Dual-Dimming Mushroom Lamp and Repair A dual-dimming desk lamp adds a small bulb to the single-dimming design. Turning the knob first turns on the power switch, illuminating the small bulb; continuing to turn the knob gradually increases the brightness of the main lamp while gradually decreasing the brightness of the small bulb, and vice versa.
Typical circuit principle of a dual-dimming mushroom lamp: A dual-dimming desk lamp is actually a combination of two single-dimming lamp circuits, but both circuits share a single dimmable potentiometer RP1 that also functions as a power switch. BCR1 and BCR2 are the thyristors in the small and large headlight circuits, respectively; SB1 and SB2 are the bidirectional trigger diodes in the two dimming circuits, respectively; R3, C3 and R4, R5 form the phase correction circuits for their respective units, used to adjust the thyristor conduction angle and widen the dimming control range; C2 and C4 determine the thyristor conduction angle. When the boom of RP1 is adjusted to the right in the diagram, C4 charges faster, the conduction angle of BCR1 decreases, thus dimming the small headlight and brightening the large headlight, and vice versa. To reduce the influence of high-order harmonics, a high-order harmonic absorption circuit consisting of C1, C5, and L is added to the circuit.
Common faults and repair methods for dual-dimming mushroom lights are as follows: 1. Both lights are not lit. First, focus on checking the potentiometer with the switch. If the power switch is damaged, replace the potentiometer with the rated resistance. Second, a burnt-out filter inductor L will also cause both lights to be off.
2. Only one lamp is not lit. The main reasons include: broken filament, poor contact of RP1 or RP2, burnt-out components on the printed circuit board, short circuit of C2 or C4, open circuit of SB1 or SB2, or open circuit of BC or BCR2. Since bidirectional trigger diodes are generally difficult to measure directly with a multimeter, a resistor short-circuit method is recommended. Connect a 4.7Ω resistor across the bidirectional trigger diode on the side of the unlit bulb. If the lamp lights up when RP1 is adjusted, then SB is faulty. When replacing it, it's best to choose a diode with similar performance to the original. In an emergency, two 3DG1 or 3DG12 diodes can be connected together with their collectors connected and their bases left floating. Then, connect the two emitters to the two ends of SB in the circuit.
3. Brightness adjustment of one or both lamps is ineffective. Check for poor contact of RP1, short circuit of R1-R3, and malfunction of BCR1 or BCR2.
4. During dimming, if the light brightness suddenly increases or decreases, or the brightness adjustment range is small, check if R4 and R5 are open-circuited, and if C3 is short-circuited or open-circuited.
5. If the thyristor continues to burn out after replacement, focus on checking if C1 and C5 are damaged or degraded. A resistor-capacitor series circuit can be connected in parallel across the bidirectional thyristor, where R is 100Ω/0.5W and C is 0.1μF/400V.
