Converting a computer power supply into a charger. Encyclopedia of radio electronics and electrical engineering

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

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In this article, the author shares his accumulated experience in converting computer power supplies into lead-acid battery chargers. The author pays special attention to the improvement of the charging current display unit, by which it is possible to determine the battery charge and the moment when charging is completed.

Since the development of a charger based on a computer power supply [1], more than a dozen such devices have been assembled. Blocks of different designs and manufacturers have been redone. I received a lot of questions about the alteration, the elimination of self-excitation of the power supply in the current stabilization mode. As practice has shown, the output current limit indication unit can be improved to work in a charger. The proposed article is devoted to these issues.

Before proceeding with the alteration of the block, it is necessary to carefully study its design. The block must be assembled on a TL494CN chip or its analogues, such as DBL494, KA7500, KR1114EU4. Other microcircuits have a number of nodes that complicate the alteration, although they do not exclude it. Next, you need to inspect all oxide capacitors. First, replace capacitors with visible signs of failure (swollen or depressurized case). For the remaining ones, measure the equivalent series resistance and replace those in which it exceeds 0,2 ohms.

As described in [1], it is better to refine the block in stages. First you need to make sure that it is functioning normally in the voltage stabilization mode. It is better if LATR or another device for regulating the mains voltage is at hand, for example, a transformer with a large number of secondary windings. The use of such a transformer from an old TV to regulate the alternating voltage is described in the article [2]. The power supply must be checked in the voltage stabilization mode at a minimum of 190 V, a nominal 220 V and a maximum of 245 V mains voltage, as well as a change in load current from minimum to maximum.

The unit must work without signs of self-excitation; it may not have an output voltage adjustment circuit, so it is better to introduce it either as in the diagram in [1], or install a variable resistor in the feedback circuit, for example, in series with the resistor R31 (see the diagram in Fig. 1 in article [1] ).

Fig. 1

For the charger, the inductor L1 can be left without rewinding if the voltage at the output of the unit is not less than 6 V, for example, only when recharging the batteries. At a voltage of less than 6 V, the device may go into intermittent mode, which will adversely affect the stability of operation. Therefore, in this case, it is better to rewind the inductor, following the recommendations of the article [1].

In some blocks, after the inductor L1, there are additional coils in the positive circuit of the output voltage. They degrade the operation of the device in the current stabilization mode. Therefore, these coils must be dismantled, replacing them with jumpers.

Instead of the MBRB20100CT (VD15) diode assembly, you can use the widely used FR302 rectifier diodes by connecting them in parallel and placing them on a common heat sink. For a maximum current of 6 A, two pairs of diodes are sufficient.

Due to the variety of designs, it is difficult to predict the complexity of the work to achieve the normal functioning of the device in the current stabilization mode.

To prevent self-excitation, capacitor C12 is best replaced with the same RC circuit as R18C9. Sometimes you have to cut the printed conductor from pin 16 of the TL494 (DA1) chip and connect this pin to the lower output of the current sensor (resistor R24) with a separate wire.

It is necessary to check how a common printed conductor is connected to pin 7 of the DA1 chip. If in the process of reworking it had to be broken, it is best to connect this output of the microcircuit with a separate wire to the negative terminal of the capacitor C20. It has been noticed that the KA7500 chip is less stable than its counterparts. Therefore, if measures to eliminate self-excitation have not been successful, you can replace this chip with a TL494 or KR1114EU4.

A slight ripple in the output voltage may be caused by the operation of the fan motor M1. If they are undesirable, then you can connect a resistor with a resistance of 1 ... 5 Ohm in series with the electric motor, and in parallel with it - a capacitor with a capacity of about 100 microfarads with a nominal voltage of 25 V. If necessary, the electric motor is cleaned of dust and lubricated, for example, with PMS100 or PMS200 silicone grease .

It is possible to facilitate the setting of the current limiting level when setting up the device by replacing the resistor R26 with a series-connected constant resistor with a resistance of 82 ohms and a tuning resistor of 220 ohms. This is due to the fact that when the board is placed into the case, another circuit of the common wire appears through the mounting screws and the case, which will affect the level of limitation.

After assembly, the device must be checked again for the absence of self-excitation when the mains voltage and load change from minimum to full, and in current stabilization mode from minimum to rated output voltage.

If the indicator on the elements DA2, R33-R35, R37, HL1 in the current stabilization mode in the laboratory power supply justifies itself, then in the charger it is not informative enough. The transition from current stabilization to voltage stabilization, indicated by the HL1 LED, does not correspond to the end of charging. It is much better to monitor the charging current. The smaller it is, the higher the battery charge. Therefore, the display unit was redesigned according to Fig. 1. The elements DA2 and HL1 are left, their designations are the same as in fig. 1 in article [1], the numbering of the added elements is continued. Resistors R33-R35, R37 removed.

The node is made on the same DA2 chip (LM393N), but now both of its comparators are used. An inverting amplifier with a gain of about 2.1 was assembled on DA500. It turned out that the comparator works great in this capacity. It amplifies the voltage from the current sensor (resistor R24) from approximately 10 mV to 5 V. This voltage is applied to the input of the second comparator DA2.2, where it is compared with the reference voltage of 5 V coming from pin 14 of the TL494 chip. When the voltage at the inverting input DA2.2 rises above the exemplary one, the HL1 LED lights up, indicating that the battery is being charged. As soon as the indicator turns off, you can turn off the charging. By moving the engine of the tuning resistor R39, the indicator operation threshold is set at a current of about 1 A. The capacitance of the capacitor C22 is not critical and can be in the range of 10 ... 100 nF. Resistor R39 - SP4-19. The LM393N chip can be replaced by the domestic analogue K1401CA3A.

The indication unit received further development in connection with the desire to see at least approximately the degree of charge of the battery. It is not much more complicated than the previous one and is made on the LM339N quad comparator chip. The node diagram is shown in fig. 2.

Fig. 2

The scheme from [3, p. 102]. An inverting amplifier similar to that shown in fig. 2.1, but with a gain of about 1. An exemplary voltage is applied to the non-inverting input of the DA100 comparator. On resistors R2.2 and R42, a divider of this voltage for the comparator DA43 is assembled. The resistance ratio of the resistors is chosen to be about 2.3:2. When the charging current is more than 1 A, the voltage at the output of the DA5 amplifier exceeds 2.1 V. The outputs of the comparators DA5 and DA2.2 have a low voltage level. Only the HL2.3 LED is lit, since the voltage on the other LEDs is less due to the voltage drop across the diodes VD1 and VD18. As soon as the charging current becomes less than 19 A, the DA5 comparator switches and the HL2.2 LED goes out, and the HL1 LED lights up. The HL2 LED is extinguished due to the voltage drop across the VD3 diode. When the charging current is less than 19 A, the DA1,7 comparator switches and the HL2.3 LED lights up, indicating the end of charging.

LEDs will fit any low-power glow of different colors, for example, AL307BM (red), AL307DM (yellow) and AL307VM (green). When establishing the display unit, move the trimmer resistor R39 slider so as to set the threshold for the operation of the comparator DA2.2 at a current of 5 A. By selecting the resistor R42, the threshold for the operation of the comparator DA2.3 is set. Resistor R39 - SP4-19. The LM339N chip can be replaced by the domestic analogue K1401CA1.

In the display unit, assembled according to the diagram in Fig. 2, due to the influence of noise and interference, two LEDs may simultaneously glow at certain voltage values ​​on the current sensor. It can be eliminated by creating a small hysteresis in the switching characteristics of comparators DA2.2 and DA2.3 by introducing positive feedback circuits through 470 kΩ resistors that are connected to the output and non-inverting input of each of these comparators.

Fig. 3

The diagram of the third variant of the indication unit is shown in fig. 3. It is assembled on the LM324N quad op-amp chip. When developing it, the scheme from the book [4, p. 77]. Indicator - one two-color LED HL1. The voltage from the current sensor is fed to an inverting amplifier assembled on the op-amp DA2.1. This amplifier has the same purpose and gain as in the previous node. The signal from the output of the amplifier passes through the low-pass filter R41C24, which suppresses high-frequency interference, and is fed to two amplifiers: an inverting DA2.2 op-amp and a non-inverting DA2.3 op-amp.

To the output of the inverting amplifier through the resistor R48 connected crystal LED HL1 green glow. A red LED crystal HL49 is connected to the output of a non-inverting amplifier through a resistor R1. The gain factors are chosen so that as the voltage on the current sensor increases, the brightness of the red color increases, and that of the green color decreases. During the adjustment, the engine of the tuning resistor R39 is moved so that at a charging current of 5 A, the HL1 LED glows only in red. As the charging current decreases, the color of the glow gradually changes from red to yellow and then to green. Green color indicates the end of charging.

Literature

1. Andryushkevich V. Alteration of a computer power supply into a laboratory and charger. - Radio, 2012, No. 3, p. 22-24.
2. Solonenko V. Autotransformer based on TS-180. - Radio, 2006, No. 5, p. 36.
3. Shelestov IP Useful schemes. - M.: "Solon-R", 1998.
4. Zikhla F. LCDs, Light Emitting and Laser Light Emitting Diodes: Schematics and Ready Solutions. - St. Petersburg: "BHV-Petersburg", 2012.

Author: V. Andryushkevich

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