TV LED backlight strip detection, strip current, lamp bead voltage

Now into the era of LED color TV, the use of LED lights as the backlight of LCD TVs, than the lamp as the backlight is much improved, one is to make the TV set thinner, the second is more power-saving, the third is the failure rate has dropped significantly. Fourth, it is easier to overhaul and judge faults, and fifth, there is no high pressure, and it is safer to repair.

LED TV backlight, is dozens of LED light-emitting tubes in series into a string, according to the screen size is different, some string is more than 20 LED tubes, some are more than 30 LED light-emitting tubes, some models set the LED string on the left and right sides of the screen, that is, a string on the right side of the screen, a string on the left side of the screen, some models place the LED light-emitting string on the bottom edge of the screen, from the center of the screen to the right edge of a string, from the center to the left edge of a string. The current of each string of LED tubes is 250mA to 350mA, which depends on the factory parameters of the LED tubes. When normal light is emitted, the voltage drop of each LED tube is about 3.3V. The power consumed by LED backlights accounts for about 85% of the power consumption of the whole machine.

In LED TVs, the detection of LED backlight is very easy, and it is convenient to detect whether the LED light is normal with a hand-cranked 500V megohmmeter that repairs the motor. Megohmmeter in the shake of the handle, will produce 500V DC voltage output, the maximum current output is 1mA, take the TV apart, find the lead of the LED string, connect the positive pole of the output of the shaking meter with the positive pole of the LED string, connect the negative pole of the meter with the negative pole of the string, shake the handle of the watch, the string will emit light, the resistance displayed by the megohmmeter is about 0.5 trillion, which indicates that the LED backlight is good, if one of the LED lights in the LED string is broken or open, the LED string will not light up. When the output of the megohmmeter is reversely connected with the LED string, shaking the megohmmeter LED string is not lit, at this time the resistance displayed by the megohmmeter is related to the structure of the LED lamp in two situations: some LED wicks do not have reverse parallel protection diodes inside, at this time the resistance displayed by the table can not be infinite, if the LED wick is connected in reverse parallel with the protection diode, the resistance displayed by the megohmmeter at this time is 0.

The voltage of a single LED backlight is about 3.3V, and the current is about 300mA. It is not possible to measure below 1K stops of the pointer meter, because the battery used by the pointer meter at this time is 1.5V, and the light-emitting tube needs 3.3V to turn on. At this time, you can use the 10K file of the pointer meter, the battery in the meter used at this time is 9V square electricity, the black watch pen is connected to the positive pole of the LED light, the red pen is connected to the negative pole of the LED, the good LED light will emit light, if it does not emit light, it is broken. If the resistance value is small, it breaks down.

 

Summary:

1) It can intelligently output the appropriate voltage according to the number of lamp beads to be tested in series.     

2) The maximum output current of the meter is slightly greater than 2MA, which is much less than the actual working current of the light bar, which will not only not damage the light bar (lamp beads), but also easier to find the lamp beads that are not lit. It turns out that when the lamp bead is slightly defective, the actual circuit can still be lit, but it cannot be lit with a megohmmeter.

3) Another advantage is that without removing the screen, you can measure on the backlight plug-in of the constant current board (first unplug the plug-in from the constant flow board), and you can quickly find the bad light bar through the comparison of internal resistance.