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Medical Instruments LCD Fault Diagnosis and Maintenance
2011-11-06 by seoer1

With the LCD in a wide range of medical equipment on the application of medical equipment engineers to understand and master the liquid crystal display unit of the common faults and maintenance is valuable and meaningful.

1 characteristics and structure of liquid crystal displays

1.1 LCD Features

(1) low-voltage, micropower :2-3V to work.

(2) flat-type structure: the basic structure of liquid crystal display device is made of two thin glass box, this structure can accommodate a limited area on the maximum amount of display, the display of the highest utilization.

(3) Passive Display: liquid crystal display device itself does not glow, it is modulated by the external light to display purposes. Therefore, no glare, no stimulation of the human eye, does not cause eye fatigue. For the dark can not see the shortcomings, if coupled with backlight, can be overcome.

(4) shows large amount of information: Compared with CRT, LCD display devices without mask constraints, so the pixels can be made smaller, more refined; compared with plasma display, liquid crystal display pixel point do not like the plasma display as to leave enough room between the pixels of the quarantine. Therefore, the size of theLCD module window in the same area within, can accommodate more pixels to display more information.

(5) easy-to-color of: liquid crystal itself, although generally there is no color, but it is easy to achieve the color of. Is more commonly used color filter method and interferometry. As the color filter method the technology matures, the color of the LCD has a more precise, more bright color and no color distortion of results.

(6) Long life: liquid crystal material is a synthetic organic polymer materials. Has a very high purity, and other materials are also high-purity substances, in very clean condition manufactured. LCD drive voltage is low, the drive current is negligible, therefore, almost no degradation of this device, long-lived. From a practical test, general use, in addition to shock, broken or damaged component parts, the liquid crystal display almost no end of life of its own (LCD backlight life is limited, we normally refer to the LCD backlight life mainly referring to the life of , but the backlight module is replaceable).

(7) No radiation, no pollution: liquid crystal display devices like CRT is not in use arising from the use of soft X-rays and electromagnetic radiation. This radiation is not only pollute the environment but also generate information disclosure, and the LCD does not have such problems, it is for personal safety and information security are very good.

1.2 The basic structure of liquid crystal displays

LCD common structure, with TFT-LCD in terms of key components including the glass substrate, liquid crystal materials, color film (color filters), polarizers, backlight modules, driver IC, ICO conductive films. First, the light from the backlight according to the polarizer, the light passes through the polarizer, the polarization will be, polarized light will pass through the LCD, because the arrangement of liquid crystal molecules produced by the voltage electrode effects, Therefore, liquid crystal can change the angle of polarization of light polarization, different polarization angles out of the light intensity caused by different intensity of light of different color film and then through the red, blue, green, 3 pixels, it will display a variety of different brightness and different colors of the pixels, and finally each pixel can be formed by the naked eye can see a variety of images and graphics.

(2) liquid crystal display of the common faults

Liquid crystal display device, almost no degradation, long life, real life is the decision LCD backlight modules. Currently used in LCD backlighting by technical division, there are halogen, light emitting diode (LED), organic light-emitting (EL), cold cathode fluorescent lamp (CCFL), cathode lamp (CLL) and metal halide lamps, etc. . One of the most widely used cold cathode fluorescent lamp, mainly because such a backlight performance, stability and cost are better, now almost all I2C LCD display used in using this light source.

I looked at some of the manufacturers data, the statistics of the current liquid crystal displays commonly used in cold cathode fluorescent lamp life basically 30000-50000 hours. Diameter 2.6-3.0 mm, tube voltage 500-1000V, the lamp current 5mA. Lamp power 0.5-7.0W, starting voltage 300-1500V.

3 instances of fault diagnosis and repair

(1) Symptom: The hospital WELLDELL-sigma2000 portable ECG monitor is turned on, power indicator light, Self-test passed (the instrument emits a short "beep" sound), there is no LCD display, sound instruments work properly.

(2) Failure Analysis and Maintenance: power indicator light, indicating that part of the normal power supply. Self-test passed, indicating that no damage to the CPU and memory. LCD screen when no self-test display, initially ruled out the possibility of damage to the system software. No hardware alarm sound, instrument sound working properly, indicating the hardware should be no damage. Observed edge-lit LCD screen, barely visible software interface. From this preliminary analysis of the backlight module failure.

Open the case, boot light guide plate is not bright, it can be concluded backlight module is faulty. Observation of the main components of a backlight module backlight lamps, high voltage circuit board, light guide and so on. Precision of liquid crystal display unit, so the first diagnosis of a separate high-voltage circuit board, the board low-voltage DC input termination board equipment, high voltage AC output termination backlight lamps. State of low-voltage DC power input measured as 11.5V, stall detection voltage with a multimeter 2500V AC output 650V (mostly due to the narrow circuit board type, be careful when measuring short-circuit to prevent the table pen). So, it could pressure the board is working properly. Next, check the backlight lamp, first open the LCD display unit on the back metal case (some instruments used in liquid crystal display unit is insulated tape back, whether it is metal shell or insulation tape, all with the LCD glass substrate in order to display unit of the external frame fixed up as a whole), then fixed the backlight lamp can be used to take out the metal tank, then carefully remove the lamp from the metal tank (mostly due to the backlight lamp diameter 2.6-3.0 mm, easily broken, so be careful). Then after the lamp still does not boot (this step is to quickly determine whether the damage to the lamp if the lamp light will not damage the lamp instructions, the only possible cause of the malfunction is the tube ends of wiring short circuit or open circuit), observed after turning off the power at both ends of the lamp casing wrapped (to prevent exposed terminals) without wear or aging, then cut open the casing, there is no disconnect terminal observation, Weld and so on. Further testing to connect high voltage output terminal of the wire and lamp, were normal conduction. Then you can finalize the backlight lamp failure due to damage or aging. Backlight lamp production process is high, generally need to buy from the manufacturer. But because of the inconvenience and delay long transport reasons,

I chose a relatively easy to buy a laptop display backlight lamp, sigma2000 ECG uses NEC's production of 10.4 "LCD, the lamp placed on the long side of the display, while the vast majority of notebook computers placed on the short side of the display tube. measuring length, the available 13.3 "LCD laptop lamp replacement. Normal light after the lamp replacement, troubleshooting.

4 Other common faults and troubleshooting procedures LCD

In addition to damaged or aging of the backlight lamp failure, the author also encountered in the maintenance of some other fault conditions:

(1) high-voltage circuit board failure;

(2) backlight lamp terminals at both ends of casing wear caused by bare, short circuit caused by contact with the metal tank;

(3) terminal off the backlight lamp, resulting in open circuit.

On this basis, the author sums up a liquid crystal display troubleshooting process.

5 Conclusion

    The excellent properties of liquid crystal display device determines its position in various types of display devices, backlight module is almost the only factors that limit their life, but there are some well-known manufacturers and a preliminary study to improve the backlight module. I believe that with the gradual improvement of liquid crystal display technology and production costs continue to decrease and eventually will replace the traditional CRT, as a closely related with people's lives, the technology.

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