Circuit diagram of an electronic clock with LED indication. How to make a watch with your own hands? Electrical diagram of a homemade clock with a thermometer
have always been important elements of any electronic devices. These devices are used in amplifiers and receivers. The main function of power supplies is considered to be to reduce the maximum voltage that comes from the network. The first models appeared only after the AC coil was invented.
Additionally, the development of power supplies was influenced by the introduction of transformers into the device circuit. The peculiarity of pulse models is that they use rectifiers. Thus, voltage stabilization in the network is carried out in a slightly different way than in conventional devices where a converter is used.
Power supply device
If we consider a conventional power supply, which is used in radio receivers, then it consists of a frequency transformer, a transistor, and several diodes. Additionally, the circuit contains a choke. Capacitors are installed with different capacities and their parameters can vary greatly. Rectifiers are usually used of the capacitor type. They belong to the high-voltage category.
Operation of modern blocks
Initially, the voltage is supplied to the bridge rectifier. At this stage, the peak current limiter is activated. This is necessary so that the fuse in the power supply does not burn out. Next, the current passes through the circuit through special filters, where it is converted. Several capacitors are needed to charge the resistors. The unit starts up only after a breakdown of the dinistor. Then the transistor is unlocked in the power supply. This makes it possible to significantly reduce self-oscillations.
When voltage generation occurs, the diodes in the circuit are activated. They are connected to each other using cathodes. A negative potential in the system makes it possible to lock the dinistor. The rectifier start-up is facilitated after the transistor is turned off. In addition, two fuses are provided to prevent saturation of the transistors. They operate in the circuit only after a breakdown. To start feedback, a transformer is required. It is fed by pulsed diodes in the power supply. At the output, alternating current passes through capacitors.
Features of laboratory blocks
The operating principle of switching power supplies of this type is based on active current conversion. Bridge rectifier in standard scheme one is provided. In order to remove all interference, filters are used at the beginning and also at the end of the circuit. Pulse capacitors laboratory block food has the usual. Saturation of transistors occurs gradually, and this has a positive effect on diodes. Voltage adjustment is provided in many models. The protection system is designed to save blocks from short circuits. Cables for them are usually used in a non-modular series. In this case, the power of the model can reach up to 500 W.
The power supply connectors in the system are most often installed as ATX 20 type. To cool the unit, a fan is mounted in the case. The speed of rotation of the blades must be adjusted in this case. A laboratory-type unit should be able to withstand the maximum load at 23 A. At the same time, the resistance parameter is maintained on average at 3 ohms. The maximum frequency that a switching laboratory power supply has is 5 Hz.
How to repair devices?
Most often, power supplies suffer due to blown fuses. They are located next to the capacitors. Repair of switching power supplies should begin by removing the protective cover. Next, it is important to inspect the integrity of the microcircuit. If no defects are visible on it, it can be checked using a tester. To remove fuses, you must first disconnect the capacitors. After this they can be removed without any problems.
To check the integrity of this device, inspect its base. Burnt fuses have a dark spot at the bottom, which indicates damage to the module. To replace this element, you need to pay attention to its markings. Then you can purchase a similar product in a radio electronics store. Installation of the fuse is carried out only after fixing the condensates. Another common problem in power supplies is considered to be faults with transformers. They are boxes in which coils are installed.
When very high voltage is applied to the device, they cannot withstand it. As a result, the integrity of the winding is compromised. It is impossible to repair switching power supplies with such a breakdown. In this case, the transformer, like the fuse, can only be replaced.
Network power supplies
The operating principle of network-type switching power supplies is based on a low-frequency reduction in the amplitude of interference. This happens thanks to the use of high-voltage diodes. Thus, it is more effective to control the limiting frequency. Additionally, it should be noted that transistors are used at medium power. The load on the fuses is minimal.
Resistors are used quite rarely in a standard circuit. This is largely due to the fact that the capacitor is capable of participating in current conversion. The main problem with this type of power supply is the electromagnetic field. If capacitors are used with low capacitance, then the transformer is at risk. In this case, you should be very careful about the power of the device. Limiters for network peak current pulse block has power supplies, and they are located immediately above the rectifiers. Their main task is to control the operating frequency to stabilize the amplitude.
Diodes in this system partially serve as fuses. Only transistors are used to drive the rectifier. The locking process, in turn, is necessary to activate the filters. Capacitors can also be used as isolation type in the system. In this case, the transformer will start up much faster.
Application of microcircuits
A wide variety of microcircuits are used in power supplies. In this situation, much depends on the number of active elements. If more than two diodes are used, the board must be designed for input and output filters. Transformers are also produced in different capacities, and their dimensions are quite different.
You can solder microcircuits yourself. In this case, you need to calculate the maximum resistance of the resistors taking into account the power of the device. To create an adjustable model, special blocks are used. This type of system is made with double tracks. Ripple inside the board will occur much faster.
Benefits of Regulated Power Supplies
The principle of operation of switching power supplies with regulators is the use of a special controller. This element in the circuit can change the throughput of transistors. Thus, the limiting frequency at the input and output is significantly different. The switching power supply can be configured in different ways. Voltage adjustment is carried out taking into account the type of transformer. Conventional coolers are used to cool the device. The problem with these devices is usually excess current. In order to solve this, protective filters are used.
The power of devices on average fluctuates around 300 W. Only non-modular cables are used in the system. In this way, short circuits can be avoided. Power supply connectors for connecting devices are usually installed in the ATX 14 series. The standard model has two outputs. Rectifiers are used with higher voltage. They can withstand resistance at 3 ohms. In turn, the maximum pulse load adjustable block accepts up to 12 A power supply.
Operation of 12 volt units
Pulse includes two diodes. In this case, filters are installed with a small capacity. In this case, the pulsation process occurs extremely slowly. The average frequency fluctuates around 2 Hz. Coefficient useful action for many models it does not exceed 78%. These blocks are also distinguished by their compactness. This is due to the fact that transformers are installed with low power. They do not require refrigeration.
The 12V switching power supply circuit additionally involves the use of resistors marked P23. They can withstand only 2 ohms of resistance, but this is enough power for a device. A 12V switching power supply is used most often for lamps.
How does the TV box work?
The operating principle of switching power supplies of this type is the use of film filters. These devices are able to cope with interference of various amplitudes. Their choke winding is synthetic. Thus, high-quality protection of important components is ensured. All gaskets in the power supply are insulated on all sides.
The transformer, in turn, has a separate cooler for cooling. For ease of use, it is usually set to silent. These devices can withstand maximum temperatures of up to 60 degrees. The operating frequency of the TV switching power supply is maintained at 33 Hz. At subzero temperatures, these devices can also be used, but much in this situation depends on the type of condensates used and the cross-section of the magnetic circuit.
Models of 24 volt devices
In 24-volt models, low-frequency rectifiers are used. Only two diodes can successfully cope with interference. The efficiency of such devices can reach up to 60%. Regulators are rarely installed on power supplies. The operating frequency of the models does not exceed 23 Hz on average. Resistors can only withstand 2 ohms. Transistors in models are installed with the marking PR2.
To stabilize the voltage, resistors are not used in the circuit. The 24V switching power supply filters are of the capacitor type. In some cases, dividing species can be found. They are necessary to limit the maximum frequency of the current. To quickly start a rectifier, dinistors are used quite rarely. The negative potential of the device is removed using the cathode. At the output, the current is stabilized by blocking the rectifier.
Power sides on diagram DA1
Power supplies of this type differ from other devices in that they can withstand heavy loads. There is only one capacitor in the standard circuit. For normal operation of the power supply, the regulator is used. The controller is installed directly next to the resistor. No more than three diodes can be found in the circuit.
The direct reverse conversion process begins in the dinistor. To start the unlocking mechanism, a special throttle is provided in the system. Waves with large amplitude are damped by the capacitor. It is usually installed of the dividing type. Fuses are rarely found in a standard circuit. This is justified by the fact that the maximum temperature in the transformer does not exceed 50 degrees. Thus, the ballast choke copes with its tasks independently.
Models of devices with DA2 chips
Switching power supply microcircuits of this type are distinguished from other devices by their increased resistance. They are used mainly for measuring instruments. An example is an oscilloscope that shows fluctuations. Voltage stabilization is very important for him. As a result, the device's readings will be more accurate.
Many models are not equipped with regulators. Filters are mainly double-sided. At the output of the circuit, transistors are installed as usual. All this makes it possible to withstand a maximum load of 30 A. In turn, the maximum frequency indicator is at around 23 Hz.
Blocks with installed DA3 chips
This microcircuit allows you to install not only a regulator, but also a controller that monitors fluctuations in the network. The resistance of the transistors in the device can withstand approximately 3 ohms. The powerful switching power supply DA3 can handle a load of 4 A. You can connect fans to cool the rectifiers. As a result, the devices can be used at any temperature. Another advantage is the presence of three filters.
Two of them are installed at the input under the capacitors. One separating type filter is available at the output and stabilizes the voltage that comes from the resistor. There are no more than two diodes in a standard circuit. However, a lot depends on the manufacturer, and this should be taken into account. The main problem with power supplies of this type is that they are not able to cope with low-frequency interference. As a result, install them on measuring instruments inappropriate.
How does the VD1 diode block work?
These blocks are designed to support up to three devices. They have three-way regulators. Communication cables are installed only non-modular ones. Thus, current conversion occurs quickly. Rectifiers in many models are installed in the KKT2 series.
They differ in that they can transfer energy from the capacitor to the winding. As a result, the load from the filters is partially removed. The performance of such devices is quite high. At temperatures above 50 degrees they can also be used.
Switching power supply 180W
The power of the power supply is about 180 W, the output voltage is 2x25 V at a load current of 3.5 A. The ripple range at a load current of 3.5 A does not exceed 10% for a conversion frequency of 100 Hz and 2% for a frequency of 27 kHz. The output impedance does not exceed 0.6 Ohm. Block dimensions - 170x80x35 mm; weight - 450 g.
After rectification by the diode bridge VD1, the mains voltage is filtered by capacitors C1-C4 (see diagram). Resistor R1 limits the charging current of the filter capacitors flowing through the rectifier diodes when the unit is turned on. The filtered voltage is supplied to a voltage converter built according to a half-bridge inverter circuit using transistors VT1, VT2. The converter is loaded with the primary winding of transformer T1, which converts the voltage and galvanically isolates the output of the unit from the AC mains. Capacitors C3 and C4 prevent RF interference from the power supply from entering the network. A half-bridge inverter converts constant pressure and a rectangular variable with a frequency of 27 kHz. Transformer T1 is designed so that its magnetic circuit is not saturated. Self-oscillating mode operation is provided by a feedback circuit, the voltage of which is removed from winding III of transformer T1 and supplied to winding I of auxiliary transformer T2. Resistor R4 limits the voltage on winding I of transformer T2. The conversion frequency depends within certain limits on the resistance of this resistor (see note at the end of the page). You can read in detail about the operation of converters with a non-saturable transformer in.
To ensure reliable startup of the converter and its stable operation, a startup unit is used, which is a relaxation generator based on a VT3 transistor operating in avalanche mode. When the power is turned on, capacitor C5 begins to charge through resistor R5 and when the voltage across it reaches 50...70 V, transistor VTZ opens like an avalanche and the capacitor is discharged. The current pulse opens transistor VT2 and starts the converter.
Transistors VT1 and VT2 are installed on heat sinks with an area of 50 cm2 each. Diodes VD2-VD5 are also equipped with plate heat sinks. The diodes are sandwiched between five duralumin plates measuring 40x30 mm each (the three middle plates are 2 mm thick, the two outer ones are 3 mm thick). The entire package is tightened with two M3x30 screws passed through the holes in the plates. To prevent the plates from being closed by screws, pieces of polyvinyl chloride tube are placed on them.
The winding characteristics of transformers are summarized in the table.
|
Transformer |
Number of turns |
Magnetic core |
||
|
Ferrite 2000NN, two rings K31x18.5x7 glued together |
||||
|
Ferrite 2000NN, ring K10x6x5 |
||||
Winding wire - PEV-2. Winding I is placed evenly along the length of the ring. To facilitate starting the converter, winding III of transformer T1 should be located in a place not occupied by winding II (see figure). Inter-winding insulation in transformers is made with varnished fabric tape. Between windings I and II of transformer T1 there is three-layer insulation, between the remaining windings of transformers it is single-layer.
Capacitors C3, C4 in the block - K73P-3; C1, C2 - K50-12; C5 - K73-11; S8,S9 - KM-5; C6, C7 -- K52-2. Transistors KT812A can be replaced with KT812B, KT809A, KT704A-KT704V, diodes KD213A with KD213B.
A correctly assembled power supply usually does not need adjustment, but in some cases it may be necessary to select a VT3 transistor. To check its functionality, temporarily disconnect the emitter output and connect it to the negative terminal of the network rectifier. The voltage on capacitor C5 is observed on the oscilloscope screen - a sawtooth signal with a swing of 20...50 V and a frequency of several hertz. If there is no ramp voltage, the transistor must be replaced.
The use of this power source does not eliminate the need to block the output power circuits of the amplifier with large capacitors. Connecting such capacitors further reduces the ripple level.
Literature
1. V. Tsibulsky Economical power supply. Radio, 1981, No. 10, p. 56.
2. Romash E. M. Sources of secondary power supply for radio-electronic equipment. - M.: Radio and Communications, 1981.
3. Biryukov S. Digital frequency meter power supply, - Radio. 1981. No. 12, p. 54, 55.
D. BARABOSHKIN
Radio, 6/85
NOTE
When turning on the power supply, measure the conversion frequency (at the terminals of winding II) - it may be significantly lower than 27 kHz (for example, 9 - 12 kHz). And although the device will work, the power transistors will fail due to overheating. Frequency adjustment is carried out by resistor R4. Moreover, the rating may differ from that indicated on the diagram by tens of ohms.
A correctly configured power supply works great; at a load of 50 - 70%, the power transistors remain cold.
The sound quality depends almost as much on the parameters of the power source as on the amplifier itself, and you should not be negligent in its manufacture. There are more than enough descriptions of calculation methods for standard transformers. Therefore, here is a description of a switching power supply that can be used not only with amplifiers based on the TDA7293 (TDA7294), but also with any other 3H power amplifier.
The basis of this power supply (PSU) is a half-bridge driver with an internal oscillator IR2153 (IR2155), designed to control MOSFET and IGBT technology transistors in pulsed sources nutrition. Functional diagram microcircuits is shown in Figure 1, the dependence of the output frequency on the ratings of the RC-drive chain in Figure 2. The microcircuit provides a pause between the pulses of the “upper” and “lower” switches for 10% of the pulse duration, which allows you not to worry about “through” currents in the power converter parts.
Rice. 1
Rice. 2
The practical implementation of the power supply is shown in Figure 3. Using this circuit, you can make a power supply with a power from 100 to 500 W, you only need to proportionally increase the capacitance of the primary power filter capacitor C2 and use the corresponding power transformer TV2.
Rice. 1
The capacitance of capacitor C2 is selected at the rate of 1...1.5 µF per 1 W of output power, for example, when manufacturing a 150 W power supply, a capacitor of 150...220 µF should be used. The VD primary power supply diode bridge can be used in accordance with the installed primary power supply filter capacitor; with capacitances up to 330 µF, 4...6 A diode bridges can be used, for example RS407 or RS607. With a capacitor capacity of 470... 680 μF, more powerful diode bridges are needed, for example RS807, RS1007.
We can talk about the manufacture of a transformer for a long time, but not everyone needs to delve into the deep theory of calculations for too long. Therefore, calculations according to Eranosyan’s book for the most popular standard sizes of ferrite rings M2000NM1 are simply summarized in Table 1.
As can be seen from the table, the overall power of a transformer depends not only on the dimensions of the core, but also on the conversion frequency. It is not very logical to make a transformer for frequencies below 40 kHz - harmonics can create insurmountable interference in the audio range. The manufacture of transformers for frequencies above 100 kHz is no longer permissible due to self-heating of the M2000NM1 ferrite by eddy currents. The table shows data on the primary windings, from which the turns/volt ratios can be easily calculated, and then it will not be difficult to calculate how many turns are needed for a particular output voltage. It should be noted that the voltage supplied to the primary winding is 155 V - the mains voltage of 220 V after the rectifier and smoothing filter will be 310 V DC, the circuit is semi-bridged, therefore half of this value will be applied to the primary winding. It should also be remembered that the shape of the output voltage will be rectangular, therefore, after the rectifier and smoothing filter, the voltage value will not differ significantly from the calculated value.
The diameters of the required wires are calculated from a ratio of 5 A per 1 sq mm of wire cross-section. Moreover, it is better to use several wires of smaller diameter than one, thicker wire. This requirement applies to all voltage converters with a conversion frequency above 10 kHz, since the skin effect - losses inside the conductor - is already beginning to affect, since at high frequencies the current no longer flows across the entire cross-section, but along the surface of the conductor, and the higher the frequency, the stronger the effect losses in thick conductors. Therefore, it is not recommended to use conductors thicker than 1 mm in converters with conversion frequencies above 30 kHz. You should also pay attention to the phasing of the windings - incorrectly phased windings can either damage the power switches or reduce Converter efficiency. But let’s return to the power supply shown in Figure 3. The minimum power of this power supply is practically unlimited, so you can make a power supply with 50 W or less. The upper power limit is limited by certain features of the element base.
To obtain higher powers, more powerful MOSFET transistors are required, and the more powerful the transistor, the greater the capacitance of its gate. If the gate capacitance of the power transistor is quite high, then a significant current is required to charge and discharge it. The current of the IR2153 control transistors is quite small (200 mA), therefore, this microcircuit cannot control too powerful power transistors at high conversion frequencies.
Based on the above, it becomes clear that the maximum output power of a converter based on IR2153 cannot be more than 500...600 W at a conversion frequency of 50...70 kHz, since the use of more powerful power transistors at these frequencies quite seriously reduces the reliability of the device. List of recommended transistors for power switches VT1, VT2 with brief characteristics summarized in table 2.
Rectifier diodes of secondary power circuits must have least time recovery and at least two times the voltage reserve and three times the current. The latest requirements are justified by the fact that the self-induction voltage surges of a power transformer amount to 20...50% of the output voltage amplitude. For example, with a secondary power supply of 100 V, the amplitude of self-induction pulses can be 120... 150 V and despite the fact that the pulse duration is extremely short, it is enough to cause a breakdown in the diodes, when using diodes with reverse voltage at 150 V. A threefold current reserve is necessary so that at the moment of switching on the diodes do not fail, since the capacitance of the secondary power filter capacitors is quite high, and charging them will require quite a bit of current. The most suitable diodes VD4-VD11 are summarized in Table 3.
The capacity of the secondary power filters (C11, C12) should not be increased too much, since the conversion is carried out at fairly high frequencies. To reduce ripple, it is much more important to use large capacitance in the primary power circuits and correctly calculate the power of the power transformer. In the secondary circuits, capacitors of 1000 μF per arm are quite sufficient for amplifiers up to 100 W (the power supply capacitors installed on the UMZCH boards themselves must be at least 470 μF) and 4700 μF for a 500 W amplifier. On schematic diagram shows a version of the secondary power supply rectifiers, made on Schottky diodes, and is wired for them printed circuit board(Figure 4). A rectifier for the forced cooling fan of heat sinks is made on diodes VD12, VD13; a rectifier for low-voltage power supply is made on diodes VD14-VD17 ( preamplifiers, active tone controls, etc.). The same figure shows a drawing of the location of parts and a connection diagram. The converter has overload protection made on the current transformer TV1, consisting of a K20x12x6 ring of M2000 ferrite and containing 3 turns of the primary winding (the cross-section is the same as the primary winding of the power transformer and 3 turns of the secondary winding, wound with a double wire with a diameter of 0.2.. .0.3 mm. If there is an overload, the voltage on the secondary winding of transformer TV1 will become sufficient to open the thyristor VS1 and it will open, closing the power supply to the IR2153 microcircuit, thereby stopping its operation. The protection threshold is adjusted by resistor R8. Adjustment is made without load, starting with maximum sensitivity and achieving stable startup of the converter. The principle of adjustment is based on the fact that at the moment of starting the converter it is loaded to the maximum, since it is necessary to charge the capacitance of the secondary power filters and the load on the power part of the converter is maximum.
About the remaining details: capacitor C5 - film capacitor 0.33... 1 µF 400V; capacitors C9, C10 - film capacitors 0.47...2.2 µF at least 250V; inductances L1...L3 are made on K20x12x6 M2000 ferrite rings and are wound with 0.8...1.0 mm wire until they are filled turn to turn in one layer; C14, C15 - film 0.33...2.2 µF for a voltage of at least 100 V with an output voltage of up to 80 V; capacitors C1, C4, C6, C8 can be ceramic, type K10-73 or K10-17; C7 can also be ceramic, but film, such as K73-17, is better.
The most widespread are push-pull secondary power sources, although they have a more complex electrical circuit compared to single-cycle ones. They allow you to get a significantly larger output output power at high efficiency.
Circuits of push-pull converter-inverters have three types of connection of key transistors and the primary winding of the output transformer: half-bridge, bridge and with a primary winding tapped from the middle.
Half bridge diagram of the key cascade construction.
Its feature is the inclusion of the primary winding of the output transformer at the midpoint of the capacitive divider C1 - C2.
The amplitude of the voltage pulses at the emitter-collector transistor transitions T1 and T2 does not exceed Upit the value of the supply voltage. This allows the use of transistors with a maximum voltage Uek up to 400 volts.
At the same time, the voltage on the primary winding of transformer T2 does not exceed the value Upit/2, because it is removed from the divider C1 - C2 (Upit/2).
A control voltage of opposite polarity is supplied to the bases of key transistors T1 and T2 through transformer Tr1. 
IN pavement In the converter, the capacitive divider (C1 and C2) is replaced by transistors T3 and T4. Transistors in each half-cycle open in pairs diagonally (T1, T4) and (T2, T3).
The voltage at the transitions Uec of closed transistors does not exceed the supply voltage Upit. But the voltage on the primary winding of transformer Tr3 will increase and will be equal to the value of Upit, which increases the efficiency of the converter. The current through the primary winding of transformer Tr3 at the same power, compared to a half-bridge circuit, will be less.
Due to the difficulty in setting up the control circuits of transistors T1 - T4, a bridge switching circuit is rarely used.
Inverter circuit with so-called push-pull output is most preferable in powerful converters-inverters. Distinctive feature in this circuit is that the primary winding of the output transformer Tr2 has a terminal from the middle. For each half-cycle of voltage, one transistor and one half-winding of the transformer alternately operate. 
This scheme has the highest efficiency, low level pulsations and weak interference emissions. This is achieved by reducing the current in the primary winding and reducing the power dissipation in the key transistors.
The voltage amplitude of the pulses in half of the primary winding Tr2 increases to the value Upit, and the voltage Uek on each transistor reaches the value 2 Upit (self-induction emf + Upit).
It is necessary to use transistors with high value Ukemah, equal to 600 - 700 volts.
The average current through each transistor is equal to half the current consumption from the supply network.
Current or voltage feedback.
A feature of push-pull self-excited circuits is the presence of feedback (Feedback) from the output to the input, in terms of current or voltage.
In the scheme current feedback
communication winding w3 of transformer Tr1 is connected in series with the primary winding w1 of output transformer Tr2. The greater the load at the inverter output, the greater the current in the primary winding Tr2, the greater the feedback and the greater the base current of transistors T1 and T2.
If the load is less than the minimum permissible, the feedback current in winding w3 of transformer Tr1 is insufficient to control the transistors and the generation of alternating voltage is disrupted.
In other words, when the load is lost, the generator does not work.
In the scheme voltage feedback
The feedback winding w3 of transformer Tr2 is connected through a resistor R to the communication winding w3 of transformer Tr1. This circuit provides feedback from the output transformer to the input of the control transformer Tr1 and then to the base circuits of transistors T1 and T2.
Voltage feedback is weakly dependent on load. If there is a very large load at the output (short circuit), the voltage on winding w3 of transformer Tr2 decreases and a moment may come when the voltage on the base windings w1 and w2 of transformer Tr1 will not be enough to control the transistors. The generator will stop working.
Under certain circumstances, this phenomenon can be used as protection against short circuit at the exit.
In practice, both circuits with feedback in both current and voltage are widely used.
Push-pull inverter circuit with voltage feedback
For example, let's consider the operation of the most common converter-inverter circuit - a half-bridge circuit.
The circuit consists of several independent blocks:
- - rectifier unit - converts AC voltage 220 volts 50 Hz to constant voltage 310 volts;
- - trigger pulse device - generates short pulses voltage for starting the autogenerator;
- - alternating voltage generator - converts 310 volt direct voltage into rectangular alternating voltage high frequency 20 – 100 KHz;
- - rectifier - converts alternating voltage 20 -100 kHz into direct voltage.
Immediately after turning on the 220 volt power supply, the triggering pulse device, which is a generator, starts working sawtooth voltage(R2, C2, D7). From it, triggering pulses arrive at the base of transistor T2. The autogenerator starts.
The key transistors open one by one and in the primary winding of the output transformer Tr2, connected to the diagonal of the bridge (T1, T2 - C3, C4), a rectangular alternating voltage is formed.
The output voltage is removed from the secondary winding of transformer Tr2, rectified by diodes D9 - D12 (full-wave rectification) and smoothed by capacitor C5.
The output produces a constant voltage of a given value.
Transformer T1 is used to transmit feedback pulses from the output transformer Tr2 to the bases of key transistors T1 and T2. 
The push-pull UPS circuit has a number of advantages over the single-cycle circuit:
- — the ferrite core of the output transformer Tr2 operates with active magnetization reversal (the magnetic core is most fully used in terms of power);
- — the collector-emitter voltage Uek on each transistor does not exceed the DC source voltage of 310 volts;
- — when the load current changes from I = 0 to Imax, the output voltage changes slightly;
- — emissions high voltage in the primary winding of the transformer Tr2 are very small, and the level of radiated interference is correspondingly lower.
And one more note in favor of the push-pull circuit!!
Let's compare the operation of two-stroke and single-cycle self-generators with the same load.
Every key transistor T1 and T2 during one clock cycle of the generator use only half the time (one half-wave), the second half of the cycle is “resting”. That is, the entire generated power of the generator is divided in half between both transistors and the transfer of energy to the load occurs continuously (from one transistor, then from the other), during the entire cycle. Transistors operate in a gentle mode.
In a single-cycle generator, the accumulation of energy in the ferrite core occurs during half the cycle, and in the second half of the cycle it is released to the load.
The key transistor in a single-cycle circuit operates four times more intensely than the key transistor in a push-pull circuit.
This watch has already been reviewed several times, but I hope that my review will also be interesting to you. Added job description and instructions.
The designer was bought on ebay.com for 1.38 pounds (0.99+0.39 shipping), which is equivalent to $2.16. At the time of purchase, this is the lowest price offered.
Delivery took about 3 weeks, the set came in a regular plastic bag, which in turn was packed in a small bubble bag. There was a small piece of foam on the indicator terminals; the rest of the parts were without any protection.
From the documentation there is only a small A5 sheet of paper with a list of radio components on one side and a circuit diagram on the other. 
1. Electrical circuit diagram, parts used and operating principle
The basis or “heart” of the watch is an 8-bit CMOS microcontroller AT89C2051-24PU equipped with a 2kb Flash programmable and erasable ROM.
Clock generator node assembled according to the circuit (Fig. 1) and consists of a quartz resonator Y1, two capacitors C2 and C3, which together form a parallel oscillatory circuit.
By changing the capacitance of the capacitors, you can change within small limits the frequency of the clock generator and, accordingly, the accuracy of the clock. Figure 2 shows a variant of a clock generator circuit with the ability to adjust the clock error.
Initial reset node serves to set the internal registers of the microcontroller to the initial state. It serves to supply, after connecting power, to 1 pin of the MK a single pulse with a duration of at least 1 μs (12 clock periods).
Consists of an RC circuit formed by resistor R1 and capacitor C1.
Input circuit consists of buttons S1 and S2. The software is designed so that when you press any of the buttons once, a single signal is heard in the speaker, and when you hold it, a double signal is heard.
Display module assembled on a four-digit seven-segment indicator with a common cathode DS1 and a resistive assembly PR1.
A resistive assembly is a set of resistors in one housing: 
Sound part circuit is a circuit assembled using a 10 kOhm resistor R2, pnp transistor Q1 SS8550 (acting as an amplifier) and piezoelectric element LS1.
Nutrition supplied through connector J1 with smoothing capacitor C4 connected in parallel. Supply voltage range from 3 to 6V.
2. Assembling the constructor
The assembly did not cause any difficulties; it was written on the board where to solder what parts.Lots of pictures - the assembly of the designer is hidden under the spoiler
I started with the socket, since it is the only one that is not a radio component: 
The next step was to solder the resistors. It is impossible to confuse them, they are both 10 kOhm: 
After that I installed it on the board, observing the polarity electrolytic capacitor, resistor assembly (also paying attention to the first pin) and clock generator elements - 2 capacitors and a quartz resonator 
The next step is to solder the buttons and the power filter capacitor: 
After this, it’s time for the sound piezoelectric element and transistor. The main thing in a transistor is to install it on the correct side and not to confuse the terminals: 
Lastly, I solder the indicator and power connector: 
I connect it to a 5V source. Everything is working!!! 
3. Setting the current time, alarms and hourly signal.
After turning on the power, the display is in "HOURS: MINUTES" mode and displays the default time of 12:59. The hourly beep is on. Both alarms are on. The first is set to operate at 13:01, and the second at 13:02.
Each time you briefly press the S2 button, the display will switch between the modes (“HOURS: MINUTES”) and (“MINUTES: SECONDS”).
When you press the S1 button for a long time, you enter the settings menu, which consists of 9 submenus, designated by the letters A, B, C, D, E, F, G, H, I. Submenus are switched by the S1 button, the values are changed by the S2 button. Submenu I is followed by exiting the settings menu.
A: Setting the current time clock
When you press the S2 button, the clock value changes from 0 to 23. After setting the clock, you must press S1 to go to submenu B. 
B: Setting the minutes of the current time
C: Turn on the hourly beep
The default is ON – a beep sounds every hour from 8:00 to 20:00. Pressing the S2 button changes the value between ON and OFF. After setting the value, you must press S1 to go to submenu D. 
D: Turn on/off the first alarm
By default, the alarm is ON. Pressing the S2 button changes the value between ON and OFF. After setting the value, you must press S1 to go to the next submenu. If the alarm is turned off, submenus E and F are skipped. 
E: Setting the first alarm clock
When you press the S2 button, the clock value changes from 0 to 23. After setting the clock, you must press S1 to go to submenu F. 
F: Setting the minutes of the first alarm
When you press the S2 button, the minutes value changes from 0 to 59. After setting the minutes, you must press S1 to go to submenu C. 
G: Turn on/off the second alarm clock
By default, the alarm is ON. Pressing the S2 button changes the value between ON and OFF. After setting the value, you must press S1 to go to the next submenu. If the alarm is turned off, submenus H and I are skipped and the settings menu is exited. 
H: Setting the second alarm clock
When you press the S2 button, the clock value changes from 0 to 23. After setting the clock, you must press S1 to go to submenu I. 
I: Setting the minutes of the second alarm
When you press the S2 button, the minutes value changes from 0 to 59. After setting the minutes, you must press S1 to exit the settings menu. 
Seconds correction
In the mode (“MINUTES: SECONDS”), you must hold down the S2 button to reset the seconds. Next, briefly press button S2 to start counting the seconds. 
4. General impressions of the watch.
Pros:+ Low price
+ Easy assembly, minimum details
+ The pleasure of self-assembly
+ Fairly low error (I was a few seconds behind during the day)
Minuses:
- Doesn't keep time after power off
- Lack of any documentation other than the diagram (this article partially solved this disadvantage)
- The firmware in the microcontroller is protected from reading