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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Charger for nickel-cadmium accumulators and batteries. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells The specialized literature substantiates the expediency of charging batteries from a fixed voltage source with current limitation. This mode is convenient in that recharging during, for example, the night guarantees their full charge by the morning, regardless of their initial state, without the danger of overcharging. The diagram of the charger is shown in Fig.1.
Zener diode VD6, operational amplifier DA1.1, transistor VT1 and elements connected directly to them form a highly stable voltage source. Its feature is the supply of the R2VD6 parametric stabilizer with the output voltage of the source, which provides it with high parameters. The R17R28 divider generates 12 voltage steps corresponding to the limit when charging single batteries and batteries made up of 2-12 nickel-cadmium batteries. The required charging voltage is selected by switch SA2. The operational amplifier (op-amp) DA1.2 together with the transistor VT2 forms an exact repeater of this voltage with a high load capacity. Its output impedance is very small, the change in voltage as the output current increases from 0 to 350 mA cannot be detected by a four-digit digital voltmeter, i.e. it is less than 1 mV, and the output resistance is accordingly less than 0,003 ohm. To limit the current at the beginning of charging, a comparison of the voltage drop across the resistor R32 (and resistors R6-R16 connected to it in parallel) and the reference voltage taken from the divider R35-R39 is used. The collector current of the transistor VT2 is equal to the charging current with sufficient accuracy. The reference voltage taken from resistors R35 and R36 is 1,2 V. The comparison of voltages is carried out by the comparator, its function is performed by the op-amp DA2.2. When the charging current creates a voltage drop of more than 32 V across the resistor R1,2, the op-amp DA2.2 opens the transistor VT3, which, with its collector current, increases the voltage at the inverting input of the op-amp DA1.2, which leads to a decrease in the output voltage of the op-amp and the transition of the entire source to the mode current stabilization. The current limit in the range from 2,5 to 350 mA is set by the SA3 switch. The output resistance of the device in the current stabilization mode is equal to the resistance of the resistor R30. The microammeter PA1 with an additional resistor R31 forms a voltmeter for a voltage of 1,2 V, therefore, when the source is operating in the current stabilization mode, its arrow points to the last division of the scale. For the voltmeter, a microammeter for a current of 100 μA was used, so this reading corresponds to a charging current equal to 100% of the setting of the SA3 switch. If a discharged battery is connected to sockets X1 and X2 of the charger by setting switch SA2 to the position corresponding to their number, at first the charging current will be determined by the position of switch SA3. After a few hours, the battery voltage will reach the value set by the SA2 switch, and the device will enter the voltage stabilization mode. The charging current will begin to decrease, which can be monitored by the indication of the PA1 device. When the current decreases to a value of approximately 5% of the set switch SA3, the comparator on the op-amp DA2.1 will switch and the HL2 LED will light up, signaling the end of charging. If the battery (or a single battery) continues to be charged even during the day, nothing will happen to it, since the current at the end of charging is very small. LED HL1 - indicator of device connection to the network. By selecting a capacitor C7, the high-frequency generation of the op-amp DA1.2 is eliminated. What is the role of diodes VD2 VD5? When charging a single battery, the voltage at the non-inverting input of the op-amp DA1.2 is 1,4 V, and in the mode of closing the output of the charger, its output voltage, which ensures the transfer of the device to the current stabilization mode, should be about 0,6 V relative to the common wire. In order for the op-amp DA1.2 to work normally in such modes, the voltage of its negative power supply must be at least 2 V in absolute value, which is ensured by the voltage drop across the VD3VD5 diodes. Similarly, for the normal operation of the op amp DA2.1 with a voltage at the inputs close to the voltage of the positive power supply, the difference between them must be at least 0,6 V - provided by the voltage drop across the VD2 diode. A drawing of a printed circuit board made of one-sided foil fiberglass 1,5 mm thick, on which most of the device parts are located, is shown in Fig. 2.
The VT2 transistor is equipped with a needle-shaped heat sink with dimensions of 60x45 mm, the height of the needles is 20 mm. Switches SA2 and SA3 together with resistors soldered to them, microammeter RA1, LEDs HL1 and HL2, output sockets X1 and X2 are installed on the front panel of the device, made of fiberglass 1,5 mm thick, and transformer T1, switch SA1, fuse FU1, diode bridge VD1 and capacitors C1 - on the rear duralumin panel of the same thickness. The panels are fastened together with duralumin ties 135 mm long, a printed circuit board is screwed to the same ties. The finished structure is installed in an aluminum case in the form of a section of a rectangular pipe. Network transformer T1 - unified type TN-30. But you can use any other similar transformer, the secondary winding of which provides a voltage of 19 ... 20 V at a current of at least 400 mA. The rectifier bridge VD1, designed for the same output current, can be assembled from four diodes with an operating current of 300 mA, for example, type D226. These can be diodes VD2-VD5. Capacitor C1 is made up of three parallel-connected oxide capacitors of the K50-29 type with a capacity of 1000 microfarads for a nominal voltage of 25 V. Capacitor C2 is K53-1, the rest are KM5 and KM-6. The thermally compensated zener diode KS191F (VD6) can be replaced with D818 with letter indices V-E or with KS191 with any letter index. Resistors R3, R5 and R17-R28 are desirable to use stable, for example, C2-29. The resistances of resistors R17 - R28 are within 160 Ohm ... 10 kOhm, but they must be the same with an accuracy of no worse than 0,3%. Resistors R6R16 do not need to be accurate. It is advisable to select them in accordance with those indicated in the diagram from resistors of close ratings, which will simplify the setup of the device. Each of the resistors R15, R16 consists of several resistors of higher rating and lower power dissipation, which are connected in parallel. Trimmer resistors R4 and R38 type SP3-19a. Any LEDs HL1 and HL2, but it is desirable to have a different glow color. Zener diodes VD7 and VD8 for a stabilization voltage of 5,6-7,5 V. Switches SA2 and SA3 PG2-5-12P1N or similar small ones. Microammeter RA1 type M4247 for a current of 100 μA. Using the device for a different current of the full deflection of the arrow, you will have to select not only the limiting resistor R31, but also R32 to provide a charging current of 2,5 mA at the extreme left (according to the diagram) position of the SA3 switch. Transistors VT1, VT2 are any silicon structures n-p-n of medium power, and VT3 - any low-power silicon structures pn-p for an allowable voltage of at least 30 V. Operational amplifiers K140UD20 (DA1, DA2) are replaceable by a double number of K140UD7 op-amps. The use of other types of op amps is determined by the possibility of their operation in the modes mentioned above, but this has not been tested. Briefly about setting up the charger. First, with a trimmer resistor R4, set a voltage of 1 V on the emitter of transistor VT16,8. Having loaded the device with a resistor of 51 ... SA68 in each next position (up in the circuit), the output voltage increases by 7,5 V. Check the absence of high-frequency generation at the output and, if necessary, select capacitor C43. Next, restore the connection of the resistor R2, and set the switch SA1,4 to position "7". When changing the position of switch SA3, make sure that the output current, measured by a milliammeter connected in series with the load resistor, is limited to the value corresponding to the position of this switch (except 350 mA). Replace the load resistor with a chain of two or three diodes (of the same type as VD2-VD5) and, setting the SA3 switch to the "100 mA" position, set the same output current with the trimming resistor R38. The arrow of the microammeter should point to the last division of the scale, if this is not the case, select the resistor R31. Now set switch SA2 to position "1" and switch SA3 to position "10 mA". Connect a 3,3 kΩ variable resistor and a milliammeter to the output of the device, then increase the resistance of this resistor from zero. With an output current of approximately 0,5 mA, the HL2 LED should turn on. When setting up the device, remember that its output impedance is asymmetrical: it is small for the outgoing current and high for the incoming one. Therefore, an unloaded device is sensitive to mains interference, and measuring the output voltage with a high-resistance voltmeter can give an unexpectedly high result. Charging the batteries is easy. You just need to set the switches to the positions corresponding to the number of batteries in it and the maximum charging current, connect the battery to the output with the correct polarity and turn on the power of the device. A sign of the end of charging is the glow of the HL2 LED. The maximum charging current should be 3...4 times less than the capacity of the rechargeable battery. What additions or changes can be made to this charger option? First of all, it is necessary to supplement it with an electromagnetic relay K1, as shown in Fig. 3, which would turn off the battery or battery after charging is completed. When the HL2 LED is turned on, the relay is activated and breaks the charging circuit with its normally closed contacts.
Resistor R44 is necessary for a clear operation of the relay and to ensure a small hysteresis of the comparator at the op amp DA2.1. Relay K1 must be for a voltage of 20 ... 27 V, transistor VT4 of any medium or high power p-n-p structure, for example, KT502, KT814, KT816. But having introduced such an addition into the device, it should be borne in mind that after the start of charging, any switching of its circuits leads to the operation of the relay, so the necessary settings must be made in advance. The device can be used to discharge batteries of seven batteries without fear of overdischarging them. To do this, switch SA2 must be set to position "5", switch SA3 - to the nearest in terms of discharge current, but greater than it, connect a resistor between the output sockets XI and X2 that provides the necessary discharge current, and connect the battery being discharged. Since the battery voltage is greater than that supplied to the non-inverting input of the op-amp DA1.2, the transistor VT2 is closed, and the battery is discharged through the resistor. When the battery voltage drops to 7 V, the op amp DA1.2 and the transistor VT1 will switch to voltage stabilization mode, the discharge will stop. The HL2 LED serves as an indicator of the completion of the battery discharge - it lights up during the discharge process, and goes out at the end. If the device is often supposed to be used to discharge batteries, besides with a different number of batteries, it is advisable to introduce an additional resistor into it, the resistance of which is 40% of the total resistance of resistors R17-R28, and, of course, a switch. The resistor is connected between the output of the reference voltage source (in the diagram of Fig. 1, the connection point of the emitter of the transistor VT1, resistors R2, R3, capacitor C3) and the fixed contact "12" of the SA2 switch connected to the resistor R17, and in parallel with this resistor - an additional switch. The battery is charged with the contacts of the switch closed, and when they are opened, when the output voltage decreases by 1,4 times (up to 1 V per battery), the battery can be discharged. The battery is discharged through the resistor with a time-varying current, which can be stabilized by the K142EN12A microcircuit by turning it on according to the circuit shown in Fig. 4.
The resistance of the resistor R46 (Ohm) is determined by the formula: R46 \u1250d 6 / Ipas, where Iraz is the discharge current (mA). The values of the resistors, on which the discharge current depends, correspond to the resistances of the resistors R16-RXNUMX at the same currents as the charge current.
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