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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Digital charger. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells The advantages of individual charging of batteries that make up the batteries for powering equipment, measuring instruments are well known: their service life is extended, it becomes possible to simultaneously charge batteries from different batteries, etc. However, radio amateurs rarely build multi-channel chargers - seeming complexity and high cost scare away. The author of the published article claims that in this case you should not regret the costs - they will pay off. Let's remember what folk wisdom says: "A miser pays twice"... In the press, for example, in [1], a description of a multi-channel charger (CH) appeared with voltage control of each of the rechargeable batteries and limitation of the charging current when the charging threshold voltage was reached. Like all such automatic battery monitoring devices, they are of course easy to use. But as experience shows, such a construction of the memory leads to a deterioration in its efficiency compared to the series connection of batteries, unjustified complication. You can still put up with the deterioration in efficiency when powered from the mains: during battery operation, the cost of electricity spent on charging it is negligible compared to the cost of the batteries themselves and the memory. The authors of the article mentioned above, in my opinion, overcame the complexity of the memory "in forehead "- when increasing the number of channels to four, they also used a quad op-amp I don't think this is the best solution to the problem. The fact is that the general trend in the development of circuitry for serial devices over the past two decades indicates a decrease in the proportion of analog devices in their composition, replacing them with digital ones, which, in mass production, have better repeatability of output parameters. Despite the fact that radio amateurs, as a rule, create single designs, repeatability is no less important for them: it is, of course, easier to assemble a device on the principle of "done and forget how it works" than to spend precious creative ardor on setting it up. It is also important that today the elements of digital technology are cheaper and more accessible. The proposed "digital" memory for four channels for nickel-cadmium batteries (see diagram) was developed precisely on the basis of such prerequisites.
Main technical characteristics:
The work of the memory is as follows. At the input CN (output 1) counter DD1 receives clock pulses with a frequency of 100 Hz. At its outputs 2 and 4 (pins 12 and 13) there is some digital combination in the binary code, which is the address, i.e., the number of the charger channel. The signal of this code is fed to the address input of the multiplexer (pins 10. 9 of the DD2 chip). Let's assume that at the moment the number I (1=1, 0, 1, 2) is written to the counter DD3. Through the multiplexer (inputs X DD2), the voltage from the 1st channel of the memory is supplied to the non-inverting input (pin 3) of the comparator DA1, which compares it with the exemplary one, corresponding to the set battery charging end voltage. At the output of the comparator (pin 6), by the time the 1st clock pulse ends, a high level voltage will be generated (the battery connected to the 1st channel is charged), or a low level (the battery is discharged), which is fed to the inputs D of the triggers of the microcircuits DD3, DD4 all four channels. At this moment, through the decoder (inputs Y of the DD2 microcircuit), a low-level pulse arrives at the clock input C of the 1st trigger, with its decline (voltage change from -3 V to +3 V), which records information from the information input D. The state of this trigger remains unchanged until the next clock pulse, i.e. until the address is repeated. The voltages from the outputs of the trigger, for example, the trigger DD3.1 of the charging node A1, are supplied to the key transistors VT2, VT3, which turn on the charging current, respectively (the battery G1, connected to the channel with the address "0", is discharged) and the indicator HL2 "No charging "Red glow (battery is charged). Thus, the described device uses the only analog "slippery" element - the DA1 comparator, which in turn (like a grandmaster during a simultaneous game session) makes a decision for each of the four batteries: whether it should be charged during the next four cycles or not. Clock pulses following with a double network frequency (98 ... 100 Hz) are fed to the input of the counter DD1 from the output of the rectifier VD1VD2 through the shaper formed by the elements R3, C5, VT1, R4. From the outputs of the counter, the clock sequence switches the memory channels with a frequency close to 6 Hz (fcycle = 2 fnet / 16 = 2-50/16 - 6 Hz), and the switching of each memory channel occurs at a frequency of about 1,5 Hz: (fswitch \u4d ftact / 250 16 / 4 / 1,5 - 2 Hz). At the same time, the "blinking" frequency of the charging indicators HL5 - HL2, with their linear arrangement and the absence of batteries in the memory (the channel turns on with the first pulse, and then turns off with the next, i.e. the blinking frequency of the indicators is 10 times lower), does not irritate user - the operation of the device in this case resembles the well-known Christmas tree garland. If the "blinking" frequency is chosen higher, for example, XNUMX kHz, then the light signals of the indicators will cease to be noticeable - the device will not attract increased attention to itself, and if it is lower, it makes it inconvenient to eliminate the often occurring non-contact when connected to a battery charger with an oxidized contact surface. Capacitor C5 prevents possible failures of the counter DD1 due to interference in the mains. To avoid failure of microcircuits when the voltage polarity of the battery being charged is reversed (due to its polarity reversal or erroneous connection), their power supply is selected bipolar. The function of the comparator (DA1) is performed by the OU KR140UD1208, which provides guaranteed parameters at low supply voltage. In addition, it is relatively "slow" and provides a delay in the change in voltage at the information input of D triggers when a clock pulse arrives at the C-input, i.e. it has a "built-in low-pass filter" at the output. The HL1 LED (green glow), which is an indicator that the device is connected to the network, together with resistors R11 - R13 forms a source of exemplary voltage. The voltage corresponding to it at the inverting input of the comparator DA1 is set by resistor R12 equal to the voltage of the charged battery. To increase the efficiency, the rectified voltage is smoothed by filter capacitors C1 and C2 only in low power supply circuits. The supply voltage of the low-power part of the device is stabilized by parametric stabilizers R1VD4 and R2VD5. All fixed resistors - C2-23, trimmer R12 - SPZ-19 or, better, multi-turn SP5-2, SP5-14. Capacitors - K10-17 and K50-35. Instead of KR140UD1208, we use its counterpart from other op-amp series, which is operable at low supply voltage. It is desirable that the powerful rectifier diodes VD1 and VD2 be with a Schottky barrier and possibly lower forward voltage drop. Transistors of the KTZ102 (VT2-VT9) series, operating in the key mode, must be with a high value of the base current transfer coefficient. When using transistors with a lower numerical value of this parameter, the load capacity of microcircuit triggers will not be enough to introduce transistors into saturation (especially VT2, VT4, VT6, VT8, including the battery charging current). In this case, you will have to use a VD4 zener diode with a high stabilization voltage, for example, KS139A. The mains power supply is made on the available transformer with a power of 3 W. The effective value of the voltage on each of its windings II and III under load is 5 V. You can use unified incandescent transformers of the TN series. Structurally, the memory is made in a case soldered from plates of foil-coated fiberglass 2 mm thick. In the upper part of the case there is a cassette for connecting rechargeable batteries, and in front of each battery there is a charging indicator corresponding to it. Ventilation holes are drilled in the upper and lower walls of the housing in the area where the mains transformer is located. Capacitors C6, C7 and C8-C10, shunting the power circuits of microcircuits, should be placed on different parts of the circuit board. Establishing a properly assembled device is easy. After turning on the power, the HL1 indicator (green glow) should light up and the HL2-HL5 indicators (red glow) should “blink”. Then, alternately closing the contacts of each of the channels of the device, check whether the indicator corresponding to it goes out. After such a preliminary check, connect a charged battery to any of the channels of the device and, using a trimmer resistor R12, set a reference voltage of 1 V at the inverting input of the comparator DA1,43. In this case, the indicator of the charging unit of this channel should light up. Working with the proposed memory is even easier. Wipe the contact surfaces of rechargeable batteries with alcohol and, observing the polarity, connect them to the spring contacts of the cassette. If the battery is low, then the corresponding LED should not light up at all. Increasingly frequent "blinking" of the LEDs indicates the imminent end of battery charging, and if one of the batteries is fully charged, then its LED lights up continuously. Briefly about the possible improvement of the described memory The exemplary voltage source (ION), built on LEDs, has a noticeable negative TCV - about 2 mV / ° C at operating temperature. Therefore, an increase in temperature by 15 ° C leads to an undercharging of the battery by about 0,03 V. This, of course, is not a serious drawback of the memory - due to the peculiarities of the current-voltage characteristic, nickel-cadmium batteries "do not get enough" for this reason, only a few percent from the total stored energy. To reduce the effect of temperature on this variant of the ION, it is placed away from heat flows. If you want to achieve even greater accuracy of the memory, you can install a more advanced ION, for example, described in [3]. But then the cost of parts of the designed memory will increase. If the mains transformer of the power supply has sufficient power reserve, you can increase the battery charging current or the number of channels of the device. To increase the charging current, it is enough to replace the transistors VT2, VT4, VT6 and VT8 with composite ones, for example, KT973A, the Zener diode VD4 - with KS139A (or KS147A) and, accordingly, change the resistance and dissipation power of the current-setting resistors R15, R17, R19, R21. The number of channels is most simply increased to eight by using the K561KP2 eight-channel multiplexer in the device. And the last. The round-the-clock operation of the device (while the batteries can simply be stored in it) requires a very careful design with the implementation of safety requirements. Literature
Author: V. Zhuravlev, Energodar, Zaporozhye region.
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