Power Circuit Diagram and Principle Analysis of Canon FAX-450 Fax Machine

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The power circuit of the Canon FAX-450 fax machine is shown in Figure 1.

Figure 1 Schematic diagram of the power circuit of the Canon FAX-450 fax machine

As shown in Figure 1, the power system is mainly composed of surge current suppression, main switch conversion, DC voltage output, voltage regulation control and automatic protection circuit. The working principle and process of the specific circuit are as follows:

1. Working principle and working process of inrush current suppression circuit The inrush current suppression circuit is mainly composed of 5~6 windings, D3, R3, R4 and other components of CR1, R2 and T1. Its function is to suppress the large charging surge current of the capacitors C7 and C8 at the instant of starting. At the moment of power-on, since the capacitors C7 and C8 correspond to an instantaneous short circuit, a large charging current is suppressed when passing through the resistor R2. When the voltages across C7 and C8 are stable, the voltage induced by the 5~6 windings of T1 is applied to the control pole of the triac CR1 via D3, R3, and R4, triggering CR1 to turn on and shorting the resistor R2. In this way, the main current no longer passes through the current suppressing resistor R2, so that R2 does not affect the normal current of the power supply after the surge current suppression effect at the instant of starting the power.

2. Working principle and process of the main switch conversion circuit The main switch conversion circuit of the fax machine is mainly composed of components such as a switch tube Q1, an integrated circuit ICl, a switch pulse transformer T1, and a capacitor C12. Its function is to convert the rectified and smooth filtered 300 V pulsating DC voltage into a pulse voltage and couple it to the secondary loop via T1. The specific work process is as follows.

After the AC power supply is turned on, the rectified and smooth filtered 300 V pulsating DC voltage is applied to the drain of the switching FET Q1 via the primary winding 1~3 of the pulse transformer T1; the other circuit is connected to the integrated circuit via the resistor R8. The roller of the ICl is powered by the Vcc terminal. When the power supply is working normally, the voltage of the ICl roller is changed by the voltage induced by the 7~6 winding of T1 through D4 rectification and C12 filtering, and the output DC voltage is supplied. After the ICl roller has a normal voltage, its internal circuit starts to work. After the pulse generated by the oscillator is processed by its internal amplification shaping tube, the switch pulse with adjustable output width is applied to the gate of Q1. , control the intermittent conduction of Q1. In this way, an alternating current is generated at the secondary of T1, and after rectification by RC2, RC3, etc., various DC voltages are obtained. The level of the output voltage can be controlled by changing the on-time of the switching transistor Q1.

3. The working principle and working process of the DC voltage output circuit are shown in Figure 1. The DC voltage output circuit of the fax machine is composed of T1 secondary high-frequency rectification, filtering and voltage stabilizing circuits, and outputs stable +5 V, ±12 V and +24 V DC voltages respectively. Among them, +24 V is the main power supply, which is used as the working power source for the thermal head and stepping motor, and is the controlled output voltage.

The pulse voltage induced by T1 secondary winding 8~roller is rectified by RC2, and filtered by L3, C19, C20, C21, C22, L4, etc., and 24 V DC voltage is output. Transistor Q3 in this loop acts as a control switch for the +24 V supply and is controlled by the main control circuit. When the main control circuit sends out the power-on signal VRC, Q3 turns on and outputs +24 V DC voltage. When the main control circuit does not send the power-on signal VRC, Q3 turns off, there is no +24 V power voltage output.

At the same time, after rectification and filtering, the output +24 V DC voltage is also regulated by T2 and Q5, and the π-type filter composed of C29, L, C31 and so on outputs 5 V DC voltage.

The pulse voltage induced by T1 secondary winding roller 輰讹~roll 輱讹~roller is rectified by D12, D15, D13, D14, filtered by C35 and C36, and then regulated by two three-terminal regulator IC3 and IC4 respectively. Output ±12 V DC voltage.

4. The working principle and working process of the voltage regulator control circuit The pulse width modulation voltage regulator circuit of the fax machine switching power supply is composed of an integrated circuit IC1 and a photocoupler PC1. The automatic stabilization of the 24 V, 5 V output voltage is achieved by controlling the on-time of the switching transistor Q1.

When the voltage at the output of 24 V rises, after the voltage divider RV1, resistor R18 and resistor R19 are divided, the voltage applied to the b pole of the transistor Q2 rises, and the e-pole voltage of Q2 passes through R16 and the Zener diode D5. The voltage is fixed. In this way, the c-pole current is increased. When the current passes through the light-emitting diode in the photocoupler PC1, the brightness of the light-emitting diode is increased, and the phototransistor current in the PC1 is increased, and the output impedance is reduced, and is applied to the integrated circuit IC16 pin (internal The voltage of the non-inverting input terminal of the error amplifier is reduced. After the internal pulse width control circuit of ICl, the pulse width of the output of the roller is reduced, the on-time of Q1 is shortened, and the energy storage of T1 is reduced when Q1 is turned on. The voltage to the secondary winding is reduced, and the output DC voltage naturally drops, thereby achieving the purpose of voltage regulation.

In addition, the 5 V voltage circuit is also provided with a voltage regulator circuit, which is composed of an integrated voltage regulator IC2, resistors RV2, R31, R32 and a transistor Q6. When the voltage at the 5 V output rises, the current flowing through the cathode-anode (KA terminal) of IC2 increases, Q6 turns on, and the b pole of the switching transistor Q5 is short-circuited to the ground through D19, and Q5 is cut off early. The voltage of the pole output is reduced, and the voltage of the 5 V DC output terminal naturally drops, achieving the purpose of voltage regulation.

When the voltage at the output of 24 V and 5 V drops, the working principle and working process of the voltage regulator circuit are opposite to the above.

5. Working principle and working process of automatic protection circuit The switching power supply system of the fax machine is equipped with overcurrent and overvoltage protection circuits. The specific protection process is as follows:

(1) Working process of overcurrent protection circuit: The overcurrent protection circuit is mainly composed of overcurrent detecting resistors R5 and R6 and integrated circuit ICl. When the current through the D and S poles of the switch Q1 is too large, the voltage drop formed on the resistors R5 and R6 increases, and the voltage is applied to the IC10 and the roll, and compared with the standard voltage in the IC1. As a result, after the treatment, the ICl roller has no pulse output, thereby turning off the switch tube Q1 to achieve overcurrent protection.

(2) Working process of 5 V overvoltage protection circuit: When the voltage regulator circuit fails and the DC voltage of the 5 V output terminal is too high, the Zener diode D11 is broken down, and the breakdown current passes through the LED in the photocoupler PC2. When the LED is illuminated, the output resistance of the phototransistor in PC2 is reduced, and the voltage applied to the IC18 pin is raised. After the pulse width control circuit in IC1, the ICl roller is forced to have no pulse output, and Ql is turned off. The purpose of pressure protection.