LCD Backlight Electro luminescent Inverter

Posted on Feb 5, 2014

There are two major types of backlights for LCDs: LEDs, which stands for light emitting diode, or EL, which stands for electroluminescent. EL backlights tend to be more efficient and have more uniform lighting than LED ones, but they require some tricky circuitry to drive. If you`ve heard of Organic LED displays, or OLEDs, that`s basically an arra

LCD Backlight Electro luminescent Inverter
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y of electroluminescent strips shunk down to the size of individual pixels. Every kit shipped before 4/07/2009 has an LCD which actually includes an electroluminescent backlight. Kits shipped on 4/07/2009 or newer have an LED-based backlight, so this video does not apply. Electroluminescent backlights are hard to drive because they actually require about 100 volts AC at around 150 Hz, and we`re just running our logic off 5 volts DC. But we can use an inductor to generate these high voltages. After a current is allowed to build up in the inductor, the switch is opened. The fundamental device law of any inductor is V=L*dI/dt, where dI/dt is the change in current per time. So if the current is asked to change from something to zero basically instantly, a huge voltage will be present. (For our actual circuit, we measured a 300V peak and roughly 100mA of peak inductor current, which means dI/dt=3, 000, 000 Amps/second, or switching off in about 33ns! This matches well to the 22ns rise time listed on the IRF730A datasheet. ) Whenever the voltage on the unconstrained end of the inductor is greater than the voltage across C1, diode D1 lets charge flow and further charge C1. This is basically a peak detector, finding the maximum to make a smooth DC signal. The 100K resistor guarantees that if Q2 is open, the electroluminescent panel will have zero voltage across it, because the 100K resistor will discharge the very small capacitance...

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