Transformerless charger. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells

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In an effort to reduce the size of the designed radio equipment, radio amateurs give an important place to the miniaturization of the power supply. Usually this problem is solved using a pulsed voltage converter. Meanwhile, significant progress in the field of electronic components allows you to create small-sized power supplies that do not contain a transformer. The relative simplicity of design and the availability of components make them attractive to radio amateurs as well.

For the first time, such a technical solution was proposed by L.M. Braslavsky from the Novosibirsk Electrotechnical Institute back in 1972, having filed an application for an invention. It turned out to be so original and non-obvious for specialists that VNIIGPE carried out an examination on the application for six whole years and only in 1978 issued a copyright certificate.

Later, other solutions were patented, allowing the implementation of capacitor power supplies. A simplified diagram of such a device is shown in Fig. 6.10. It allows you to implement the "training" of batteries - a mode in which the battery is charged during one half-cycle of the mains voltage and then discharged with a lower current to the ballast resistor.

(click to enlarge)

The described capacitor voltage converter is designed to charge car batteries with a capacity of up to 70 Ah, so the maximum average output current of the device should be 7 A. This value is consistent with the limitation of the variable component at the level of 20 ... 30% of the nominal voltage for the applied oxide capacitors.

Rectifier diode VD38, capacitor C13 and zener diodes VD39, VD40 form the supply voltage of the control unit, which synchronizes the operation of switching transistors VT2 and VT3 with the polarity of the mains voltage and stabilizes the output current.

The device works as follows. With a positive half-wave of the mains voltage, a block of capacitors C1.C12 and a power storage capacitor C13 are charged. With a negative half-wave, the LED of the optocoupler U1 turns on, and its phototransistor, opening, shunts the emitter junction of the transistor VT1. The transistor VT1 closes and through the resistor R5 connects the non-inverting input of the op amp DA1 to the output of the capacitor unit.

At the same time, the op-amp itself switches and opens the transistors VT3, VT2 and the LED of the optocoupler U2 The op-amp DA1 operates in comparator mode, so its output signal can only take two values ​​- close to the supply voltage and to zero. If the voltage at its inverting input is greater than at the non-inverting one, the output voltage will be close to zero and the transistor VT3 will be in the closed state. Otherwise, the voltage at the output of the op-amp is close to the supply voltage, the transistor VT3 opens, and through the resistor R10 - the transistor VT2 and the optocoupler U2.

The input signal for stabilizing the output current is the voltage on the capacitor unit. Thus, the change in voltage on the capacitor unit (its decrease) is directly proportional to the charge given to the load, therefore, by stabilizing the charge given off by the capacitor unit during a single discharge cycle, the device stabilizes the output current. Its value is regulated by the resistor R7. After closing the transistor VT1, the voltage from the capacitor unit is supplied to the non-inverting input of the op-amp DA1 and compared with the exemplary one supplied to the inverting input from the divider R6...R8. When the voltage on the capacitor unit becomes less than the exemplary one, the op-amp DA1 switches to the zero state and closes the transistor VT3, and through it (and the load of the device) the optocoupler photodistor U2.

If for some reason the voltage on the capacitor unit has not decreased to the exemplary one (i.e., the charge, determined by the position of the resistor R7 slider, has not gone into the load), and the time allotted for discharging has ended, the operation of the unit to prevent the mains voltage from entering the output device is organized like this. The voltage of the negative half-wave of the network decreases until the LED of the optocoupler U1 turns off and, consequently, its phototransistor closes. This leads to the opening of the transistor VT1, shunting the non-inverting input and switching the comparator DA1 and, as a result, closing the transistors VT3, VT2 even before the appearance of a positive half-wave of the mains voltage. Thus, there is a forced synchronization of the current stabilization unit with the polarity of the mains voltage. The U2 optocoupler is needed only as a security enhancement and may not be available in built-in power supplies.

Charging the battery takes a relatively long time and requires some control. Therefore, the device provides the ability to automatically turn off the battery being charged at a voltage of 14,2 ... 14,4 V. The function of the threshold element for disconnecting a fully charged battery is performed by the electromagnetic relay K1 (RES10), which operates at a voltage of about 10,5 V. The relay is connected to the output terminals X2 and X3 through a wire trim resistor R11. This resistor, together with capacitor C14, forms a filter that suppresses the AC component of the pulsating charging voltage, but passes the slowly rising DC component of the battery voltage. Therefore, when the threshold voltage is reached, relay K1 is activated and, by opening contacts K1.1, turns off the power to the capacitor unit and the control system. The relay winding itself remains energized by the battery being charged and, due to the presence of hysteresis, turns off when the voltage drops to 11,8 V. After that, the battery is automatically recharged. Switching on / off the automatic end of charging is carried out by the switch SA2.

The use of the RES10 series relay is due to its low consumption current and, consequently, the low battery discharge current in the charging stop mode. The low-power contacts of the relay used also reflect the features of the described device associated with the capacitive nature of the load. Therefore, a break in the power supply circuit of the capacitor unit occurs without sparking. The use of two mains fuses (FU1, FU2) and a two-section SA1 switch is associated with increased electrical safety requirements due to the lack of galvanic isolation of the device from the mains.

It is possible to use any oxide capacitors in the capacitor unit, but preferably of the same type. In the case of using imported capacitors, the dimensions of this block can be significantly reduced. The diodes of the block can also be any, designed for the same current and reverse voltage - even diodes D226B and D7Zh will do, but the dimensions of the block and its mass will increase significantly. Optocoupler T0325-12,5-4 will be replaced by T0125-10 or T0125-12,5 not lower than class 4. Instead of KP706B (VT3), it is possible to use similar domestic field-effect transistors or imported IGBT for the same current and voltage, and preferably with a minimum channel resistance.

When choosing an electromagnetic relay, it must be taken into account that the nameplate rated voltage is approximately 1,5 ... 1,7 times higher than the trip voltage and that the trip voltage can be somewhat different even for relays from the same batch. It is possible to use relays RES9, RES22, RES32 and others with a sufficiently low current consumption, for an operation voltage in the range of 8 ... chatter" relay contacts and false positives.

A properly assembled device starts working immediately. It will take, basically, only a selection of resistors R6 and R8 to adjust the charging current adjustment range. To do this, a discharged battery must be connected to the output of the unit and, using a selection of resistors R6 and R8, set the range of regulation of the charging current by resistor R1 using the ammeter RA7.

If, at the initial position of the resistor R7 slider, the current is different from zero, then you need to reduce the resistance of the resistor R8. If the charging current becomes equal to zero not in the extreme position of the R7 engine, the resistance of this resistor should be increased. Next, the engine of the resistor R7 is set to its final position. If now the charging current is less than the maximum, the resistance of the resistor R6 will have to be reduced, and if it exceeds, it will be increased. After that, by setting the SA2 switch to the "Manual mode" position, you need to bring the battery to full charge, controlling the voltage on it with a DC voltmeter. Then you should disconnect the device from the network, switch the SA2 toggle switch to the "Auto" mode, and the R11 resistor slider to the maximum resistance position. Reconnecting the device to the network, by reducing the resistance of the resistor R11, they achieve a clear operation of the relay K1 - the device is ready for operation.

When setting up and operating the charger, you must remember that there is no galvanic isolation from the mains. Therefore, you can connect and disconnect it from the battery only when the power cord is disconnected from the mains.

Author: Semyan A.P.

See other articles Section Chargers, batteries, galvanic cells.

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