Dual dimmable mushroom light

The double-dimming desk lamp is based on the single-dimming system and adds a low-power light bulb. When turning the knob, first turn on the power switch and the small light bulb will light up. As you continue to turn the knob, the brightness of the big light will gradually increase and the brightness of the small light will gradually decrease. ,vice versa. The circuit principle of a typical double-dimming mushroom lamp is shown in the figure below: The double-dimming desk lamp is actually two single-dimmer lamp circuits combined into one, but the two circuits share a dimming potentiometer RP1 that also serves as a power switch. BCR1 and BCR2 are the thyristors of the small and headlight circuits respectively, SB1 and SB2 are the bidirectional trigger diodes of the two sets of dimming circuits respectively; R3, C3, R4 and R5 respectively constitute the phase correction circuit of their respective units for controllable Silicon conduction angle broadens the dimming control range; C2 and C4 determine the thyristor conduction angle. When the boom of RP1 is adjusted to the right in the figure, C4 charges faster and the conduction angle of BCR1 decreases, so Small lights dim, big lights brighten, and vice versa. In order to reduce the influence of high-order harmonics, high-order harmonic absorption loops of C1, C5 and L are added to the circuit. Common faults and repair methods of double dimming mushroom lamps are as follows: 1. If both lamps do not light up, first check the potentiometer with switch. If the power switch is damaged, replace the switch potentiometer with rated resistance. Secondly, if the filter inductor L burns out, both lights will not light up. 2. The main reasons for only one light not lighting up are: broken filament, poor contact of RP1 or RP2, burned components in the printed board, short circuit of C2 or C4, open circuit of SB1 or SB2, open circuit of BC or BCR2, etc. Since it is generally difficult to directly measure the quality of bidirectional trigger diodes with a multimeter, it is advisable to use the resistor short circuit method for identification, that is, use a resistor with a resistance of 4.7Ω and connect it across both ends of the bidirectional trigger diode on the side of the non-lighting bulb. If the light is on during RP1, it can be determined that the SB is damaged. It is best to choose one with performance similar to the original model when replacing it. In an emergency, two 3DG1 or 3DG12 can also be replaced by overlapping. The connection method is that the collectors of the two tubes are connected. The base is left floating, and the two emitters are connected to both ends of SB in the circuit. 3. If the brightness adjustment of one or two lamps has no effect, check whether RP1 is in poor contact, whether R1~R3 ​​is short-circuited, whether BCR1 or BCR2 has poor performance, etc. 4. During the dimming process, if the light brightness suddenly becomes brighter or darker, and the light and dark adjustment range is small, you can generally check whether R4 and R5 are open circuits, and whether C3 is short-circuited or open-circuited. 5. After replacing the thyristor, if it still burns out frequently, you should focus on checking whether C1 and C5 are damaged or their performance has deteriorated. A resistor-capacitor series circuit can be connected in parallel at both ends of the triac, where R is 100Ω/0.5W and C is 0.1μF/400V.