The author offers options for converting a cell phone charger into a stabilized power supply with an adjustable output voltage or into a stable current source, for example, for charging batteries.
One of the most numerous electronic devices that are widely used in everyday life is undoubtedly chargers for cell phones. Some of them can be improved by improving parameters or expanding functionality. For example, turn the charger into a stabilized power supply unit (PSU) with an adjustable output voltage or a charger with a stable output current.
This will allow you to power various radio equipment from the network or charge Li-Ion, Ni-Cd, Ni-MH batteries and batteries.
A significant part of the memory for cell phones is assembled on the basis of a single-transistor self-oscillating voltage converter. One of the variants of the circuit of such a charger using the example of the ACH-4E model is shown in Fig. 1. It also shows how to turn it into a power supply with adjustable output voltage. Designations of standard elements are given in accordance with the markings on printed circuit board.
Rice. 1. One of the variants of the charger circuit using the example of the ACH-4E model
Newly introduced elements and improvements are highlighted in color.
In simple memory devices, which include the one being modified, a half-wave mains voltage rectifier is often used, although in most cases there is space on the board to accommodate diode bridge. Therefore, at the first stage of modification, the missing diodes were installed, and resistor R1 was removed from the board (it was installed in place of diode D4) and soldered directly to one of the pins of the XP1 plug. It should be noted that there are chargers that do not have smoothing capacitor C1. If this is the case, it is necessary to install a capacitor with a capacity of 2.2...4.7 μF for a rated voltage of at least 400 V. Then capacitor C5 is replaced with another with a larger capacity. In this version, modifications to the memory are shown in Fig. 2.
Rice. 2. Modified memory
In the original charger, a 1N4937 diode was used in the output rectifier, which was replaced by a 1N5818 Schottky diode, which made it possible to increase the output voltage. After this modification, the dependences of the output voltage on the load current were removed, which are shown in blue in Fig. 3. The amplitude of the output voltage ripple with increasing load current increases from 50 to 300 mV. When the load current is more than 300 mA, pulsations with a frequency of 100 Hz appear.
Rice. 3. Dependence of output voltage on load current
The dependencies show that the stability of the output voltage in the memory is low. This is due to the fact that its stabilization is carried out indirectly by controlling the voltage on winding II, namely, by rectifying the pulses on winding II and applying the closing voltage through the zener diode ZD (stabilization voltage 5.6...6.2 V) to the base of transistor Q1 .
To increase the stability of the output voltage and the possibility of its adjustment, the DA1 microcircuit (parallel voltage stabilizer) was introduced at the second stage of refinement. Control of the converter and provision of galvanic isolation are implemented using transistor optocoupler U1. To suppress impulse noise at the frequency of the self-oscillator, an L1C6C8 filter is additionally installed. Resistor R9 has been removed.
The output voltage is set with variable resistor R12. When the voltage at the control input of the DA1 microcircuit (pin1) exceeds 2.5 V, the current through the microcircuit and, accordingly, through the emitting diode of the optocoupler U1 will increase sharply. The phototransistor of the optocoupler will open, and the gate of the base of transistor Q1 will receive the closing voltage from capacitor C4. This will lead to the fact that the duty cycle of the self-oscillator pulses will decrease (or generation will fail). The output voltage will stop increasing and begin to decrease smoothly due to the discharge of capacitors C5 and C8.
When the voltage at the control input of the microcircuit becomes less than 2.5 V, the current through it will decrease and the phototransistor will close. The duty cycle of the oscillator pulses will increase (or it will start working), and the output voltage will begin to increase. The output voltage range, which can be set with resistor R12, is 3.3...6 V. A voltage of less than 3.3 V, taking into account the drop on the emitting diode of the optocoupler, is not enough for normal operation microcircuits. The dependences of the output voltage (for different values) on the load current of the modified device are shown in red in Fig. 3. The amplitude of the output voltage ripple is 20...40 mV.
The elements (except for the variable resistor) of the second stage of refinement are placed on a single-sided printed circuit board made of foil fiberglass laminate with a thickness of 0.5...1 mm, its drawing is shown in Fig. 4. Installation - from the side of printed conductors. You can use fixed resistors MLT, C2-23, P1-4, capacitors C6, C7 - ceramic, C5 - oxide imported, it is removed from motherboard personal computer, C8 - oxide low-profile imported. Since the output voltage has to be adjusted infrequently, not a variable resistor is used, but a trimmer PVC6A (POC6AP). This made it possible to install it on back wall memory housings. Choke L1 is wound in one layer with PEV-2 0.4 wire on a cylindrical ferrite magnetic core with a diameter of 5 mm and a length of 20 mm (from the computer's SMPS choke). You can use optocouplers of the PC817 series and similar ones. The board with the parts (Fig. 5) is inserted into the free space of the charger (partially above the capacitor C1), connections are made with pieces of insulated wire. For the tuning resistor, a hole of appropriate dimensions is made in the rear wall of the charger, into which it is glued. After checking the device, resistor R12 is equipped with a scale (Fig. 6).
Rice. 4. Printed circuit board and elements on it
Rice. 5. Board with parts
Rice. 6. Scale on memory
The second option for modifying the charger is to introduce a current stabilizer (or limiter) into it. This will allow you to charge Li-Ion or Ni-Cd, Ni-MH batteries and batteries containing up to four batteries. A diagram of such a modification is shown in Fig. 7. Using the switch, you can select operating modes: power supply or one of two “storage” modes with current limitation. The 220 µF capacitor (C5) has been replaced by a capacitor with a capacity of 470 µF, but with a higher voltage, since in the “memory” modes without load the output voltage can increase to 6...8 V.
Rice. 7. Scheme of the second option for finalizing the memory
In "BP" mode, the device operates normally. When switching to one of the “memory” modes, the output current flows through resistor R10 (or R11). When the voltage across it reaches 1 V, part of the current will begin to branch into the emitting diode of the optocoupler U1, which will lead to the opening of the phototransistor. This will lead to a decrease in the output voltage and stabilization (limitation) of the output current I out. Its value can be determined using approximate formulas: Iout = 1/R10 or Iout = 1/R11. By selecting these resistors the desired current value is set. Field-effect transistor VT1 limits the current through the emitting diode of the optocoupler and thereby protects it from failure.
Most of the parts are placed on a single-sided printed circuit board (Fig. 8 and Fig. 9) made of foil fiberglass laminate with a thickness of 0.5...1 mm. The field-effect transistor must have an initial drain current of at least 25 mA. The switch is any small-sized slider for one or two directions and three positions, for example SK23D29G, it is placed on the rear wall of the charger and is equipped with a scale. If you apply a switch to a larger number of positions, you can increase the number of rated current values and thereby expand the range of chargeable batteries.
Rice. 8. Printed board and elements on it
Since charging is carried out with a stable current, it should be carried out for a certain time, which depends on the type and capacity of the battery or battery being charged.
Publication date: 11.12.2017
Readers' opinions
- Alius / 07/22/2019 - 07:06
1. Is it possible to increase the output voltage to 12-15 volts with a simple modification (installing a zener diode at 12-15 V, or TL431...)? 2. The zener diode must be removed from the circuit (Fig. 1, Fig. 7) during the modification described...? (it’s just not clear on the diagram...) 3. Thank you for your answer in advance; and the author! - Anatoly / 12/23/2017 - 19:22
very useful information given detailed description the ongoing refinement, understandable to any "teapot". Thank you.
Hello to all visitors to this site! Probably everyone at home had or still has a phone charger without a USB connector on it. Such chargers are still connected without using any extraneous cables, but have their own wire and connect directly to the phone through its own connector. You don’t see these often anymore, since they already use power supplies for phones with built-in usb port ohm I also have such a power supply from an old push-button telephone that no longer works. And I decided to convert this power supply into a power supply with a USB connector. This alteration does not take much time and is not difficult in the process. To convert the power supply to a power supply with a USB port, I needed:
Tools:
1) Sharp stationery knife,
2) Hammer,
3) Scissors,
4) Electric soldering iron,
5) Glue gun and hot glue,
6) Lighter,
7) A simple pencil.
Materials:
1) The power supply itself is 5 volts from the phone,
2) USB connector,
3) Heat shrink tubes,
4) Wires.
The process of converting a regular charger into a usb charger.
We take our power supply from the phone and use scissors or a utility knife to cut off the cable from it.
Now you need to open the power supply housing. There weren't any screws on mine, so I had to open the case in other ways. To do this, take a hammer and hit the glued seams of the power supply with light, not strong blows. There is no need to hit it hard, because the power supply may crack. But after this method, small dents may remain on the charger. Therefore, I recommend doing this process either with a rubber mallet or a wooden mallet.
Then we use an electric soldering iron to unsolder the remaining wires from the cable from the board.
Next, take a USB connector and two short wires (from the power supply cable itself).
Using an electric soldering iron, solder the wiring to the power supply board.
We solder the USB port to these wires soldered to the block, while maintaining the polarity, that is, plus and minus. To insulate the cable, we already put heat-shrinkable tubes on it in advance.
After soldering the connector to the wires, we put heat shrink on the bare spots and use a lighter to open them with a hot flame.
Now, using a simple pencil, we draw marks on the body of the power supply for the future hole for the USB connector.
Using a utility knife, we cut a recess along the lines on the body of the power supply for the USB connector.
Using a glue gun and hot glue, glue the USB port to the charger body.
We also glue the board to the case so that it holds more tightly.
Power source - from a cell phone charger
I. NECHAYEV, Kursk
Small-sized portable equipment (radios, cassette and disc players) are usually powered by two to four galvanic cells. However, they do not last long, and they have to be replaced quite often with new ones, so at home it is advisable to power such equipment from a power supply. Such a source (in common parlance it is called an adapter) is not difficult to purchase or make yourself; fortunately, there are many of them described in amateur radio literature. But you can do it differently. Almost three out of every four residents of our country today have a cell phone (according to the research company AC&M-Consulting, at the end of October 2005 the number of subscribers cellular communication in the Russian Federation exceeded 115 million). Its charger is used for its intended purpose (for charging battery phone) only a few hours a week, and the rest of the time is idle. The article describes how to adapt it to power small-sized equipment.
In order not to spend money on galvanic cells, owners of wearable radios, players, etc. equipment use batteries, and in stationary conditions they power these devices from the network alternating current. If you don’t have a ready-made power supply with the required output voltage, you don’t have to buy or assemble such a unit yourself; you can use a cell phone charger for this purpose, which many people have today.
However, you cannot directly connect it to a radio or player. The fact is that most chargers included with a cell phone are an unstabilized rectifier, the output voltage of which (4.5...7 V at a load current of 0.1...O.ZA) exceeds that required to power a small-sized apparatus. The problem can be easily solved. To use the charger as a power supply, you must connect a voltage stabilizer adapter between it and the device.
As the name itself says, the basis of such a device should be a voltage stabilizer. It is most convenient to assemble it on a specialized microcircuit. The large range and availability of integrated stabilizers allow us to produce a wide variety of adapter options.
The schematic diagram of the adapter-voltage stabilizer is shown in Fig. 1. DA1 chip is selected 
depending on the required output voltage and current consumed by the load. The capacitance of capacitors C1 and C2 can be in the range of 0.1...10 µF (rated voltage - 10 V).
If the load consumes up to 400 mA and the charger can supply such current, the KR142EN5A (output voltage - 5 V), KR1158ENZV, KR1158ENZG (3.3 V), KR1158EN5V, KR1158EN5G (5 V) microcircuits can be used as DA1, as well as five-volt imported 7805, 78M05. Microcircuits of the LD1117xxx, REG 1117-xx series are also suitable. Their output current is up to 800 mA, the output voltage is from the range 2.85; 3.3 and 5 V (for LD1117xxx - also 1.2; 1.8 and 2.5 V). The seventh element (letter) in the designation LD1117xxx indicates the type of housing (S - SOT-223, D - S0-8, V - TO-220), and the two-digit number following it indicates the nominal value of the output voltage in tenths of volts (12 - 1.2 V, 18 - 1.8 V, etc.). The number attached through a hyphen in the designation of REG1117-xx microcircuits also indicates the stabilization voltage. The pinout of these microcircuits in the SOT-223 package is shown in Fig. 2, a.
It is also acceptable to use stabilizer microcircuits with adjustable output voltage, for example, KR142EN12A, LM317T. In this case, you can get any output voltage value from 1.2 to 5...6 V.
When powering equipment that consumes a small current (30...100 mA), for example, small-sized VHF FM radios, the adapter can use the KR1157EN5A, KR1157EN5B, KR1157EN501A, KR1157EN501B, KR1157EN502A, KR1157EN502B, KR11 microcircuits 58EN5A, KR1158EN5B (all with rated output voltage 5 V ), KR1158ENZA, KR1158ENZB (3.3 V). Drawing of a possible version of the adapter printed circuit board using
using microcircuits last episode shown in Fig. 3. Capacitors C1 and C2 - small-sized oxide capacitors of any type with a capacity of 10 μF.
The dimensions of the adapter can be significantly reduced by using miniature microcircuits of the LM3480-xx series (the last two digits indicate the output voltage). They are produced in the SOT-23 package (see Fig. 2.6). The printed circuit board drawing for this case is shown in Fig. 4. Capacitors C1 and C2 - small-sized ceramic K10-17 or similar imported ones with a capacity of at least 0.1 μF. Appearance adapters mounted on boards manufactured in accordance with Fig. 3 and 4, shown in Fig. 5. 
It should be noted that the foil on the board can serve as a heat sink. Therefore, it is advisable to make the area of the conductor for the microcircuit terminal (common or output), through which heat is removed, as large as possible.
The assembled device is placed in a plastic box of suitable dimensions or in the battery compartment of the powered device. To connect to the charger, the adapter must be equipped with an appropriate socket (similar to the one installed in a cell phone). It can be placed on a printed circuit board with a stabilizer or mounted on one of the walls of the box.
The adapter does not require any installation; you just need to check its operation with the connecting wires that will be used to connect to the charger and the powered device. Self-excitation is eliminated by increasing the capacitance of capacitors C1 and C2.
LITERATURE
1. Biryukov S. Microcircuit voltage stabilizers for wide application. - Radio, 1999, No. 2, p. 69-71.
2. LD1117 Series. Low Drop Fixed and Adjustable Positive Voltage Regulators. -
3. REG1117, REG1117A. 800mA and 1A Low Dropout (LDO) Positive Regulator 1.8V, 2.5V, 2.85V, 3.3V, 5V and Adjustable. -
4. LM3480. 100 mA, SOT-23, Quasi Low-Dropout Linear Voltage Regulator. -
Over the course of several years, my house has accumulated a large number of chargers from cell phones, which are no longer used for their intended purpose due to connectors that are unsuitable for new models of smartphones.
There are five power supplies from Nokia alone. It was decided to take advantage of them - make a couple of spare chargers.
Some of these units have an output voltage of 5 Volts, which is suitable for modern digital equipment with low charging current. But I didn’t have the task of making a super charger.
Also, among the unnecessary “things from the past,” I found a couple of mouse adapters - from ps/2 to usb, plus a connector for micro usb- that’s all the components for my homemade product.
Charging assembly
The adapter body can be easily disassembled. Having removed everything unnecessary, we leave only the plug itself.
Only the outermost pair of contacts will be used. We make a couple of small holes for plastic clamps, which will later be used to tighten the case and secure the cables.
With the charger everything is simple: cut off the old plug. I had a couple available faulty cables(the cat bit me), but with a whole micro-connector, and there were new collapsible connectors.
Having soldered the cables according to the diagram, we fix them with a clamp.
We close the housing and also secure it with a tie. The adapter body serves as both a distribution box and a functioning usb.
UPGRADE CHARGERS
Many people have cheap Chinese chargers for AA batteries. At one time, I, tempted by the low price (about 3 euros), purchased such a device. After working for about an hour, the charger began to melt and smoke. The cause turned out to be a power transformer the size of Matchbox. Naturally, it turned out to be impossible to continue using this charger - but it’s also a pity to throw it away.
Let's try to open and change the charger to a better one. Little inside free space, and installing a larger transformer is not possible - and it is not necessary! We will install a board from the charger to the mobile phone.
I'm sure everyone has these unused chargers lying around. A charger for absolutely any phone model is suitable. We insert the IP board into the case, and it fits most cases perfectly in size,
And we connect the low-voltage supply output of 5 Volts, 0.3 Amperes to the contacts of the battery holder through resistors and diodes that are already installed there. To obtain different charging currents, you can select the value of these resistors by monitoring the current with an ammeter.
Another weak point is the low-quality power plug on the case, which is replaced by a wire with a plug. As a result, we have a compact, powerful, and most importantly galvanically isolated charger from the mains. This charger has been successfully used for 5 years.