Powering monitor LED backlight separately.

[amadeok]

Hello,
Since i suffer from eye strain lately i've been trying to reduce the brightness of my monitor and also use a analog voltage instead of usual PWM of the monitors backlight driver. Inside the monitor the power supply was essentialy working as a LED driver and providing constant current rather than constant voltage to the backlight, so it was pointless to try to reduce the voltage/current from there as it would just increase the voltage and eventually turn off. So after reading the documentation of the panel i succeeded in turning the backlight(which is essentially a LED strip) on by using a step up variable voltage converter circuit board based on the XL6009 (one of those you find on ebay or aliexpress) attached to a 12v 5A AC DC power supply.
So i'm now able to dim the brightness to the level i want, but since the backlight now is being fed by a different source than the monitor, the two are no longer synced together, when the monitor goes on standby for example, the backlight keeps on. I'm mostly concerned that i may damage the panel by this arrangement, is it safe to run the monitor in this way?
thanks

 

[Nigel Goodwin]

It shouldn't do any harm, but it's wasting power and wasting life on the LED's.

 

[Diver300]

It won't hurt to run the backlight with the monitor turned off. You could use the old backlight driver to run a relay or something to turn the new backlight turn on and off.

 

[ChrisP58]

If you can identify the current sense element in the existing driver, you should be able to change the default current.

Most likely it would be a low value (< 1 Ohm) resistor in the power supply-LED loop. Replacing that resistor with a lower higher resistance should reduce the current.

Corrected as per Diver300's comment below.

 

[Diver300]

If you can identify the current sense element in the existing driver, you should be able to change the default current.

Most likely it would be a low value (< 1 Ohm) resistor in the power supply-LED loop. Replacing that resistor with a lower resistance should reduce the current.

All the current control systems that I have seen work the other way round to that. I would suggest a larger resistor to reduce the current.

 

[ChrisP58]

All the current control systems that I have seen work the other way round to that. I would suggest a larger resistor to reduce the current.

You are right. I had my thinking cap on backwards.
A higher resistance value will produce the same feedback voltage from a lower current.

 

[gophert]

It shouldn't do any harm, but it's wasting power and wasting life on the LED's.

In the winter months, I wouldn't worry about wasting power. The energy just goes into heating your office. As for the LED life, your new setup is specifically designed to run them at reduced power. My contact at Osram said that lifetime more than quadruples when you cut current to half of max rated current. And if you cut to a quarter of half-rated current, the LEDs May run forever.

I wouldn't change anything if your circuit works (illuminates). My question is, does it work as far as reducing eye strain?

 

[Nigel Goodwin]

If you can identify the current sense element in the existing driver, you should be able to change the default current.

Most likely it would be a low value (< 1 Ohm) resistor in the power supply-LED loop. Replacing that resistor with a lower higher resistance should reduce the current.

Usually it's all done inside an IC, which even if you can identify which one it's rare that a datasheet would be available.

As long as it's working fine with what he's done?, the only real change needed is to make it switch OFF when the monitor turns OFF - easiest way is just a relay - there's probably a 12V rail that switches ON and OFF, connect the coil across that (with a reverse diode across it) and use the relay contact to switch the mains to the LED power supply.

 

[gophert]

As long as it's working fine with what he's done?, the only real change needed is to make it switch OFF when the monitor turns OFF

As said, there is no harm or concern with leaving the light on except for a few people on this site that are bothered by the idea of some LEDs running at reduced power behind a blacked-out LCD panel. Really no concern.