – I don't have to worry about which way it's plugged into the socket. I tested it with the supplied current set resistor (0.15 ohms), then replaced it with a slightly higher value (0.20 ohms) to reduce the output – and the heat – a bit.
84% including a rectifier is pretty good. From the specs of the SS14 Schottky diodes used for the rectifier the driver would be ~90% efficient if the rectifier was bridged/shorted out.
Update installed 6 November 2010
As with the boot light, I swapped the dome light's EQB8L driver for one with a PT4115, after changing the set resistor on the PT4115 driver to give 500 mA.