Energy-saving lamp circuit and maintenance

There are two specifications of PHILIPS 14W energy-saving lamps, one is 2U1 lamp tube, balanced arrangement, the other is 3U1 lamp tube, triangle arrangement. Now take 2U1/14W lamp tube as an example to introduce its circuit principle and common fault troubleshooting for reference.

1. Circuit Working Principle

The circuit principle is drawn according to the actual object as shown in the attached figure. The number of components is the same as that of the circuit board. This circuit belongs to a half-bridge high-frequency inverter circuit. The mains 220V is added to the rectifier tubes D1~D4 for bridge rectification after passing through the fuse resistor TR. C2, inductor L1, and C1 form a π-type high- and low-frequency filter circuit, which not only prevents the interference of high-order harmonics on household audio-visual appliances when the energy-saving lamp is working, but also further reduces the AC ripple of the output DC voltage. It is beneficial to the operation of the subsequent circuit. After filtering, the output DC voltage of about 300V is added to the power tubes Q1 and Q2. The starting circuit is composed of R1 and C3; the high-frequency oscillation circuit is composed of O1, Q2, C4, C5 and pulse transformer T1 (windings N1, N2, N3); the series resonance/lighting circuit is composed of self-inductance transformer T2, C6, and lamp 2U1. When the power is just turned on, the 300V DC voltage charges C3 through R1. When the charging voltage of C3 reaches a certain value, it is added to the base of Q2 through R2, Q2 is triggered to turn on, and then through the inductive coupling of the windings of the pulse transformer T1, Q2 and Q1 are triggered to turn on and off in turn, and the circuit enters an oscillation state, generating an output pulse similar to a rectangular wave. The pulse voltage resonates through T2 and C6, and obtains sufficient starting voltage at both ends of 2U1 to ignite and emit light. When the lamp is lit, due to the self-inductance of T2, the lamp current is constant, which not only reduces the flicker of the lamp, but also plays a role in current limiting protection. Ensure the safe operation of the energy-saving lamp.

The energy-saving lamp is equipped with multiple protection circuits to improve the reliability of the energy-saving lamp and extend the service life of the energy-saving lamp. The fuse resistor TR is used for overcurrent protection of the whole machine. The unidirectional conduction function of the diodes D7 and D8 is mainly used to absorb the reverse voltage added to Q1 and Q2 during operation to prevent the two power tubes from being damaged by high reverse voltage. The function of the resistors R5 and R6 is to limit the overcurrent of the base of Q1 and Q2. The resistors R3 and R4 and the diodes D5 and D6 are connected in series to form a reverse voltage absorption protection circuit for the be junction of the two power tubes to protect Q1 and Q2 from being damaged. The capacitor C7 has a DC isolation function to prevent the DC high voltage from entering the lamp tube and burning the filament.

2. Common fault repair

Example 1: When the power is turned on, there is a beep but the light does not turn on.

This is generally a serious short circuit fault in the circuit. First, use a flat screwdriver to pry open the lamp housing and lamp holder, then remove the circuit board and observe the components on the circuit board. It is found that TR is burnt and blackened, and its two ends are not connected. Then check the rectifier filter circuit. D1~D4, C2, L1, C1 are all normal. It is initially determined that the power tube Q1 or Q2 is short-circuited. Then solder Q1 and Q2 to measure. It is found that the two power tubes ce are broken down (short-circuited) and cause the oscillation circuit to stop oscillating, causing the above fault. After replacing TR (1Ω/2W), Q1, Q2 (model DK51), the fault is immediately eliminated. If you can't find the DK51 tube, you can choose the high reverse voltage tube 13002 as a replacement. Note that the be pin arrangement of this tube is opposite to the original tube, so you can't make a mistake when welding.

Example 2: When the power is turned on, the lamp cannot light up.

This fault often occurs in the starting circuit. Check TR and the DC voltage (300V) across C1. Then check the starting circuit R1 and C3. It is found that C3 is invalid. Replace C3 and try it out. The energy-saving lamp lights up normally. Troubleshooting.

Example 3: After power is turned on, both ends of the lamp are dark red.

The main reasons for this failure are: (1) C6 has serious leakage or breakdown short circuit. The voltage of the replaced capacitor must be greater than 630V (it is best to choose a 1200V polyester capacitor). (2) C7 capacity is reduced or has serious leakage. (3) D5 or D6 has poor performance. Both should be replaced.

Example 4: The light is sometimes on and sometimes not.