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The common fault diagnosis and maintenance of the medical equipment liquid crystal display (LCD)

Source:本站 Date:2015-09-07 

With the widespread use of the liquid crystal display in medical equipment, it is worthy and meaningful for medical equipment engineers to understand and obtain the knowledge of common fault diagnosis and maintenance of LCD.

1.The characteristics and structures of the LCD

1.1The characteristics of the LCD

(1)Low pressure, weak power consumption: it can work with 2-3V.

(2)The flat type structure: The basic structure of the liquid crystal display is a thin box made by 2 pieces of glass substrates. This kind of structure can contain the maximum displaying content in the limited area, so it has the highest utilization of the displaying content.

(3)The passive display: The LCD itself can not give out light, but it reaches the goal of display through utilizing the outside light. So there is no dizzy light, and will not stimulate human eyes or lead to eyestrain. In order to avoid the shortcoming that can not see in the darkness, we just need to provide the backlight for it.

(4)It can display more information. Comparing with CRT, the LCD doesn’t have shadow mask restrictions, so the pixel point can be smaller and more meticulous. Comparing with plasma display, the pixel point of the LCD does not need to leave definite isolation region likes plasma display. So the LCD can contain more pixel and display more information in the display window area with same size.

(5)Easily colored: Even though the liquid crystal does not have color itself, it is easy to be colored. And we of ten use color filtration method and interferometric method to realize it. With the mature development of color filtration method technology, the effect of being colored can become more accurate, bright-colored and true.

(6)Long life span: the liquid crystal material is the organic high polymer synthetic material. It has extremely high purity and the other materials are all high purity substances that made in an extremely purified condition. The driving voltage of liquid crystal is very low and the driving electrical current is much less.

(7)Without radiation and pollution: the LCD will not produce X-ray and electromagnetic wave radiation as CRT when it is used. This kind of radiation will not only pollute the environment but also can reveal the important information. However, the LCD will not make these problems; it can well ensure human safety and keep information secret.

1.2The basic structure of LCD

1.The construction of the common LCD: For the TFT-LCD, the key components include glass substrate, liquid crystal material, color films (color filters), polarizer, backlight module, driving IC, ICO electric conductive film and so on. Firstly, the backlight shines on the polarizer and the light will become polarized after the ray of light pass through the polarizer. Then the polarized light will cross over the liquid crystal. Because the arrangement of the liquid crystal molecules is influenced by the voltage of electrode, so the liquid crystal can change the angle of polarized light. The different polarized light can create different intensity. And the light with different intensity will display various brightness and colors through red, blue and green of color filters. At last, we can see many visible images and pictures that are made by many colors. 

2.The common faults of LCD

There is barely degradation for the liquid crystal display, and it has an extremely long life span. The life span of the LCD is truly decided by the backlight module. At present, the backlight that is used by LCD can divide into halogen lamp, LED, EL, CCFL, CLL, metal halide lamp and so on. And the CCFL is the most widely used backlight among them. It is mainly because that this kind of backlight has best features, stabilization and cost. So nearly all the LCD displays adopt this kind of backlight nowadays.

After searching many manufacturers’ materials, we can conclude that the life span of CCFL which adopted by common LCD is 30000-50000 hours. The external diameter is 2.6-3.0 mm, the voltage of lamp is 500-1000V, the electric current of lamp is 5mA, the lamp power is 0.5-7.0W, and the trigger voltage is 300-1500V.

3.The examples of fault diagnosis and maintenance

(1)The failure phenomenon: after turning on the WELLDELL-sigma2000, the power indicator lamp is bright, the system pass through self-test (the equipment gives out a short and fast sound). But there isn’t any image in the liquid crystal screen. The sound is normal when the equipment works.

(2)The faults analysis and maintenance: the power indicator lamp is bright, that means the power part is normal. The system pass through self-test, that means CPU and memory does not damage. There isn’t any image in the liquid crystal screen, so the system software turns out to be not damaged. There is no alarming voice of the hardware and the sound is normal when the equipment works, so it means the hardware is normal. Seeing the liquid crystal screen under the backlight, we can see a blurry system software operation interface. So the preliminary analysis is the fault of backlight module.

Opening the cabinet, the light guide does not shine after turns on the machine. So it can ensure that the backlight module is broken. The main components of the backlight module are backlight lamp, high voltage circuit board, light guide and so on. The liquid crystal display unit is very precise, so we should check the independent high voltage circuit board at first. The low voltage direct-current input of this circuit board connects the equipment motherboard, and the high voltage alternating current output connects the backlight lamp. The input of low voltage direct current is 11.5V in the opening situation and the output of high voltage alternating current that tested by 2500V universal meter is 650V. So we can ensure that the high voltage circuit board can work normally. Then we will check the backlight lamp. At first, opening the back metal skin of the liquid crystal display, and then take out the metal shaft that is used to fasten the backlight lamp. Next, carefully remove the lamp from the metal shaft (Because the external diameter of the backlight lamp is mostly 2.6-3.0mm and it is very fragile, we need to be careful). At this time, the lamp is still not bright (This step is used to quickly judge the lamp is broken or not. If the lamp is bright, we can see the lamp is not broken. And the only fault reason is maybe the short circuit between two sides of the lamp). Turning off the power and then watching the sleeve to check if there is abrasion or aging phenomenon. Next, cutting open the sleeve and watching if there is disconnection or rosin joint phenomenon. Then we check the wire that connects the high voltage output and the lamp terminal, and it works normally. At this moment, we can definitely ensure that the fault reason is the damage or the rosin joint of the backlight lamp. The backlight lamp is not easy to produce, and we often need to purchase from the manufacturer. However, due to the inconvenience of the transportation and the long term delay, the author chooses laptop display screen backlight lamp which is easier to buy. The sigma2000 electrocardiogram monitor uses the 10.4 inches LCD that manufacturing by NEC Company. The lamp put in the long side of the display screen, but most laptop lamp put in the short side of the display screen. After measuring the length, it can be replaced with the laptop lamp of 13.3 inches LCD. Then the lamp can bright normally, so the fault is totally resolved.

4.The other common faults and the fault diagnosis procedures of LCD

Except for the faults of backlight lamp damage and ageing, the author has still been confronted with other faults in the maintenance work:

(1) The faults of high voltage circuit board

(2) The two side sleeves of backlight lamp are damaged and leads to the expose of the terminal. And thus it can result the short circuit when touching with metal shaft

(3) The disconnect of the backlight lamp terminal, this it results the short circuit

The author summarizes a set of LCD fault diagnosis procedures on this basis.


The outstanding features of the liquid crystal display decide its high position in the variety of displays. The backlight module is nearly the only factor that restricts its life span. But at present, some famous manufacturers have researched and initially improved the backlight module. We believe that it will certainly replace the traditional CRT and become the technique that closely related to human’s life with the gradual improvement of liquid crystal display technology and the constant reduction of the manufacturing cost.