LS7538/TT6061/4KA514 touch step dimming desk lamp circuit diagram

Fault phenomenon: A SL-430 luxury decorative desk lamp made in Guangdong. After plugging the power cord into the power socket, the bulb always lights up, and the brightness of the bulb does not change when the lamp holder is touched.

Maintenance: Open the base of the desk lamp and take out the control circuit board. It is found that the control chip of the dimming desk lamp is a circuit board (COB) eight-pin package device (marked on the board is 4KA514). After careful inspection, it was found that there were cracks on the side of the thyristor Q1 (BT134). Using a multimeter to check the electrical resistance, it was found that the T1 and T2 poles of the thyristor were short-circuited. This is the reason why the bulb always lights up after powering on. I thought I had found the fault point, but after replacing Q1 with the same model of thyristor, the light bulb stopped lighting up. No matter how I touched the lamp holder, there was no response. It seems that the fault is not that simple.

In order to facilitate detection and analysis, the electrical schematic diagram is drawn according to the circuit board as shown in the attached figure. But what kind of control chip U1 will be and what its working principle is unknown. I couldn't find any information about 4KA514 on the Internet. I guess it may be a model defined by the manufacturer. When searching for "dimmer light control chip", I found that the pin functions of LS7538 and TT6061A (touch step dimming dedicated chip) in the application circuit are basically the same as this chip, so I regarded u1 as LS7538 to analyze the circuit in the attached picture. : R2 is connected between Vdd and the ① pin of U1 to determine the initial brightness of the light: R1 and C4 are connected to the ② pin of the chip to realize the zero-crossing detection function of the AC mains power to achieve reliable control of the conduction and cut-off of the thyristor Q1 ;The ④ pin of the chip receives the touch sensing signal through R4, C5~C7; the RC network composed of R5 and C1 is connected between the chip ⑤ pin and Vdd to realize the sensitivity control of the touch signal; the potential of the chip ⑥ pin determines the step of brightness adjustment Sequence: Pin ⑧ of the chip outputs a thyristor control signal with adjustable pulse width to control the on and off of thyristor Q1.

From the working principle of the touch-type dimming lamp, we can know that the ⑧ pin of the control chip (u1) should output a pulse width modulation signal of tens of KHz, and control the on-off time of the thyristor Q1 through R7 and C3 to realize the brightness adjustment of the bulb. Use a multimeter (DC voltage gear) to measure the voltage between this pin and the ground (neutral line). Under normal circumstances, there should be a certain DC voltage value displayed. And the voltage value should change with the touch shift. But the actual measurement shows that no matter how you touch the desk lamp shell, the voltage displayed is only a few tens of millivolts and there is no change. Obviously the chip does not output a pulse modulation signal and the thyristor Q1 cannot work. Based on this analysis, it is believed that there may be three reasons why the ⑧ pin of U1 does not output a pulse signal: first, the peripheral resistor-capacitor components are damaged; second, the chip working voltage is abnormal; third, the chip itself is damaged. First, check the peripheral resistor components of the line. After testing, the zero-crossing detection circuit R1 and c4 are normal, the brightness control component R2 is normal, the touch sensitivity control components R5 and C1 are normal. R4, C5~C7, R7, R8, and C3 are all normal; start Check the working voltage of the chip and measure the voltage between pin ③ (power supply terminal) of the chip and ground (pin ⑦). It is only 4.4v. Obviously the voltage value is too low. After replacing the filter capacitor C2, the voltage of this pin remains unchanged. Try connecting a 56kΩ resistor in parallel to both ends of R3 to increase the supply current. As a result, the voltage increases slightly. However, the voltage value of pin ③ has not reached the 6v required for the chip to work. Pin ⑧ of the chip still has no output. So I disconnected the connection between the ③ pin of the chip and the power supply. The power supply was measured to be 9v, so I suspected that the chip was damaged. Since the "chip" labeled 4KA514 cannot be purchased, I purchased an Ls7538 chip and soldered it on a breadboard with a suitable width. According to the pin sequence of the 4KA514 chip on the original circuit, weld the corresponding pins of the Ls7538 chip and solder the original Remove R6 on the circuit board, and connect the ⑥ pin of the chip to Vdd. Power on and touch the lamp holder. The brightness of the bulb will change in five brightness states: low bright, low medium bright, medium bright, brightest, and off.