In recent decades, electronic technology has developed so quickly that the equipment becomes obsolete much earlier than it fails. As a rule, outdated equipment is written off and, falling into the hands of radio amateurs, becomes a source of radio components.
Some of the components of this equipment are quite possible to use.

How to assemble a laboratory power supply from a printer

During one of my visits to the radio market, I managed to buy several printed circuit boards from decommissioned equipment for almost nothing (Fig. 1). One of the boards also included a power transformer. After searching the Internet, we were able to establish (presumably) that all the boards were from EPSON dot matrix printers. In addition to many useful parts, the board has a good dual-channel power supply. And if the board is not intended to be used for other purposes, an adjustable laboratory power supply can be built on its basis. How to do this is described below.

The power supply contains channels +24 V and +5 V. The first is built according to the circuit of a step-down pulse-width stabilizer and is designed for a load current of about 1.5 A. When this value is exceeded, the protection is triggered and the voltage at the output of the stabilizer drops sharply (short circuit current - approximately 0.35 A). An approximate channel load characteristic is shown in Fig. 2 (black curve). The +5V channel is also built according to a pulse stabilizer circuit, but, unlike the +24 V channel, according to the so-called relay circuit. This stabilizer is powered from the output of the +24 V channel (designed to operate from a voltage source of at least 15 V) and does not have current protection, so if the output is short-circuited (and this is not uncommon in amateur radio practice), it may fail.

And although the stabilizer current is limited in the +24 V channel, during a short circuit the key transistor heats up to a critical temperature in about a second. The +24V voltage stabilizer circuit is shown in Fig. 3 (letter designations and numbering of elements correspond to those printed on the printed circuit board). Let's consider the operation of some of its components that have features or are relevant to the alteration. A power switch is built on transistors Q1 and Q2. Resistor R1 serves to reduce power dissipation across transistor Q1. Transistor Q4 is used to build a parametric voltage regulator for the supply voltage of the master oscillator, made on a microcircuit designated on the board as ZA (we will further consider it as DA1).

Laboratory power supply diagram

This microcircuit is a complete analogue of the famous TL494 computer power supply. Quite a lot has been written about its operation in various modes, so we will consider only some circuits. Stabilization of the output voltage is carried out as follows: a reference voltage from the internal source of the microcircuit (pin 14) is supplied to one of the inputs of the built-in comparator 1 (pin 2 DA1) through resistor R6. The other input (pin 1) receives the output voltage of the stabilizer through a resistive divider R16R12, and the lower arm of the divider is connected to the reference voltage source of the current protection comparator (pin 15 DA1). As long as the voltage at pin 1 of DA1 is less than at pin 2, the switch on transistors Q1 and Q2 is open.

As soon as the voltage at pin 1 becomes greater than at pin 2, the switch closes. Of course, the key control process is determined by the operation of the microcircuit master oscillator. Current protection works similarly, except that the load current is affected by the output voltage. The current sensor is resistor R2. Let's take a closer look at current protection. The reference voltage is supplied to the inverting input of comparator 2 (pin 15 DA1). Resistors R7 participate in its formation. R11 and also R16. R12. As long as the load current does not exceed the maximum value, the voltage at pin 15 of DA1 is determined by the divider R11R12R16.

Resistor R7 has a fairly high resistance and has almost no effect on the reference voltage. When overloaded, the output voltage drops sharply. At the same time, the reference voltage also decreases, which causes a further decrease in the current. The output voltage drops almost to zero, and since now the series-connected resistors R16, R12 are connected in parallel with R11 through the load resistance, the reference voltage, and therefore the output current, also decreases sharply. This is how the load characteristic of the +24 V stabilizer is formed.

The output voltage on the secondary (II) winding of the step-down power transformer T1 must be no lower than 29V at a current of up to 1.4 A. The +5V voltage stabilizer is made using transistor Ob and an integrated stabilizer 78L05, designated on the board as SR1. A description of a similar stabilizer and its operation can be found in. Resistors R31, R37 and capacitor C26 form a PIC circuit to form steep pulse fronts.
To use a power source in a laboratory unit, you need to cut out the area on which the stabilizer parts are located from the printed circuit board (separated by light lines in Fig. 1).

To be able to regulate the output voltage of the +24 V stabilizer, it should be slightly modified. First, you need to disconnect the +5 V stabilizer input, for which you need to unsolder resistor R18 and cut the printed conductor going to the emitter terminal of transistor Q6. If the +5 V source is not needed, its parts can be removed. Next, you should unsolder the resistor R16 and connect a variable resistor R16* instead (like other new elements, it is shown in the diagram with thick lines) with a nominal resistance of 68 kOhm.

Then you need to unsolder resistor R12 and solder it on the back side of the board between pin 1 of DA1 and the negative terminal of capacitor C1. Now the output voltage of the block can be changed from 5 to 25 V. You can lower the lower limit of regulation to approximately 2V if you change the threshold voltage at pin 2 of DA1. To do this, remove resistor R6, and apply voltage to pin 2 of DA1 (about 2 V) from trimming resistor R6’ with a resistance of 100 kOhm, as shown in the diagram on the left (opposite the previous R6).

This resistor can be soldered from the parts side directly to the corresponding pins of the microcircuit. There is another option - instead of resistor R6, solder R6″ with a nominal value of 100 kOhm, and between pin 2 of the DA1 chip and the common wire, solder another resistor - R6″’ with a nominal value of 36 kOhm. After these modifications, the stabilizer protection current should be changed. Having removed resistor R11, solder in its place variable R11* with a nominal resistance of 3 kOhm with resistor R11″ connected to the motor circuit. The roller of resistor R1 V can be displayed on the front panel for quick adjustment of the protection current (from approximately 30 mA to a maximum value of 1.5 A).

With this switching on, the load characteristic of the stabilizer will also change: now, if the load current is exceeded, the stabilizer will go into its limiting mode (blue line in Fig. 2). If the length of the wire connecting resistor R11′ to the board exceeds 100 mm, it is advisable to solder a capacitor with a capacity of 0.01 μF parallel to it on the board. It is also advisable to provide transistor Q1 with a small heat sink. A view of the modified board with adjusting resistors is shown in Fig. 4.

Such a power supply can be operated with a load that is not critical to voltage ripples, which at maximum load current can exceed 100 mV. The ripple level can be significantly reduced by adding a simple compensation stabilizer, the diagram of which is shown in Fig. 5. The stabilizer is based on the widely used TL431 microcircuit (its domestic analogue is KR142EN19). The regulating element is built on transistors VT2 and VT3. Resistor R4 here performs the same function as R1 in a switching regulator (see Fig. 3).

On transistor VT1, a voltage drop feedback unit is assembled and soldered from the parts side directly to the corresponding pins of the microcircuit. There is another option - instead of resistor R6, solder R6″ with a nominal value of 100 kOhm, and between pin 2 of the DA1 chip and the common wire, solder another resistor - R6″’ with a nominal value of 36 kOhm.

After these modifications, the stabilizer protection current should be changed. Having removed resistor R11, solder in its place variable R11* with a nominal resistance of 3 kOhm with resistor R11″ connected to the motor circuit. The roller of resistor R1 V can be displayed on the front panel for quick adjustment of the protection current (from approximately 30 mA to a maximum value of 1.5 A). With this switching on, the load characteristic of the stabilizer will also change: now, if the load current is exceeded, the stabilizer will go into its limiting mode (blue line in Fig. 2). If the length of the wire connecting resistor R11′ to the board exceeds 100 mm, it is advisable to solder a capacitor with a capacity of 0.01 μF parallel to it on the board. It is also advisable to provide transistor Q1 with a small heat sink. A view of the modified board with adjusting resistors is shown in Fig. 4.

Such a power supply can be operated with a load that is not critical to voltage ripples, which at maximum load current can exceed 100 mV. The ripple level can be significantly reduced by adding a simple compensation stabilizer, the diagram of which is shown in Fig. 5. The stabilizer is based on the widely used TL431 microcircuit (its domestic analogue is KR142EN19). The regulating element is built on transistors VT2 and VT3. Resistor R4 here performs the same function as R1 in a switching regulator (see Fig. 3). Transistor VT1 contains a feedback unit based on the voltage drop across resistor R2. The collector-emitter section of this transistor must be connected instead of resistor R16 in the circuit in Fig. 3 (of course, variable resistor R16’ is not needed in this case).

This node works as follows. As soon as the voltage across resistor R2 exceeds approximately 0.6 V, transistor VT1 opens, which causes the comparator of the DA1 chip in the switching regulator to switch and, therefore, close the switch on transistors Q1.02. The output voltage of the switching stabilizer decreases. Thus, the voltage across this resistor is maintained at a level of about 0.65 V. In this case, the voltage drop across the regulating element VT2VT3 is equal to the sum of the voltage drop across resistor R2 and the voltage at the emitter junction of transistor VT3. i.e. about 1.25... 1.5V depending on the load current.

In this form, the power supply is capable of delivering a current of up to 1.5A to the load at a voltage of up to 24V, while the ripple level does not exceed a few millivolts. It should be noted that when the current protection is triggered, the ripple level increases, since the DA1 microcircuit of the compensating stabilizer closes and the control element is completely open.

A printed circuit board for this stabilizer has not been developed. Transistor VT3 must have a static current transfer coefficient L21E of at least 300, and VT2 - at least 100. The latter must be installed on a heat sink with a cooling surface area of ​​at least 10 cm².
Setting up a power supply with this addition involves selecting output divider resistors R5-R7. When the unit is self-excited, you can bypass the emitter junction of transistor VJ1 with a capacitor with a capacity of 0.047 μF. A few words about the +5 V channel stabilizer.

It can be used as an additional source if transformer T1 has an additional 16...22 V winding. In this case, you will need another rectifier with a filter capacitor. Since this stabilizer does not have protection, the load must be connected to it through an additional protection device, for example, described in, limiting the current of the latter to 0.5 A. The article describes the simplest modification option, but you can further improve the characteristics of the source by adding the compensating stabilizer with its own adjustable protection according to current, for example, on an operational amplifier, as is done in .

We present to our readers a review of the power supply of the Canon LaserBase MF-5630 multifunctional device, which is one of the latest generation devices. As is already becoming a tradition, acquaintance with the circuitry of the device begins with a review of its power supply. And, in principle, this is logical, because the operation of any electronic device begins with the startup and normal functioning of the power supply.

Device power supply Canon LaserBase MF-5630 is a single-cycle pulse converter that generates five supply voltages:

- voltage +3.5V1;

- voltage +3.5V2;

- voltage +5V1;

- voltage +5V2;

- voltage +14V;

- voltage +24V.

In addition, on the power supply board, as befits laser devices, there is a stove control circuit, which, in turn, is controlled by signals FSRD And RLYD, coming from the microprocessor to connector CN1 of the power supply.

Signal FSRD controls triac TRA1 through a galvanic isolation element - optocoupler PC2, and the signal RLYD designed to control relays RL1.

The power supply board is connected to the controller board using two interface connectors: CN101 and CN102.

The power supply is controlled by a microprocessor via a signal ON/OFF. This signal allows or, conversely, prohibits the formation of two voltages: +3.5V2 and +5V2. These voltage channels are switched off when the device goes into standby mode.

The LaserBase MF-5630 power supply cannot be classified as a very complex and extraordinary circuit, although it uses several solutions that deserve special mention.

A general block diagram of the power supply, giving an idea of ​​its main components and their interaction, is shown in Fig. 1. The block diagram shows not only the main components of the power source, but also the main electronic elements that make up this node.

Fig. 1 Block diagram of the power supply of the Canon LaserBase MF-5630 MFP

If we correlate this block diagram with the circuit diagram presented in Fig. 2 and Fig. 3, then the purpose of all electronic components of the power supply, in principle, will become clear. However, it is still necessary to make some comments.

Fig.2 Primary part of the power supply of the Canon LaserBase MF-5630 MFP

The primary part of the pulse converter is shown in Fig. 2. The converter is made according to a self-oscillator circuit, i.e. the switching moments of power transistor Q1 are determined by EMF pulses induced in the additional winding ( pin 1-pin 2) transformer T1, and the ratings of the timing circuit, consisting of capacitor C10 and resistor R6. The duration of the control pulses on the gate of Q1 can be limited by the transistor Q2, which, in turn, is controlled by the feedback signal received from the optocoupler PC1.

A very interesting feature of the primary part of the power supply is the use of an active snubber (a snubber is a damping circuit). The snubber provides limitation of voltage pulses arising in the primary winding of transformer T1 ( pin 7-pin 5) at the moment of closing the power transistor Q1. These pulses can damage Q1, so they must be limited. The main element of the snubber is a powerful transistor Q20, which opens when Q1 is turned off. Opening, Q20 connects capacitor C20 in parallel with the primary winding, which bypasses this winding, thereby limiting the EMF pulse.

Fig.3 Secondary part of the power supply of the Canon LaserBase MF-5630 MFP

All secondary voltages are obtained by half-wave rectification of pulses induced in the secondary windings of transformer T1. To obtain voltages with a nominal value +5V controlled stabilizers are used PQ05RD11(IC201 and IC202). Stabilizer PQ05RD11 has the following main characteristics:

- low voltage drop: no more than 0.5V;

- output current up to 1 A;

- input voltage up to 20V;

- power dissipation: 14W;

- output voltage value: from 4.85V to 5.15V.

The stabilizer is controllable, i.e. turning it on/off can be done by sending a corresponding signal to pin 4. Setting a high level signal on this pin causes the stabilizer to start, and setting the signal ON/OFF a low level blocks its operation and output voltage +5V absent.

Stabilizer IC201 is designed to generate voltage +5V1 and it starts only after the channel voltage appears and reaches the specified level +14V. This is provided by zener diode D202 and resistive divider R204/R201. In addition, the zener diode also provides protection against short circuits and overloads in the channel +14V. When the channel voltage +14V decreases significantly, then the zener diode D202 closes, which leads to the switching off of the stabilizer IC201 and a loss of voltage +5V1. Naturally, the corresponding circuits of the device are switched off, protecting it from operation in the event of a short circuit.

Stabilizer IC202 is designed to generate voltage +5V2 and it starts only after voltage appears at the output of the power supply +3.5V2.No voltage +3.5V2 will lead to a lack of tension +5V2 .

The voltage generation channels are also controllable +3.5V2 And +24V. These channels contain switches that allow or prohibit the supply of these voltages to the output of the power supply, i.e. into the load.

Key Q333, the opening of which causes voltage to appear at the output of the power supply +3.5V2, controlled by signal ON/OFF, generated by the central microcontroller of the device. Setting this signal to a high level results in two voltages appearing at the output of the power supply. +3.5V2And +5V2 .

Key Q303 switches channel voltage +24V and turns on only after voltage appears +5V2 .

Thus, the power supply in question uses alternate connection of loads of different channels. The sequence of appearance of output voltages is as follows:

+3.5V1/+14V – +5V1 – Activation ON/OFF – +3.5V2 – +5V2 – +24V.

The feedback circuit in this power supply is typical. It uses optocoupler PC1 as a galvanic isolation element. The LED current of this optocoupler is regulated by a controlled stabilizer microcircuit of type TL431 (only this circuit uses its analogue TA76432 - IC101). The channel voltage is applied to the control input of IC101 +3.5V1 through the divider R115, R117, VR101, i.e. voltage +3.5V1 is the main voltage of the power supply, on which feedback operates.

In addition, the LED current of optocoupler PC1 can be controlled by a trigger on transistors Q112/Q113. To be more precise, this trigger, when triggered, creates a maximum current through the LED of the optocoupler, which leads to the feedback signal being set to the maximum value and, as a result, turning off the power source. Transistors Q112/Q113 are a trigger for protection against exceeding the output voltage of the power supply. Overvoltage protection is implemented, as usual, using zener diodes:

- zener diode D106 – protection against exceeding +14V in the channel;

- Zener diode D109 – protection against excess in channel +5V1;

- Zener diode D105 – protection against excess in the +5V2 channel;

- Zener diode D107 – protection against excess in the +24V channel.

Opening any of these zener diodes triggers the trigger and further turns off the power supply.

The Canon LBP-1120 laser printer source has classic construction option for this type of printer, but there is also a peculiarity, this is the use of a special PWM controller as a control chip. It is worth noting that sources based on this chip are very often found in other laser printers and MFPs, for example from HP. Structurally, the printer power supply is located on the printer control board. On the same board there are high-voltage power supplies for the primary charge, development and transfer rollers, see Fig. 1. The block diagram of the power supply is shown in Fig. 2.

The printer power supply generates stabilized +24V voltage used to power motors, high voltage sources, solenoids, relays, fans, etc.; as well as +5V and +3.3.V, necessary to power controller and formatter microcircuits, memory, optocoupler LEDs, sensors, laser, interface circuits, etc. Let's consider the operation of the components of the power supply unit (see Fig. 3).

The connector for connecting the printer network cable is indicated in the diagram as INL101. The printer input circuits are represented by an input noise filter and control circuits for the image capture unit. The printer is turned on using the power button SW101. The surge protector is formed by elements (R101, C101, VZ101, L101, L102, C104, C106, C105 and L103). Its purpose is to suppress and filter symmetrical and asymmetrical impulse noise from a household electrical network.

The FU101 mains fuse is designed to protect the supply network from overloads that occur when the mains rectifier or power cascade malfunctions. Varistor VZ101 protects the primary part of the power supply from increased voltage in the network and short-term high-voltage surges. If the mains voltage exceeds the operating threshold of this varistor, its resistance decreases and a significant current begins to flow through it. As a result, the input fuse blows. A thermistor with negative TKS (TH201) serves to limit the surge of charging current of capacitors C109, C107 at the moment the power source is turned on. When the power supply is turned on, at the initial moment of time, the maximum charging current of the capacitors flows through the diode bridge, and this current can damage the DA101 diode rectifier. Since in a cold state the resistance of the thermistor is several Ohms, the current through the bridge rectifier diodes is limited to a level that is safe for them. After a certain period of time, as a result of the charging current flowing through the thermistor, it heats up, its resistance decreases to fractions of an Ohm and no longer affects the operation of the circuit.

The rectification of the alternating current of the network is carried out by the DA101 diode bridge. The conversion of direct current, after rectification and smoothing, into a pulsed high-frequency current flowing through the primary winding of transformer T501 is carried out by the IC501 microcircuit (STR-Z2756). The microcircuit includes a PWM controller with its inherent circuits and a powerful key transistor that switches the primary winding pulse transformer.

The microcircuit is powered by applying voltage to its pin 5 (Vcc). The starting voltage at the initial moment of switching on is formed by a divider from the rectified mains voltage taken from the diode bridge. The voltage divider is formed by resistors R542, R541, R544, R545, R540. This circuit creates a minimum starting current to start the microcircuit; in case of startup, additional recharge of the microcircuit in operating mode is carried out by the circuit R505, D502, C503. This circuit rectifies the pulsed EMF removed from the secondary winding (pins 1-2) of transformer T501.

The output power buses +5V and +24V in the power supply are formed by rectifying pulsed EMF from the secondary windings of transformer T501 with diode assemblies (DA501, DA502). The +3.3V output bus is formed using a voltage stabilizer from the +5V channel. It is assembled using elements Q502, IC505, R537, R539.

Stabilization of the output voltages is carried out using the PWM method using a feedback signal supplied to pin 5 (CONT) of the IC501 microcircuit. The feedback signal is generated by the RS501 optocoupler, the LED current of which is controlled by the IC504 stabilizer. The feedback signal is proportional to the output voltage +5V, which is generated using a resistive divider R516 and R530, the middle point of which is connected to the control input of the IC504 chip.

Blocking the IC501 microcircuit can be done by applying a “high” level signal to its input pin 7 (CD). The signal at this contact is controlled by a second optocoupler (PC502), which protects the power source from emergency operating conditions. The safety lock is triggered in the following cases:

Excess current in channel +5V;

Excess voltage in channels +5V and +24V;

The excess current in the +5V channel is monitored by the comparator IC302-1. Its inverse input (pin 2) is supplied with voltage from the +5V channel through a divider R525 and R523, and the voltage from the +5V channel is also supplied to the non-inverse input (pin 3) through a resistor R526; between the two controlled points, current sensors R514 and R513. The voltage drop across these resistors corresponds to the current in the channel. If the current in the channel increases, then the potential difference between pin 2 and pin 3 of the comparator IC302 increases, the comparator switches, and a “low” level voltage is formed at its output (pin 1), which opens the transistor Q501, and flows through the LED of the optocoupler PC502 current from the +24V channel, as a result, the PWM controller IC501 is then blocked.

Increasing voltages +5V and +24V using zener diodes ZD505 and ZD502. If one of them is triggered, current begins to flow through the LED of the optocoupler PC502, then a blocking voltage is applied to pin 7 of the IC501 chip.

The power source also includes a control circuit for the image capture unit. The heating element is connected to connector J102, and the alternating current of the primary network flows through the heating element, controlled by the triac Q101. The triac is controlled by the microprocessor via the FSRD signal. The FSRD signal is supplied to the base of transistor Q102, which, in turn, controls triac Q101 through a galvanic isolation element - optocoupler SSR301. The FSRD signal consists of pulses that follow at a very low frequency during periods of heating of the stove. The maximum operating temperature for heating the heating element is 190*C. Temperature control is carried out using a temperature sensor, which is a thermistor located on the back side of the heating element. The thermistor is included in the circuit of the resistive divider, the voltage of the middle point of which is supplied to the analog input of the microcontroller that controls most of the printer units, and to the comparison circuit that controls the protective relay. The control chip analyzes the analog voltage level from the temperature sensor and generates FSRD control pulses for the triac. Control is organized in ON/OFF mode.

In the event of uncontrolled heating of the fixation unit, the control unit provides protection implemented using a relay. It will be in an open state when:

• the printer is in standby mode;
• overheating is detected;
• any fatal error occurs;
• A paper jam occurs.

Relay RL101 is switched by transistor Q103, which is controlled by comparator IC302. This comparator receives a signal (on pin 5) from the stove temperature sensor and compares it with the reference voltage generated on pin 6. The temperature sensor voltage decreases as its temperature increases. Therefore, when the voltage on pin 5 of comparator IC302 drops below the threshold on pin 6 (0.67V), this means overheating of the stove, and leads to the switching off of transistor Q103, opening of the relay and, accordingly, breaking the power supply circuit of the heating element. The signal from the temperature sensor is also supplied to pin 38 of the microcontroller. Additionally, the relay can be controlled by the /RLYD signal from the microcontroller (pin 27). This signal is generated at the moment when the heating process of the stove should begin. At the moment when the relay should close, the /RLYD signal is set to a low level by the microprocessor, and to open the relay and turn off the stove, the /RLYD signal is set to a high level. Typical power supply faults are presented in table. 1.

Table 1.

Manifestation of malfunction	Items to be checked
The printer does not turn on. There is no +310V voltage at the output of diode bridge 101.	1. Fuse FU101 2. Thermistor TN101
Burnt fuse.	1. Varistor VZ11 2. Diode bridge D101 3. Chip IC601 STR-Z2756
The printer does not turn on. At the output of the diode bridge D201 there is a voltage of +300V. There is no supply voltage of approximately +16V on pin 8 of the IC501 chip.	1. Starting circuit R541, R542, R544, R545, R540. 2. Additional feed circuit C503, D505, R505.
The printer does not start. Output voltages +5V, +Z.ZV, +24V appear briefly. The characteristic sound of a short start is heard.	1. Presence of a short circuit in the load. 2. Makeup circuit IC501 3. Secondary rectifiers: DA501, DA502. 4. Current sensors: R514, R513, 5. Protection circuit: ZD505, ZD502, Q501. 6. Feedback circuit: IC502.

Troubleshooting in the printer power supply must first be done by checking the serviceability of fuse FU201. This is done visually and using a tester, because... Fuses in ceramic housings are mainly used. Next, the integrity of the housings of the VZ101 varistor, the TN101 thermistor, and the IC501 microcircuit is visually assessed. At this stage, the quality of the capacitors is immediately assessed. After this, it is necessary to collect information when turning on the printer, namely, check the voltage at the output of the diode bridge, at pin 8 of the IC501 chip, at the output of the power supply (voltage +3.3V, +5V, +24V). Next, you need to check the image capture unit, the resistance of the heating element, the serviceability of the triac (triac), the condition of the relay (sticking contacts), and the thermal fuse. At the diagnostic stage, it is even possible to start the printer with the image fixing unit disabled. The printer turns on, but a printer error is displayed on the operator panel; in this mode, the power supply is in operating mode, i.e. generates all output voltages. Naturally, with such diagnostics, it is necessary to follow all safety rules in order to avoid electrical damage.