Charger for car battery from computer power supply.

Hello, dear ladies and dear gentlemen!
On this page, I will briefly tell you how to convert the power supply unit of a personal computer into a charger for car (and not only) batteries with your own hands .
The charger for car batteries should have the following property: the maximum voltage supplied to the battery is not more than 14.4V, the maximum charging current is determined by the capabilities of the device itself. It is this method of charging that is implemented on board the car (from the generator) in the normal mode of operation of the car’s electrical system.
However, unlike the materials from this article, I have chosen the concept of maximum simplicity of modifications without using homemade printed circuit boardstransistors and other "frills".
A power unit for rework was presented to me by a friend, he himself found it somewhere at work. From the inscription on the label it was possible to make out that the total power of this power supply unit is 230W, but the channel 12V can consume no more than 8A current. Opening this power supply, I found that it does not have a chip with the numbers "494" (as described in the article offered above), and its basis is the UC3843 chip. However, this microcircuit is not included according to the standard scheme and is used only as a pulse generator and driver of a power transistor with overcurrent protection function, and the functions of the voltage regulator on the output channels of the power supply unit are assigned to the TL431 microcircuit installed on the additional board: style="text-align: center;">
A trimmer is installed on the same option board to adjust the output voltage in a narrow range.
So, to convert this power supply to a charger, you first need to remove all unnecessary. The excess is:
1. Switch 220 / 110V with its wires. These wires just need to unsolder from the board.At the same time, our unit will always operate from 220V voltage, which eliminates the danger of burning it if this switch is accidentally switched to the 110V position;
2. All output wires, except One bundle of black wires (in a bundle of 4 wires) is 0V or "common", and one bundle of yellow wires (in a bundle of 2 wires) is "+".
Now we need to ensure that our unit always works if it is connected to the network (by default it only works if we close the necessary wires in the output bundle of wires), as well as eliminate the overvoltage protection action, which disconnects the unit if the output voltage is above a certain predetermined limit. This is necessary because we need to get 14.4V output (instead of 12), which is perceived by the built-in block protections as overvoltage and it turns off.
As it turned out, the on-off signal, and the overvoltage protection action signal passes through the same optocoupler, of which there are only three - they connect the output (low-voltage) and input (high-voltage) parts of the power supply. So, in order for the unit to always work and be insensitive to output overvoltages, it is necessary to close the contacts of the desired optocoupler with a solder jumper (i.e.the state of this optocoupler will be "always on"):
Now the power supply will always work when it is connected to the network and regardless of what kind of voltage we will do at his output.
Next, set the output of the unit, where it used to be 12V, the output voltage is 14.4V (at idle go). Since it is not possible to install 14.4V at the output (it only allows something somewhere around 13V) using rotation of the trimmer resistor mounted on the optional power supply board, it is necessary to replace the resistor connected in series with the trimmer to a slightly smaller resistor face value, namely 2.7kOm:
Now the output voltage setting range has shifted to a larger direction and it became possible to set output 14 .4В.
Then, you need to remove the transistor, which is next to the TL431 chip. The purpose of this transistor is unknown, but it is turned on in such a way that it has the ability to impede the operation of the TL431 chip, i.e. to prevent the output voltage from stabilizing at a given level.This transistor was here at this place:
Next, to make the output voltage more stable at idle, you need to add a small load to the output of the block via the + 12V channel (which we will have +14.4 B), and on the + 5V channel (which is not used here). As a load on the + 12V channel (+14.4), a 200 ohm 2W resistor was used, and a + 5V channel used a 68 ohm 0.5W resistor (not visible in the photo, since it is located behind the additional charge):
Only after installing these resistors, you should adjust the output voltage at idle (no load) at 14.4V.
Now you need to limit the output current to the level allowed for this power supply (i.e., about 8A). This is achieved by increasing the value of the resistor in the primary circuit of the power transformer used as an overload sensor. To limit the output current at 8 ... 10A, this resistor must be replaced with a resistor of 0.47 Ohm 1W:
After this replacement, the output the current will not exceed 8 ... 10A even if we short the output wires.
Finally, you need to add part of the circuit,which will protect the unit from connecting the battery with reverse polarity (this is the only "homemade" part of the circuit). This will require a conventional 12V automotive relay (with four contacts) and two diodes for 1A current (I used 1N4007 diodes). In addition, to indicate the fact that the battery is connected and charged, you will need an LED in the case for installation on the panel (green) and 1kOhm 0.5W resistor. The scheme should be like this:
Works as follows: when the battery is connected to the output with the correct polarity, the relay is triggered by the energy remaining in the battery, and after it is triggered, the battery starts charging from the power supply through the closed contact of this relay, which is indicated by a lit LED. A diode connected in parallel to the relay coil is needed to prevent overvoltages on this coil when it is disconnected due to self-induction EMF.
The relay is glued to the radiator of the power supply using silicone sealant (silicone - because it remains elastic after "drying" and well withstands thermal loads, i.e.compression-expansion during heating-cooling), and after "drying" of the sealant, the remaining components are mounted on the relay contacts:
Wires to the battery selected flexible, with a cross section of 2.5mm2, have a length of about 1 meter and end in "crocodile" to connect to the battery. To fasten these wires in the body of the device, two nylon ties, threaded into the radiator holes, are used (the holes in the radiator must be pre-drilled).
That's all:
Finally, all labels were removed from the power supply case and a self-made sticker with new device characteristics was pasted:
The disadvantages of the received charger include the absence of any indication of the state of charge of the battery, what makes the ambiguity - is the battery loaded or not? However, in practice it has been established that for a day (24 hours) the usual car battery with a capacity of 55A · h has enough time to fully charge.
The advantages includethat with this charger the battery can “stand on the charge” for an arbitrarily long time and nothing bad will happen - the battery will be charged, but not “recharged” and will not deteriorate.

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