10 March 2010

PSOne screen led-mod

The PSOne screen is great, a major drawback though, is it's high power consumption. Out of the box, it draws about 750mA at 7.5V, almost all of that is to power the backlight.

In this post I'll describe how I saved power with a custom led backlight.

Ben Heck popularised the "4-led" mod for these screens to reduce their power draw. He used 3mm white leds in each of the corners.

Whilst this does drastically reduces the power consumption, it's a little uneven and a fair bit darker than stock. You sacrifice screen brightness and uniformity, for lower power consumption. I tried it on one of my screens:

In comparison you can see the "4-led" is dimmer overall and uneven at the corners, it isn't too bad when playing a game, and some people can live with it, but it's not ideal.

I wanted high brightness and good uniformity, whilst still saving power. Nowadays, most lcds are lit from the edges by a row of leds.
If I could replicate this in the PSOne screen, it should solve the uniformity and brightness issues of the '4-led' mod and if I was careful, I could still save power.

I found I could fit ten PLCC2(or 3528) leds along the top and bottom edges of the screen.
Some rough calculations showed I might expect ~170nits from those 20 leds, while probably not quite as bright as the stock ccfl, it'd should be acceptable. (the stock screen is probably 250-300nits.)
  • screen area = 8.4x10^-3 m^2
  • each led ~1.8lm
  • 20 x 1.8lm = 36lm,
  • lightguide/polariser efficiency ~4%
  • 36lm x 4% = 1.44lm,
  • 1.44 / 8.4x10^-3 m^2 = 171.4nits 
I built the strips on a small piece of pcb, and made slits in the copper for each led. These strips would fit where the original bulb was, either side of the light-guide, behind the lcd panel.

A typical white led might be rated "3.4V @ 20mA". To power it from a higher voltage you need to limit the current, most simply, you can use a resistor.
A regulated 5V source is available on the PSOne screen board, to work out the resistor needed you find the voltage drop across the led; 5V-3.4V=1.6V. Then divide by the current you want; 1.6V/0.02A=80Ω. Then choose the closest higher value resistor, in this case 82Ω.
This is fine for a couple of leds, however for my led strips it would be too inefficient. When you drop voltage across a resistor like that, you're wasting a small amount of power in heating it up. If you wired 20 leds in that way, you would be drawing 400mA @ 7.5V just for the leds, that's not a whole lot better than the stock ccfl backlight draw.
A better way to drive the leds is to have them in series and limit the current for the whole string. This means there's only one place where you're wasting power instead of twenty.
Since I have two strings of 10, I would need around 34V @ 40mA to light up both strings.
I needed to step-up the voltage from my 7.5V source, to around 34V. To do this I used an MC34063a switching regulator.
I took the reference diagram from the datasheet and adjusted the component values to get the right output, and came up with this circuit:
I built it on some Veroboard and it worked quite well and was pretty efficient. (~85-90%)
Running both strings of leds at 20mA, the screen and backlight together only pulled around 250mA from 7.5V, this is excellent compared to the ~750mA with the stock backlight
Here's a comparison between a screen with my led mod(left) and a stock screen.(right) 
Overall the image is a lot better than the "4-led" mod, it's much brighter and doesn't have the same unevenness in the corners.
It is slightly dimmer when compared with the stock screen, although the camera exaggerates it a bit, they're pretty close in real life. In any case, considering it's only using a third of the power it's more than acceptable.

step-by-step diagrams

1 comment:

  1. Hello,
    Did you buy a regulator pcb on ebay? I'm not 100% sure what exact components to use. Could I get you to build me one?