A Liquid Crystal Display (LCD) is a display technology that works based on the principle of directing liquid crystals placed between two transparent surfaces through electrical effects. Unlike traditional screens like cathode ray tube (CRT) displays, LCD technology creates flat, thin, lightweight, and low-energy-consuming display systems. As a result, LCDs are widely used today in many areas, including televisions, laptops, calculators, and car dashboards. LCD systems regulate the passage of light through liquid crystal molecules to form an image, and they do not emit light directly; instead, they use a backlit structure.

^{Visual of an LCD Screen Example(Anadolu Ajansı)}

Structure of Liquid Crystals

Liquid crystals are a special class of substances that possess characteristics of both liquids and solids. These structures exhibit liquid fluidity within specific temperature ranges while displaying a crystalline-like regularity at the molecular level. They are typically composed of long, thin molecules and are sensitive to electric fields. Liquid crystals can be categorized into three main arrangements: Smectic, Nematic, and Cholesteric.

• Smectic liquid crystals are structures where the molecules are arranged in a specific order both horizontally and vertically.
• Nematic liquid crystals have molecules arranged only along the vertical axis and represent the most commonly used arrangement in LCD technology.
• Cholesteric (or chiral nematic) crystals are structures where the nematic arrangement is twisted into layers in a spiral shape.

The Twisted Nematic (TN) structures commonly used in LCD systems align these nematic crystals by rotating them 90°. This alignment allows the controlled change of the light's direction, enabling image formation.

Basic Structure of an LCD

An LCD screen typically consists of a liquid crystal layer sandwiched between two glass panels. Inside these glass surfaces, there are electrode layers made of transparent conductors. On the outside of the glass, two polarized filters are present. Color filters (RGB: red, green, blue) are placed in front of the top glass, while the back of the bottom glass contains a backlighting system (typically an LED lamp).

The basic display mechanism of an LCD screen is based on the alignment of the liquid crystals when an electric field is applied, and whether the light passes through the polarized filters according to their orientation. This structure is well insulated to prevent environmental effects, as liquid crystals are highly sensitive to external factors such as heat, UV light, moisture, and oxygen.

^{Visual Representing the Function of an LCD Screen (ptcled)}

Operating Principle

In LCD technology, image formation is achieved by directing light. The white light from the backlight source passes through a polarizer to become linear. As this linear light passes through the liquid crystal layer, its direction changes according to the arrangement of the crystals. If no voltage is applied to the electrodes, the crystals remain twisted at 90°, and the light, having changed direction, passes through the top polarizer. In this state, the cell is in the "open" position, and the light reaches the front surface of the screen.

When voltage is applied to the electrodes, the crystals align due to the electric field, and the twist is removed. In this case, the incoming light passes through without bending and cannot pass through the second polarizer since it arrives at a right angle. This results in a dark appearance in the relevant pixel of the screen.

Through this system, the brightness and color of each pixel can be controlled. Using color filters, the components of light are separated, and red, green, and blue sub-pixels are created. By controlling these sub-pixels at different intensities, the desired color image is formed.

LCD Types

LCD screens are classified into three main types based on the arrangement of color filters:

• Delta-type LCD: RGB sub-pixels are arranged in a triangular form. This type is preferred for displays with a more circular design.
• Stripe-type LCD: Sub-pixels are arranged in horizontal or vertical stripes. It is the most commonly used standard arrangement.
• Mosaic-type LCD: RGB pixels are arranged in a mosaic pattern, irregular but evenly distributed.

These arrangement differences directly affect the color accuracy, resolution, and image clarity of the screen.

Driver Methods

The electronic drivers that enable the operation of LCD screens work based on two main principles:

• Static Driving Method: Requires a separate signal line for each cell. This method is not preferred in complex and high-resolution systems due to its costly and difficult-to-implement structure.
• Dynamic Driving Method: This method is further divided into two subcategories:
• Passive Matrix System: Direct control is not applied to each pixel. It is used in simpler and slower systems, such as calculators.
• Active Matrix System (TFT - Thin Film Transistor): Each pixel has its own transistor (usually MOSFET) behind it. This design provides high resolution, fast refresh rates, and wide viewing angles.

TFT LCDs are commonly used in computer monitors, televisions, and smartphone screens.

Applications of LCD Technology

LCD technology is used in a variety of electronic devices today. LCD panels are found in televisions, monitors, laptop screens, navigation systems, digital clocks, and calculators, among others. The thin design, low energy consumption, and high image quality of LCD screens make them widely preferred in many areas.

LCD televisions use digital components like video analyzers, image processors, and analog-to-digital converters (ADC) instead of cathode ray tubes (CRT) to form images. With these components, images are synchronized at the pixel level and spread uniformly across the screen. Since there is no scanning line issue, a homogeneous image is presented.

Power Supply System and I/O Ports

LCD televisions use Switched Mode Power Supply (SMPS) systems to generate different DC voltages such as 33V, 12V, 9V, 5V, 3.3V, and 1.8V. These voltages are used in various subunits like the tuner, inverter, audio amplifiers, and image processors. These values may vary depending on the size of the LCD panel.

The I/O connection ports of LCD devices are quite diverse. The most common ones are as follows:

• PC IN (15-pin VGA): Used for computer connection.
• ANTENNA INPUT: Allows television signal reception via coaxial cable.
• AUDIO/VIDEO OUT: Transmits audio and video signals via RCA connectors.
• Y/C VIDEO IN (S-Video): Used for camera connections.
• COMPOSITE VIDEO IN: Allows connection of analog video sources.
• AUDIO IN (L-R): Receives audio signals from external audio sources.
• PC SOUND INPUT: Allows sound from a computer to be transmitted to the LCD TV.

Common Faults and Causes

Faults seen in LCD-equipped devices are generally categorized into three main groups: power-related faults, image (raster) faults, and sound faults. Most faults occur in the power supply section. Faults can arise due to overheating, exposure to high current, or environmental conditions affecting components in the SMPS circuit.

Fault detection is usually carried out systematically: first, the integrity of external connections is checked, followed by the internal circuit components. The color or flashing status of the LED indicators on the screen may provide clues about which circuit is faulty. For example, if the screen light is off but sound is present, the lighting circuit should be checked; if there is no sound, the sound processor or amplifier should be checked.