Attachment for automatic shutdown of the charger. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Automobile. Batteries, chargers

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The article describes a set-top box designed to work with a charger that does not have the function of disconnecting from the network when the battery is charged. This prefix should be of interest, first of all, to those motorists who, having the simplest factory-made or home-made charger, would like to automate the charging process with minimal time and money.

It is known that the voltage at the terminals of a lead-acid battery being charged with a stable current almost stops increasing as soon as it is fully charged. From this point on, almost all the energy supplied to the battery is spent only on electrolysis and heating of the electrolyte. Thus, at the moment of cessation of the increase in the charging voltage, it would be possible to disconnect the charger from the network. The instruction manual for car batteries [1] recommends, however, to continue charging in this mode for another two hours. This is how the automatic charger I described earlier [2] works. However, practice shows that this recharging is really necessary only during the annual control and preventive charge-discharge cycle in order to determine the technical condition of the battery.

In everyday use, it is quite enough to withstand the battery under constant voltage for 15 ... 30 minutes. This approach makes it possible to significantly simplify the automatic charger without any noticeable effect on the completeness of the battery charge. If you charge the battery with an unstabilized current, then along with a smooth increase in the charging voltage (pronounced weaker than in the first case), the charging current decreases. The evidence of a fully charged battery is the cessation of changes in both voltage and current.

This principle underlies the operation of the proposed prefix. It contains a comparator, one of the inputs of which is supplied with a voltage that increases proportionally with an increase in the charging voltage on the battery (and decreases with a decrease) and simultaneously proportionally decreases with an increase (increases with a decrease) of the charging current. The same voltage is applied to the second input as to the first, but with a significant time delay. In other words, as long as the voltage on the battery increases and (or) the charging current decreases, the voltage value at the second input of the comparator will be less than the voltage value at the first one, and this difference is proportional to the rate of change of the charging voltage and current. When the voltage on the battery and the charging current stabilize (which will indicate that the battery is fully charged), the voltage values ​​​​at the inputs of the comparator will equalize, it will switch and give a signal to turn off the charger. This idea is borrowed from [3].

The prefix is ​​made on widely used elements. The maximum operating current is 6 A, however, if necessary, it can be easily increased.

The schematic diagram of the attachment is shown in fig. one.

The device consists of an input op-amp DA1, two voltage comparators on the op-amp DA2.1, DA2.2, a two-input electronic relay VT1 - VT3, K1 and a power supply unit consisting of a network transformer T1, diodes VD1-VD4, a smoothing capacitor C6 and a parametric voltage stabilizer VD5R19. The output of the charger is connected to the terminals X1, X3, and the battery being charged - to the terminals X2, X3. The mains plug of the charger is connected to the socket X5 of the set-top box.

When you press the SB1 button, the mains voltage is supplied to the charger and to the mains winding I of the T1 transformer of the set-top box. The unstabilized voltage from the diode bridge VD1-VD4 is fed by an electronic relay, and the output voltage of the parametric stabilizer is supplied by the DA2 microcircuit (DA1 is powered by a charger). Battery charging starts.

The voltage drop created by the charging current across the resistor R1 is fed to the input of the op-amp DA1, which is connected according to the inverting amplifier circuit. The voltage at its output will increase with a decrease in the charging current. On the other hand, the output voltage of the op-amp is proportional to its supply voltage. And since the amplifier is powered directly from the battery being charged, the output voltage of the op-amp will be a function of both the voltage at the terminals of the battery being charged and the charging current. This construction of the set-top box made it possible to use it in conjunction with a variety of chargers, including the simplest ones.

A low-pass filter R4C2 is connected to the output of the op-amp, from which the voltage through the integrating circuits R7C3 and R5R6R8C4 is fed to the inputs of a comparator made on the op-amp DA2.2. The R8C4 circuit has a time constant many times greater than the R7C3 circuit, so the voltage at the non-inverting input of this comparator will be less than at the inverting one, and the output will go low.

The comparator on the op-amp DA2.1 is a conventional threshold device, the inverting input of which is supplied with an exemplary voltage from the R15R16 resistive divider, and to the non-inverting input from the R11R12R13 divider connected to a rechargeable battery. The comparator switches when the voltage reaches 14,4 V on the battery and serves to eliminate the possibility of premature shutdown of the charger in conditions of insignificant dynamics of changes in the voltage on the battery.

As a result, until the voltage on the battery being charged reaches the specified value, the set-top box will not turn off the charger, even if the DA2.2 comparator has switched. This situation is possible when setting the charging current to an underestimated value and, as a result, with a very slow change in the charging voltage and current. Initially, the output of the comparator DA2.1 also has a low voltage.

The outputs of both comparators through resistive dividers R17R18 and R20R21 are connected to the bases of transistors VT2 and VT1. Thus, when you press the SB1 button, these transistors remain closed, and VT3 opens. Relay K1 is activated and contacts K1.1 blocks the contacts of the button. The prefix remains on after the button is released.

Since transistors VT1 and VT2 are connected according to the AND logic circuit, they open only at a high voltage level simultaneously at the output of comparators DA2.1, DA2.2. This can only happen when the battery is fully charged. In this case, the transistor VT3 closes and the relay K1 releases the armature, opening the power supply circuit of the set-top box and the charger.

On fig. 2 shows graphs of voltage changes at the inputs of the DA2.2 comparator, as well as the charging current in the process of recharging the 6ST-60 battery using a simple charger with an unstabilized charging current. The initial state of charge of the battery is about 75%.

In the case when the set-top box will operate in conditions of strong interference, the power supply circuit of the DA2 op-amp should be shunted with a ceramic capacitor with a capacity of 0,1 μF.

The prefix is ​​characterized by reduced sensitivity to fluctuations in mains voltage. If, for example, it increases, then the voltage on the battery being charged also increases, but at the same time the charging current also increases. As a result, the voltage at the output of the op-amp DA1 will change slightly.

The prefix is ​​mounted in a metal box measuring 140x100x70 mm. On its front panel there are clamps X1-X3, fuse FU1 and socket X5. Most of the parts of the attachment are placed on a printed circuit board with dimensions of 76x60 mm, made of foil fiberglass with a thickness of 1,5 mm. The drawing of the board is shown in fig. 3. Transformer T1 and relay K1 are mounted separately next to the board. Resistor R1 is soldered directly to terminals X1, X2.

Resistor R1 is composed of two resistors C5-16V connected in parallel with a resistance of 0,1 ohms and a rated power dissipation of 1 W; other constants - MLT. Trimmer resistors R9, R12 - SPZ-16v.

Capacitor C1 - KM5, the rest - K50-35. Capacitor C4 is desirable to be trained before installation on the board by connecting it for several hours to a constant voltage source of 10 ... 12 V.

Instead of KD105B, you can use diodes KD106A, and instead of KD522B - any of the KD521 series. Zener diode VD5 - any low-power with a stabilization voltage of 11 ... 13 V.

KT3102B transistors are replaceable by any low-power ones of the corresponding structure with a static base current transfer coefficient of at least 50, and when replacing the VT3 transistor, one should focus on the operating current of the existing relay K1. When choosing a replacement for the K553UD2 OU, it must be taken into account that not all operational amplifiers allow operation with an input voltage equal to the supply voltage.

The set-top box uses a ready-made low-power network transformer with an alternating voltage of the secondary winding of 14 V at a load current of up to 120 mA. Relay K1 - RMU, passport RS4.523.303, but any one with a response voltage of 12 ... 14 V is suitable, the contacts of which are designed for switching an alternating voltage of 220 V at a current of 0,3 ... 0,5 A.

To set up the set-top box, you will need a stabilized voltage source regulated within 10 ... 15 V, and a digital voltmeter with a measurement limit of 20 V. First, the slider of the resistor R12 is set to the bottom, and R9 to the left position according to the diagram. A source is connected to terminals X1 and X3, a voltage of 14,4 V is set at its output and the set-top box is connected to the network.

Press the button SB1, while the relay K1 should work. Make sure that at the outputs of the op-amp DA2.1 and DA2.2 (pins 10 and 12) there is a low voltage level (1,3 ... 1,5 V). Then measure the voltage at the output of the op-amp DA1 (pin 10). It should be approximately equal to the voltage of the connected power supply.

The terminals of the resistor R30 are closed for 40 .. .8 s, ensuring fast charging of the capacitor C4, and then after a ten-minute exposure, the voltmeter is connected to the output of the op-amp DA2.2 and smoothly rotate the knob of the resistor R9 until the comparator switches, i.e., an abrupt increase in voltage on its output is up to 11 ... 11,5 V. Then the voltage is measured at the inverting input of the op-amp DA2.2 and the resistor R9 reduces it by 15 ... 20 mV.

It should be noted that it is necessary to measure the voltage in the input circuits of the comparator with a digital voltmeter with an input resistance of at least 5 ... 10 MΩ in order to prevent the discharge of the capacitor C3. Since the input resistance of many popular digital avometers does not exceed 1 MΩ, you can turn on a ten-megaohm resistor at the input of the existing voltmeter, which together with the input resistance of the device forms a voltage divider with a ratio of 1:10.

In conclusion, rotate the knob of the resistor R12 until the op-amp DA2.1 is switched. In this case, relay K1 should release the armature.

If the radio amateur does not have a digital voltmeter and no power source, you can set up the set-top box directly in the process of actually charging the battery. To do this, connect the charger and the battery to the set-top box, set the charger switch to the "On" position, and the sliders of the resistors R9, R12 of the set-top box - as indicated above. Press the button SB1, make sure the relay K1 is activated and set the charging current in accordance with the instruction manual for the charger.

Next, they monitor the process of charging the battery, periodically measuring the voltage at the terminals. When it reaches 14,4 V, rotate the knob of the resistor R12 until the op-amp DA2.1 is switched.

When the voltage stops increasing, continue charging in this mode for another 20 ... 30 minutes and then smoothly rotate the knob of the resistor R9 until the op-amp DA2.2 trips and the set-top box and charger are disconnected from the mains. This completes the setup.

In conclusion, it should be noted that in order to guarantee a full charge of the battery, it is desirable to set the maximum allowable values ​​of the charging current in order to ensure good dynamics of the voltage change at the output of the op-amp DA1. This is especially true for chargers with unstabilized output current and heavily discharged batteries.

Literature

1. ZHUITs.563.410.001IE. Batteries lead starter with a capacity of more than 30 Ah. User manual. 1987.
2. Kupriyanov K. Automatic charger. - Radio, 2000, No. 12, p. 33-37.
3. Tenev L. A device for detecting moving metal objects. - Radio, 1987, No. 5, p. 61.

Author: K. Kupriyanov, St. Petersburg

See other articles Section Automobile. Batteries, chargers.

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