Analysis of common problems of LCD screen: black screen
LCD screen black screen is a common fault, which may be caused by many factors. The main components of LCD screen include liquid crystal layer, backlight, polarizing filter and circuit board.
A lit LCD screen can be imagined as a stage performance. If you suddenly can’t see the performance, there are generally two possibilities, either the stage light is broken or the actor did not appear. The backlight is equivalent to the stage light, and the liquid crystal layer is the actor, so we can divide the conventional display screen black screen into two situations: one is the backlight is not bright, and the other is FOG (liquid crystal, FPC, IC) failure.
- Backlight is not bright
1.Backlight source failure:
2.The backlight source is usually composed of LED lights, which provide the necessary light source for the liquid crystal layer. If the current and voltage of the backlight source fail to meet the requirements, the backlight will not light up normally, resulting in a black screen. This may be due to:
- The LED light fails or burns out.
- Insufficient power supply.
Take the basic backlight circuit diagram below as an example
When the current voltage of the circuit is lower than 27V and the current is 90mA, the backlight will not light up or will be very dim and basically invisible.
- Circuit problem:
If an LED in the backlight circuit is open or short-circuited, the backlight will not light up. Check whether the backlight circuit connection is firm, and use a multimeter to test whether the current and voltage are normal.
Poor soldering or poor plugging of the backlight pads are also common problems, resulting in poor signal transmission. Ensure that the welding is firm and check the contact between the plug and the socket.
Common backlight connection methods are shown in the figure below:
- FOG failure
Interface connection problem:
Poor interface connection between the LCD screen and the motherboard may cause a black screen. Check the plugging of the interface to ensure good contact.
Data line failure: Damage or loosening of the signal line interface (such as HDMI, VGA) may result in no signal output.
Abnormal driving voltage: The normal display of the LCD screen depends on multiple voltage signals, including:
VDD: main power supply voltage.
AVDD: Analog power supply voltage, used to drive liquid crystal molecules.
VCOM: Common voltage, used to control contrast.
VGL/VGH: Driving voltage for liquid crystal switches.
VGMN/VGMH: Mainly used to adjust the display voltage curve.
In addition, the timing and related proch (processing) parameters of the LCD screen must be configured correctly. If these parameters do not match, the LCD screen may not work properly. Use an oscilloscope to monitor whether the timing signal meets the requirements.
Initialization failure: The failure to successfully load the initialization code of the LCD screen will also cause a black screen. Check the running status of the control program and restart or reload the initialization code if necessary.
Damage to the LCD layer: If there is physical damage inside the LCD layer, the signal may not be transmitted, resulting in a black screen. Check whether there are obvious cracks or damage to the LCD layer.
3. Summary The problem of LCD screen black screen is usually related to the circuit or interface of the backlight source and the LCD layer. When troubleshooting, it is recommended to start with the following steps: Check whether the backlight source is lit normally. Confirm whether the connection between the LCD screen and the motherboard is firm. Measure the driving voltage of the LCD screen to ensure that the signal is normal. Check whether the initialization code is loaded successfully.
