Hi, I signed up a while ago with the intention of making an n64 portable but never really made any progress, anyway i've got a bit of spare time now so i've started up again and the first thing i did was address this exact led lighting issue.
I wasn't going to post until i had a bit more work done but I think i can help you guys out here.
I don't want to sound preachy but it seems many people don't really know how to drive an led properly and correct me if im wrong but most of you run 3 or 4 white 3mm leds in parallel from a 7805 with a single current limiting resistor of 10 ohms or so, this puts about 40 mA through each led massively overdriving them, and will dramatically reduce their lifespan.
Because leds can differ in their forward voltage they should be driven with a constant current, when you run them in parallel you risk sending more current than you intend through the led with the highest Vf this could burn them out and cause uneven lighting.
At the very least if you run sets of 2 in series from the 7.2V line with an appropriate current limiting resistor, this would double the efficiency of your backlight straight away because instead of having 4 leds each drawing say 20mA from a 7805, you would have 4 leds of the same brightness with each PAIR drawing 20mA.
Anyway, sorry for the lecture, this is the reason i posted, here is my solution to the backlight problem:
These are a couple of led arrays i made up on a piece of pcb from the edge of the n64 mobo, they are each a set of 10 plcc-2 sized white leds wired in series. They fit where the ccfl did in the psone screen:
The most efficient way to drive leds is in series so i made a little step-up circuit from an MC3406 to get the 30 or so volts and control the current with a couple of LM317 regulators.
With the current set at 10mA per side the brightness is about the same as the original backlight and only a little worse for uniformity, at 20 mA the picture is really quite vibrant although the battery draw is a little higher.
At 10mA the total draw from the screen and backlight at 7.4V was about 250mA . This should be on a par or better power-wise than the traditional 4 led mod.
Here's a shot of the screen with the new backlight to show it off at it's worst
As you can see there is a small amount of uneveness near the top on a dark background but that is a worst case scenario and the camera exaggerates it a bit. I think that it is an acceptable sacrifice for the 500mA or so power saving i'm getting over the ccfl and no where near as bad as the 4 led mod.
any thoughts?
