Charger with current stabilization. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Charger with current stabilization. Encyclopedia of radio electronics and electrical engineering

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

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We bring to your attention a charger (charger) with stabilization of the set charging current for car batteries with a current of up to 10 A. It also provides for an automatic shutdown of the charging current when the battery reaches the set voltage. This device can also be used as a stand-alone power supply with adjustable output voltage and load current limiting for circuits that do not require strict voltage ripple standards.

The operation of this device is quite close according to the principle of operation of pulse voltage stabilizers with pulse-width regulation of the output voltage. Currently, switching power supplies (UPS) are the most promising, but for many radio amateurs, their manufacture is fraught with great difficulties.

In this circuit, an attempt was made to apply the ideas of the UPS using a thyristor power regulator. At the same time, measures were taken to achieve the highest efficiency. For this purpose, a circuit of a full-wave rectifier with a midpoint of the output winding of a power transformer was chosen, where thyristors are directly connected instead of diodes, which, along with current rectification, also perform the functions of its regulation. For this circuit, we need only two radiators to cool two thyristors, and not four, as in the circuit with the inclusion of diodes in the bridge.

Charging currents are high - such a device begins to gradually turn into a heating device.

Of course, in the secondary winding of a power transformer, it will be necessary to wind twice as many turns as in a rectifier bridge circuit, but on the other hand, the cross section of the winding wire is half as much, which can even be an advantage when winding a transformer.

The figure shows the memory circuit ("ground" is shown conditionally, and it is not communicated with the body).

(click to enlarge)

The scheme consists of several parts:

1. Power step-down transformer T1 with thyristors VS1, VS2, smoothing power supply filter on capacitors C1C4 and inductor L1.

2. A pulse generator that controls the opening phase of thyristors VS1 and VS2. The generator is assembled according to a typical circuit on an analog of a unijunction transistor on the elements VT1 and VT2, a timing capacitor C6 and a matching pulse transformer T2.

3. An adjustable current source on transistors VT3, VT4 and a capacitor C7 with a resistor R13, which acts as a variable resistor, with which the phase of the pulses generated by the generator is regulated.

4. Current and voltage tracking circuits for controlling an adjustable current source on operational amplifiers DA1.1 and DA1.2 according to the voltage comparator circuit. This also includes the shunt of the ammeter R14.

5. A rectifier for powering the pulse generator circuits and microcircuits, consisting of diodes VD1, VD2, a parametric voltage regulator on the VD6 diode and a resistor R11, a smoothing power supply filter on capacitors C8, C9, as well as reference voltage sources for the operation of voltage comparators DA1 on resistors R24 -R27.

6. To improve the accuracy of disconnecting a fully charged battery, an additional unit is used, made on the DDI chip and R8R10, VD4, VD5, VD9 and VD10 elements.

It is necessary to say special about this node, it can not be installed. In the manufacture of chargers for car batteries, especially when charging with high currents, when trying to automate them, they encountered the problem of voltage instability at which they are turned off, and everything worked fine at the stand. After observing, the author noticed that the owners of the memory very incorrectly connect them to the batteries, they can use random conductors (once I saw a connection with wires more than 10 m). A significant voltage drop is formed on these wires, and the device that monitors the output voltage begins to turn off the charger erroneously ahead of time, and sometimes turns on and off cyclically.

This influencing factor can be excluded, given that the charging current in the circuit flows pulsating, i.e. then, when the rectifier emf exceeds the battery emf, there are periods of time when there is no charging current, at which time it is necessary to control the output voltage. This algorithm can be implemented in various ways. By introducing this method of monitoring the output voltage, it was possible to significantly increase the accuracy of turning off the charger when the battery reaches the set voltage level.

The principle of operation of the memory circuit At the initial moment, when turned on, the controlled current source VT3-VT4 will open with a plus through the resistor R7, so the phase delay of the pulses generated by the generator on transistors VT1-VT2 is minimal. Thyristors VS1 and VS2 open almost immediately with the appearance of a half-wave of the AC sine wave, and the power consumed from the transformer is maximum. As the capacitors C1-C4 charge, the charging current of the battery will appear, which will cause a voltage drop across the shunt of the ammeter R14. This voltage is fed through the resistor R20 to the inverting input of the voltage comparator DA1.1, compared with the set reference voltage from the variable resistor R27.

As soon as the voltage drop across the shunt R14 exceeds the exemplary one, the DA1.1 comparator will switch and a low level (almost "ground") will appear at its output. This low level is fed through the diode VD7 and the resistor R13 to the base of the transistor VT4, and the controlled current source begins to close, increasing its resistance in the capacitor circuit Sat. The generator pulses are generated later, the thyristors VS1-VS2 open less, and the power consumption also decreases. With a decrease in the charging current, the comparator returns to its original position again, without affecting the transistors VT3-VT4. Thus, pulse-width regulation of the charging current is carried out.

On the DAI comparator. 1 is a circuit for monitoring the output voltage. As soon as it exceeds the set value (usually 14,6 V), the DA1.2 comparator will also switch and similarly, only through the VD8 diode, then through the resistor R13 it will close the transistors VT3-VT4, and the pulse generator will turn off, the charge current will stop. Due to a sufficiently wide hysteresis loop formed by resistors R27, R28, only when the voltage at the charger terminals drops to 12,7 V, the comparator will return to its original position again, and the charger will start working. LED HL2 signals the end of the charge.

As mentioned above, a new voltage control principle is applied here, which improves the tripping accuracy. The voltage is controlled only during narrow periods of time between half-waves of the AC sinusoid, the rest of the time the sensitivity of the comparator is greatly underestimated. The node is made on a DDI chip and auxiliary elements VD4, VD5, VD9, VD10, R8, R9, R10.

On the DD 1.1-DDI.2 microcircuits, a pulse shaper is made, isolated from the positive half-waves of the current sinusoid, taken from the secondary winding of the transformer T1 through the rectifier diodes VD1-VD2, which are fed through the resistor R8 and the zener diode VD4 to the input of the DD1.1 microcircuit. Thanks to the VD4 zener diode, which cuts off part of the voltage, as well as due to the threshold properties of the DDI chip, the DDI .2 output will have pulses with a frequency of 100 Hz and a duration of 7 ... 8 ms (the duration depends on the supply voltage). At the output of the DDI .3 chip, there will be inverted pulses with a duration of 2 ... 3 ms with a period of 10 ms. During these time intervals (2 ... 3 ms) there is guaranteed no charging current, and the applied pulses from the outputs of the DDI .3 microcircuit through the VD10 diode do not affect the non-inverting input of the DA1.2 comparator. In this period of time, the output voltage is controlled.

In the period when there are no pulses at the output of DDI .3, i.e. a low level is present, it will significantly bypass the voltage control input, effectively turning off the DA1.2 comparator. When the comparator DA1.2 is triggered, its low level, applied to the input of the DD 1.3 chip through the VD9 diode, prohibits the passage of pulses through the DDI .3 chip, there is a high level at its output, and it does not affect the comparator. In practice, the introduction of such a voltage control principle made it possible to achieve a very accurate disconnection of the battery from the charger.

The requirements for the parts installed in the memory are not critical; various interchanges of transistors and diodes are possible here. It is better to replace thyristors with more modern ones like T-112, etc. Inductor L1 is installed in order to protect the thyristors from significant currents when charging capacitors C3C4. The inductor is made on a Ш12x25 core with a gap of 0,1 mm, wound with PEL 2,02 wire until it is filled.

Without power filter capacitors, the current control circuit is inoperative, and their presence is even desirable, because. charging will be close to DC charging, which will benefit the battery. The capacitances of capacitors, especially C3 and C4, can be increased, thereby reducing the voltage ripple, which at the output of the memory at the indicated ratings of C1-C4 is 1,5 V at a load current of 5 A.

For the pulse generator, the circuit was chosen with a transformer output, because long-term practice of servicing various devices based on thyristors has shown their good reliability, in contrast to circuits with galvanic coupling to thyristor control electrodes. Here, thyristors quickly fail even in very unloaded power control circuits. Transformer T2 used a typical MIT-3 (FIT4 can be used), but you can also make it yourself on a Sh7x6 core, all turns are wound with PEL 0,15 wire, each winding contains 40 turns.

The circuit for monitoring and setting the output voltage, assembled on resistors R17, R19, R20, was chosen for ease of installation, they are installed on the panel near the output terminals.

Power transformer T1 is made of U-shaped iron 35 mm wide, set thickness 38 mm. The primary winding is wound with PEL wire 0,7, 890 turns; the secondary winding is PEL-1,7 wire, 70 turns per half winding.

A shunt for an ammeter, in its absence, can be easily made from a piece of steel wire with a diameter of 1,8 ... 2 mm, a length of 15 ... 18 cm, twisted in a spiral. Then the resistor R15 calibrates the scale of the measuring instrument for a current of 10 A or another selected scale. It is easier and easier to do this than to select a shunt for the device. Also, an additional resistance R16 is adjusted to the device to measure the voltage under the selected scale of the device.

If necessary, the hysteresis of the voltage comparator can be removed by excluding resistor R22 from the circuit, then when the set voltage is reached, the current will decrease to the current of the battery batteries, the value of which depends on the type of battery and its wear. Then there is no particular need to install the DD1 chip. In this capacity, the memory can work as a separate power supply. Resistor R18 can adjust the output voltage, and resistor R27 - set the current limit in the power circuit.

References:

Integrated circuits. operational amplifiers. Volume /. - M: Fizmatlit, 1993.-240 p.

Author: B.G. Erofeev

See other articles Section Chargers, batteries, galvanic cells.

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