Analysis of laser printer power adapter


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Analysis of laser printer power adapter

The power supply circuit of laser printer is mainly to convert 220V mains power into a variety of stable direct current needed by the printer, and provide working voltage for various control circuits in the printer. The working stability of the power supply circuit directly affects whether the printer can work normally. Laser printers use many different voltages, so the power supply system is complex. The first thing to do is to convert 220V alternating current into low-voltage direct current. For example, in HP33440 laser printer, the power supply part provides +5V,? Dc voltage of 5V and 24V. +5V powers all logic integrated circuits and is also used to detect printer switch circuits. If the +5V voltage is normal, the printer can be started, input is received, and various indicator lights on the control board can be lit. The +24V voltage is used for cooling fans, scanning motors, erasing lights and clutch switches, as well as for power adapters for high-voltage power systems. There is an internal locking switch on the printer cover. When the printer cover is opened, the switch disconnects the 24V power adapter circuit.
The following is an analysis of the power adapter of HP1010 laser printer, which adopts a parallel self-excited power adapter with switching tube VT501 and switching transformer T501 as the core.

1. Rectifying filter circuit
The rectifier circuit (rectifying circuit) is can convert ac power to dc circuit. After switching on the power switch SW101, the mains voltage is filtered through the circuit composed of fuse FU101, SW101, C101, R101, L102, C104, C105, C106, etc., and rectified through the rectifier stack composed of VD101 ~ VD104, which is filtered by C107 to generate 300V voltage at both ends of C107. In the circuit, TH101 is a thermistor with negative temperature coefficient, which can limit the large impact current generated by charging C107 at the moment of starting up. R101 is a piezosensitive resistor, which is used to protect the power adapter from excessive mains voltage.

2. Oscillation circuit
Oscillating current is a kind of current whose magnitude and direction change with the period. The circuit that can produce oscillating current is called oscillating circuit. The 300V voltage is added to the drain electrode of the switching tube VT501 through the 1-3 primary winding of the safety resistor R523 and T501. The other way passes through the starting resistors R501 and R502 to the gate of VT501 to provide the starting voltage for VT501. The 5-6 positive feedback winding of T1 and VT501 together constitute the switch oscillation circuit. During the conduction of VT501, energy is stored in the primary winding of the switching transformer T501. During the cut-off period of VT501, the energy stored in the primary winding of T501 is released through the secondary winding. The pulse voltage generated in the secondary winding is rectified by schottky type rectifier tube VD501 and filtered by C506 to obtain 24V voltage, which supplies power to the load.

3. Voltage control circuit
Voltage stabilizing circuit refers to the power supply circuit that can keep the output voltage basically unchanged when the voltage of the input network fluctuates or the load changes. When rising mains voltage or load lighter, C506 on both ends of the voltage rise, after sampling error amplifier circuit in the IC501 treatment, make PC501 (3) feet of the output voltage increases, the C503 charging speed, VT502 conduction in advance, VT501 as ahead of time, T501 for VT501 conduction time and energy storage, power adapter output voltage drop to the provisions of the voltage value, so as to achieve the goal of a stable output voltage. On the contrary, the control process is opposite.

4. Overvoltage protection circuit
In view of the instability of the power supply circuit, the circuit designed to prevent such instability from affecting the effect of the circuit is called protection circuit. When error sampling amplifier circuit or PC501 abnormal, resulting in the power adapter output voltage is too high, IC501 internal circuit detection, output from its high level control voltage. This voltage provides about 1.1v working voltage for PC502's (1) pin via R527, which makes the led in PC502 start to emit light and the light-sensitive triode to conduct. The voltage output by (3) pin makes VT502 saturated to conduct and VT501 to cut off, which avoids the harm caused by the high output voltage of the power adapter to the load and switch tube.

5. Power supply and control circuit of fixing system
The power supply used by the fixing roller heater is 220V ac, and its heating circuit is set in the ac input circuit. The working and control process is as follows.
When the machine is energized, the pins of connector J201 are put into high level. The foot inputs the control signal to conduct VT103, the RL101 drive coil to generate the magnetic field, the RL101 contact suction. The control voltage input by the foot is amplified by VT102 to power SSR101 and make it work. The thyristor VT101 is switched on due to the trigger signal. At this time, the local voltage through the connector J102 (1), (2) foot for fixing roller heater power supply.
When the temperature of fixer roller heater reaches about 185℃, the foot of J201 enters a low level to make VT102 cut-off, then VT101 is turned off and the fixer roller heater stops heating. At the same time, the microprocessor commands the "ready" light to come on. If the printer does not work within the set time, the J201 foot input low level, make VT103 cut-off, RL101 contact release, the machine into standby state.

| release time：2019.09.11 Source：

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Analysis of laser printer power adapter