Draw a local line according to the object as shown in figure 1. As can be seen from the figure, the secondary induced voltage of the main power transformer is rectified by DM85 1/2/7/8 bridge and filtered by CM85 1, and a stable voltage of 235V is obtained (measured). RM85 1-RM860 is 10 470kΩ resistors connected in parallel to form a 47kΩ resistor as the main power supply. R9 102 ~ R9 104 are the input voltage sampling resistors of the backlight chip. When the input voltage is too high or too low, the chip stops working. Q9 10 1 and LM80 1 form a step-up switching power supply under the control of SLC3011m. High frequency and high voltage are rectified by D9 10 1 and filtered by CM852, and then used as backlight voltage to supply power to lamp beads. Because the lamp beads are driven by constant current, the backlight power supply circuit does not need voltage stabilization. Here, r9117 ~ r9120 are voltage-stabilized sampling resistors and backlight overvoltage protection sampling resistors. When the backlight voltage is too high, the chip stops working. R9 105 is the over-current protection sampling resistor of Q9 10 1. The sampling voltage is sent to pin ③ of the chip. If the voltage is too high, the chip stops working. Q9 102 is the bulb constant current control tube, R9 1 13 is the current sampling resistor, and the sampling signal is sent to the chip pin. VR 1 is a constant current setting potentiometer. When the current sampling signal deviates from the set value, the chip will automatically adjust to the set value. Strip, one light strip *** 100 light beads, 50 in series as a group, two groups in parallel, as shown in figure 1. Using a self-made tester, it was found that a light beam broke 9 light beads, but one actually broke 28 light beads. I found the problem. I can't find the same type of light bar on the Internet, but there are similar light beads. I want to try to replace the damaged light beads.

The LED light bar generates a large amount of heat when it works, so it is installed on a special aluminum strip, which has a good heat dissipation effect, but manual welding is very difficult. First remove the damaged lamp bead, use an electric soldering iron with power greater than 35W (the welding head is relatively thick, which is about the size of the whole lamp bead), eat more tin, put it on the luminous surface of the lamp bead, and gently rotate the welding head. Burn the plastic on the surface of the lamp bead first, while continuing to heat it, let the welding head tilt slightly, and at the same time gently push the lamp bead outward, because the bottom of the lamp bead and the substrate are welded with heat sinks, and the lamp bead can only be removed after the tin on the heat sinks melts. Remove the lamp beads and clean the pads with a pointed electric soldering iron. Because it is impossible to weld the heat sink and the substrate at the bottom of the lamp bead by hand, a little silicone grease is coated on the bottom of the substrate and the lamp bead to help heat dissipation (the lamp bead is very small, and the welding feet and pads are very small. Firstly, the lamp beads are positioned by using the privacy of silicone grease, and then the gap between the two lamp beads is filled with a pointed soldering iron and rosin welding wire. If the heat dissipation is too fast, the lamp bead pin and the substrate pad cannot be reliably welded, and it can be used normally as long as it can be reliably connected with the adjacent lamp bead. After all replacement, let the light bar light up and bend the light bar slightly to both sides. If there is a badly welded lamp bead, it will be reflected.

A voltage stabilizing tube is reversely connected in parallel in the LED lamp bead for backlight. When the lamp bulb fails, the voltage stabilizer tube breaks down to provide a current path (actually, the broken tube still has a residual resistance of about10Ω ~ 20Ω, which is not a direct short circuit), and other lamp bulbs can still work normally. However, if the internal voltage stabilizing tube is open when the lamp bead is damaged, the whole light bar will not work. When the LED passes through different currents, the color temperature of light will change, which will cause the color change of the screen, so the LED needs constant current driving. Adjusting the brightness of backlight is actually controlling the intermittent lighting time of LED. Because the intermittent frequency is high, the eyes can't feel the flicker, and the constant current is constant during the lighting period. Generally, a string of light bars is controlled by a constant current, and a few light beads in the light bar are damaged, while the current in other light beads is basically unchanged. The machine adopts two parallel connections and two series connections. When individual lamp beads in a street lamp string are damaged and the voltage regulator is short-circuited, the light bar can still emit light, but the voltage drop at both ends of the string with damaged lamp beads will be reduced (the resistance will be reduced). Due to constant current drive, one current with small impedance in parallel light string will increase and the other current will decrease. The increase of current will improve the brightness of the lamp bead to make up for the damaged lamp bead. But it also shortens the life of the lamp bead. If there are lamp beads damaged in the light bar, it will cause excessive current, accelerate the damage of other lamp beads, and the current imbalance will be aggravated. If more than 10% of the lamp beads are damaged, serious current imbalance will cause the remaining lamp beads to burn out quickly. Four strings of light bulbs were inspected, and it was found that among the two strings connected in parallel, group A had 1 broken, group B had 27 broken, group C had 2 broken, and group D had 8 broken ... Among them, group C 1 and group D 1 were damaged in the open circuit, which led to the failure of all light bulbs. The machine has only been used for more than three years, and the working hours are not very long. There may be three reasons why so many lamp beads are damaged: 1. The parallel structure causes current imbalance; 2. Output 330V/280mA according to the backlight. Using1.2kΩ (six 200 12/ 10W resistors in series) as a dummy load, the measured constant current value is 300mA, which is 20mA greater than the nominal value; The maximum brightness of this machine is 20. After the repair, the backlight brightness of the viewing menu is adjusted to 18, and it works under high brightness for a long time.