In all LCD panels, there is a light diffuser behind the liquid crystal display. This has to have very diffuse light to light the screen evenly. This will be used to create a light table suitable for photography, drawing, and many other uses.
Overview
It is fortunate that the panel came with a CCFL driver so it will be much less work to get it to light up. The main controller chip’s part number is ‘MP1009’ which is a half-bridge CCFL inverter controller. The next chip is a jellybean dual n-channel MOSFET which handles the grunt of converting DC to AC. The transformers seen wrapped in Kapton tape are high-voltage low current types. The primary winding is visibly identified by the thicker copper windings. The secondary consists of many wraps of very small copper wire.
Reverse Engineering Circuit
Using the datasheet I started reverse engineering the circuit and came up with a basic diagram of how the circuit works. The Mosfets are represented using switches. The circuit topology is a half-bridge DC-AC converter (Inverter) meaning that there are only two switches instead of four in a full-bridge configuration. When the system is at rest, the voltage on the primary floats to the middle of the voltage rail, due to the resistor divider. There is a lot left out in this simplified circuit, there is circuitry to provide a high voltage pulse to start the arc of the CCFL light, then the lower potential current can pass through from the transformer. There is also a voltage sense line tapping off the high voltage line (top of the secondary of the transformer). Circuit protection is also not included in this diagram.
When the top switch is closed, the current flows from the voltage rail through the top winding of the transformer and then to ground. This induces current to flow out from the top winding of the secondary transformer through the cold-cathode fluorescent light (CCFL), and finally returning to ground, emitting light.
When the bottom switch is closed, the current flows the opposite way through the transformer. This induces current to flow into the transformer which lights up the lamp.
When a voltage/current meter is placed into the circuit, the following waveforms will result. The positive direction of current flow is defined as flowing to the voltage divider.
Making the Panel Work
After the reading the datasheet, the inverter chip has the lowest voltage rating of 30V, and a minimum voltage of 8V, making the voltage range of the circuit 8-30V. The connections to the board are as follows
| Pin Number | Description |
|---|---|
| 1 | Von/Voff |
| 2 | VAdj |
| 3 | GND |
| 4 | GND |
| 5 | Vin |
| 6 | Vin |
| 7 | N.C. |
Von/Voff is a enable/disable pin. The datasheet states that when the voltage is below 0.4v, the chip is disabled. A higher voltage will enable the chip. The VAdj pin is used to control the brightness of the CCFL, this will be connected to Vcc to disable this functionality.
I will start powering the circuit with 12v since it is a standard voltage for monitors/computers. The monitor lights up at around 10v then increases in brightness until it reaches max brightness at around 20v before the over-voltage protection kicks in. It seems to run fine at 12v.
Here is the effect I was able to achieve with the camera settings of F36,200, ISO200. This will work to create the ‘cropped’ effect.
