A device for charging car batteries. Encyclopedia of radio electronics and electrical engineering

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

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In relatively low-power electrical networks, the simultaneous operation of many power tools and welding machines causes such surges and dips in the mains voltage that all chargers that I have previously assembled simply refused to work or required continuous monitoring. In a device with manual regulation of the charging current, with a strong decrease in the mains voltage - up to 170 V - it was necessary to set the current regulator to the maximum. If he did not keep track of the mains voltage, then the charging current exceeded the limit value and, at best, the fuse blew, at worst, the transformer. Stabilized regulators were unable to track such a wide range of changes in mains voltage, and with sharp jumps and dips, they led to the consequences described above.

I had to approach this problem more thoroughly, and, as practice has shown, not in vain. Several years of operation of the new charger have confirmed that only a complete absence of mains voltage can prevent the battery from charging. The use of a proportionally integrating (PI) controller in the new device made it possible to more accurately maintain the specified charging current under the action of any destabilizing factors.

The PI controller is a system in which a special frequency response of the filter is formed in the feedback circuit to ensure the stability of the regulation. With a slow departure of the controlled parameter from the set value, the filter behaves as an integrator, and with a fast departure - as a non-inertia link. The transition from one mode to another is determined by the value of the cutoff frequency, at which the phase shift in the control ring does not exceed the allowable value and the stability of the system is ensured.

Schematic diagram of the charger is shown in fig. one.

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(click to enlarge)

The source of the charging current is two secondary windings IV and V of the network transformer T1, forming with diodes VD1, VD2 and VD3, VD4, respectively, two full-wave rectifiers connected in parallel. The current can be smoothly changed by a variable resistor R14 in the range from 1 to 10 A with stabilization of the set value. This node is made according to the traditional phase-controlled scheme, with the only difference being that not a thyristor, but a powerful field-effect transistor VT1 is used as a regulatory element. This decision led to ease of control and design convenience.

The phase control method involves the use of a sawtooth voltage to generate control pulses for the regulating element. To synchronize this voltage with the moments when the mains voltage passes through zero, a node is used, assembled on the elements VD6-VD8, R1, R2, R9, R10 and the comparator DA4, powered by the transformer connected in series according to the half-windings II I I-2.

When the voltage on the winding II is zero, the VD7 diode is closed by the reverse voltage coming through the resistors R9, R10 from the outputs of the auxiliary power supply of the microcircuits, and the comparator switches to a state where the open collector output (pin 9) is low voltage.

Through this output and the current-limiting resistor R13, the capacitor C8 is discharged, which is constantly charged through the resistor R8 from the same auxiliary source. Thus, a sawtooth voltage is formed on the capacitor C8 with reference to the zero phase of the voltage in the network.

Comparator DA5 controls the regulating transistor VT1 in accordance with the sawtooth voltage applied to the inverting input and the output voltage of the PI filter at the non-inverting input. After the sawtooth voltage reaches the level present at the non-inverting input, a voltage close to zero will be established at the open collector output, which will close the transistor VT1.

Two resistors R3 and R5 connected in parallel and performing the function of a current-measuring element are included in the positive circuit of the rechargeable battery.

The charging current pulses taken from these resistors are fed to the input of the active Bessel low-pass filter assembled on the op-amp DA3. The choice of the filter type is due to the uniformity of its frequency response as well as the high linearity of the phase response and a short settling time.

The cutoff frequency of the low-pass filter is about 8 Hz. It is determined by the elements R4, R6, C3, C4. The filter effectively suppresses the fundamental harmonic of the 100 Hz charging current, however, its inertia should not be excessively large.

A microammeter RA1 with additional resistors R12, R16 is connected to the output of the low-pass filter, the readings of which are directly proportional to the average value of the charging current. Calibrate the microammeter in amperes of the charging current with a trimming resistor R16. From the output of the low-pass filter, the voltage is also supplied to the adder formed by resistors R11, R14, R15. The variable resistor R14 regulates the charging current. The difference of the signals supplied to the connection point of the resistors R11 and R15 is fed to the input of the PI filter.

The PI filter is assembled on the op-amp DA6 and elements R17, R19. C10. Based on the inertia of the low-pass filter, the cut-off frequency of the regulator was chosen close to 8 Hz. As the frequency decreases, the filter gain increases and, near zero frequency, theoretically increases to infinity. This achieves a minimum discrepancy between the set and actual values ​​​​of the charging current. At a frequency of 8 Hz or more, the transmission coefficient is determined only by the values ​​of the resistors R17, R19. It is equal to approximately 27 dB.

Thus, the mismatch signal, acting on the control transistor VT1 through the comparator DA5, nullifies the difference in the voltage values ​​of the above signals at the junction point of the resistors R11 and R15.

To power comparators, operational amplifiers and other components of the device, an auxiliary bipolar source is provided, formed by half-windings III.1, III.2 of transformer T1, rectifier VD5, voltage stabilizers DAI, DA2 and smoothing oxide capacitors C1, C2, C5 C6. The HL1 LED is an indicator that the device is connected to the network. A fan with an electric motor M1 is used for forced cooling of a block of powerful diodes VD1-VD4 and a transistor VT1.

Most parts of the device are placed on a universal technological board, the installation is made by pieces of insulated wire. Resistors R3 ... R5 - wire C5-16V. The remaining constants are OMLT, MLT or MT. Variable R14 - wire with a linear characteristic. PPB-1 tuning R16 - SPZ-39A.

Oxide capacitors are best used designed for operation at elevated temperatures. The rest of the capacitors - any.

Transformer T1 - TS-180 from an old tube TV. The magnetic circuit must be disassembled, all windings, except for the primary I, must be wound from the coils, retaining the paper interlayer gaskets, and winding new ones. First, the windings II.1 are laid on one coil and II.2 on the other, 37 turns of wire each. PEV-2 0,18. and then also III.1 and III.2, 55 turns of wire each. PEV-2 0.38. The windings IV and V are wound last, 150 turns of wire each. PEV-2 0,86 with a tap from the middle. Interwinding and interlayer gaskets are required.

The half-windings located on different coils and wound in the same direction should be connected in opposite directions (i.e., end to end), as indicated in the diagram.

Diodes VD1-VD4 and transistor VT1 are installed without insulating gaskets on a common heat sink from the computer processor assembly with the DL-43 fan. A heat sink in the form of a plate with an area of ​​​​about 5 cm2 should also be provided with a stabilizer DA1.

Microammeter RA1 - M4206 with a full deflection current of the arrow 100 μA. Network toggle switch SA1 - MT-1. The clips for the leads of the rechargeable battery are large spring-type, "crocodile" type, they can be purchased at a radio parts or auto parts store.

The view of the charger with the cover removed is shown in fig. 2.

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For an initial check of the charger's performance, an active load with a power of 100 W is connected to its output (a car headlight lamp with filaments connected in parallel). Before this, the charging current regulator R14 is set to the position of maximum resistance, which will correspond to the minimum current.

The load was connected in series with a control ammeter to the output of the charger. They are convinced that the regulator R14 allows you to change the charging current within the established limits, which, if necessary, can be adjusted by selecting the resistor R15.

Then, a battery is connected to the output of the device in series with a control ammeter. A charging current of 10 A is set on the control ammeter and, by moving the slider of the resistor R1, the pointer of the microammeter RA1 is set to the final division.

Author: Dymov A.

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