LCD SCREENS CCFL TESTER
hello friends today I am gonna telling you about some easy and valuable electronics projects for school and college practical
Liquid crystal displays (LCDs) are used in a wide range of products including flat screen computer monitors, laptop computers, tablet PCs, PDAs, digital cameras, and portable devices. Compact cold cathode fluorescent lamp (CCFL) based backlight (illumination source for LCD screen)The arrangement in these applications enables a well-observable display in both dim and bright ambient light conditions. The LCD backlight mainly consists of a light source, diaphragm, light-leading board and plastic frame. It has high brightness, long lifetime and good uniformity characteristics.
The CCFL often uses a fluorescent lamp, which has a phosphor coated glass cylinder with a cathode at each end. The CCFL tube is powered by a small electronic inverter (CCFL inverter) circuit that flashes the screen electronically. The inverter circuit accepts a low-level DC input voltage and provides a high-level AC output to drive the backlight CCFL tube (s). Figure 1 shows the CCFL for LCD. The need often arises to check for a faulty LCD backlight circuit to find dead components..
Fig1; circuit diagram
Absence of back light means either the CCFL inverter and / or the CCFL tube is in dead state. Here is a simple circuit to test the CCFL tube in a LCD backlight unit, which might help the hardware technician to speed up the repair work. This basic circuit (go / no go test) is portable, 6V battery operated and can be used to test almost all types of LCD backlight CCFL tubes.
The working of the CCFL circuit is shown in Fig. 2 A timer IC NE555 (IC1) is wired as astable multivibrator (AMV) to drive a standard MOSFET T1 (IRF 512 / IRF 830). Components R2, R3 and C3 determine the operating frequency of the AMV. MOSFET T1 switches the inverter output transformer (X1) to produce high-voltage AC supply at its output terminals. Here an ordinary step - down transformer is used as the inverter transformer by reversing its primary and secondary windings.
Assemble the circuit on a general - purpose PCB and en close in a suitable cabinet. Fix the 2 - pin connector on front side of the cabinet in such a way that the CCFL under test can easily be connected here. Fix the test switch on top of the frame. Keep the 6V battery in side the cabinet. To test a CCFL, connect its ter minals to the primary leads of transformer X1 and press test switch S1 momentarily. If the CCFL is good, you will notice a dim / bright glow in it (depends on the wattage rating of the CCFL under test). Always try to keep the test time as short as possible.
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