A voltage divider is a series of two resistors one can use to lower an input voltage to your desired voltage. Let’s take the BKL_EN circuit, for example. Many circuits inside of a laptop don’t come on until they see voltage at their enable pin – think of the enable pin like their power switch. The backlight boost circuit runs off of 12 volts; however, most of these chips ask for 2.5-3.5 volts for enable. The backlight circuit has a set of transistors that enable 12v to flow through to it when it receives the proper signals, but no 3v to be found in this circuit!

So, what do we do? Do we create an entirely new circuit along the 3.3v rail that knows to come on when backlight is supposed to come on just to create this BKL_EN 3 volts? No, waste. We use a voltage divider to turn 12v into 3v just for the BKL_EN pin!

Here’s a video demonstrating the concept on an 820-2879 board, on a board we recently repaired under our Macbook logic board repair service.