ThinkPad hacking: LED backlight conversion

[xiphmont]

In case anyone was wondering what I was doing with all the LEDs recently, I've been converting classic Thinkpads with old, dying CCFL backlights to LED. It's a popular mod, and there are plenty of generic DIY backlight kits out there. Unfortunately, most of these kits don't work as-is, and there are no good instructions, walkthroughs, reviews or comparisons of various kits anyway.

I've just had a great deal of fun fixing that. Consider this a beta-test invitation :-)

Also... I touched the white tape. I touched it really quite a lot.

Comments

Advisable backlight PWM frequency?

[thatlarsguy (from livejournal.com)]

Hi Monty,

A lot of people find the eystrain caused by flickering of LED backlights - used to control brightness by PWM - to be worse than CCFL. This can be mitigated by increasing the PWM frequency as high as possible, which is software-controllable via the intel graphics driver (see http://devbraindom.blogspot.com.au/2013/03/eliminate-led-screen-flicker-with-intel.html).

As someone well versed (compared to me!) in LED backlight and inverter hardware - do you have a view on what a maximum safe PWM frequency would be without causing damage to the hardware? (In my case, the LED backlight I'm controlling is from a 1st-gen ThinkPad X1 Carbon.)

Thanks for any input...

Cheers,
Lars

Re: Advisable backlight PWM frequency?

[xiphmont.livejournal.com]

Hi!
(BTW, I deleted the dupe, hope that's OK. I have screening set because my posts attract a ton of spam).

Setting higher backlight frequency *should* be safe for LED backlights-- the hardware drivers for those are usually very robust switching topologies that aren't relying on (or prone to undesired) resonance. As I understand it, the safety problem is usually setting higher frequencies for the high-voltage boost inverters needed by CCFL.

Modern/recent Linux kernels set (I believe) 200Hz as the default backlight PWM frequency when using the Intel backlight driver. There was a mailing list discussion at the time that concluded it was probably a safe call for any LED or CCFL backlight. That seems to be working out OK.

Even if it's safe, once you start getting PWM frequencies up into the sensitive part of the hearing range, you'll often hear the inverter switching. So I'd keep the PWM frequency under 500Hz, or up over 20kHz (which wouldn't be an option in software config-- the inverter hardware would need to do that natively).

Re: Advisable backlight PWM frequency?

[thatlarsguy (from livejournal.com)]

That's reassuring! I've found no issue running mine at 700hz but have been a bit worried about it given the default of ~220hz, under both Windows and Linux, let alone running it in the 1-2khz range, which I would like as it reduces the visible flicker even more (using the 'pencil test' of waving a pencil in front of the screen). I've not heard any audible sound from this, though I have heard of other people reporting this at frequencies around 1khz.

Thanks for your input!