Repair and modification of NOKIA cell phone charger. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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With the increase in the fleet of mobile phones, the number of chargers that come with the phones is proportionally growing. Given the low quality of our electrical networks, these devices often fail. This is especially true for models of chargers from unknown manufacturers, purchased on radio markets due to their low cost.

As a rule, in order to maintain profitability, such manufacturers use cheaper components in their devices, which inevitably entails a decrease in their reliability.

After a similar charger for a NOKIA phone bought on the radio market failed after working for a week, it was decided to find out the cause of the malfunction and make the necessary changes to the circuit to improve the reliability of the device as a whole.

It should be noted that when comparing two chargers - certified and "gray" it is not easy to find the difference (Fig. 1). The body of the device from an unknown manufacturer (on the top in Fig. 1) is characterized by a less deep embossing of the NOKIA logo and technical characteristics of the device, as well as the absence of a silk-screened icon regulating the method of disposal of the device at the end of its service life. On fig. 2 shows the circuit board of the device.

Fig. 1

Fig. 2

The circuit diagram of the device was restored from the circuit board. It is a classic pulse flyback converter (Fig. 3).

Rice. 3 (click to enlarge)

Such simple circuits are widely used in switching power supplies and chargers (up to 25 W).

The declared characteristics of the device are an output voltage of 5,7 V and a load current of 800 mA.

Now let's briefly consider the principle of operation of the power supply in the circuit diagram (Fig. 3).

The mains voltage is supplied through the current-limiting resistor R1 to the input of the rectifier on diodes D1-D4. Transistor Q1 has a self-oscillator, the frequency of which is mainly determined by the characteristics of the pulse transformer TF1 used here. Resistor R3 sets the operation mode of transistor Q1. The output voltage is stabilized by using the feedback winding of the pulse transformer TF1 and the D7 C4 ZD1 circuit. Transistor Q2 and resistor R2 serve to limit the current of transistor Q1 at the time of starting the oscillator, as well as in the event of an overload or short circuit at the output of the device. The circuit contains a half-wave output voltage rectifier based on diode D8 and capacitor C5. Resistor R6 is used to discharge the capacitor C5 after the device is turned off.

As a result of checking the charger described above, a faulty transistor Q1 marked 1003 and a burnt resistor R3 were found. The burnt coating of the resistor made it impossible to determine its resistance. In order to increase the reliability of the circuit, a more powerful and widespread domestic transistor KT 1A was used as transistor Q940 (Fig. 4). It should be noted that due to the large spread in the characteristics of the KT 940A transistors, in some cases it may be necessary to change the resistance value R3 indicated in the diagram.

Fig. 4

It should be noted that on the board, in the place provided for this, there is no oxide capacitor C, which must be connected at the output of the diode rectifier D1-D4. In this case, the oscillator of the device actually operates in the modulation mode with the rectified mains voltage. For this reason, in many cases, such devices may not provide the advertised output current needed to charge a mobile phone battery. The consequence of this can be, for example, an increase in the total charging time.

If necessary, you can install this missing capacitor - its capacitance can be no more than 10 microfarads for an operating voltage of at least 450 V. It is recommended immediately with the installation of the capacitor to solder a resistor with a resistance of about 300 kOhm parallel to its legs on the mounting side (to discharge this capacitor after disconnecting the device from networks). In addition, for reliability, it is desirable to use a resistor R1 with a higher power dissipation, as it limits the charging current of the above capacitor at the moment the device is connected to the network. The board has a place for an LED to indicate the operation of the device and, if necessary, it can be installed on the board through a current-limiting resistor with a resistance of 680 Ohm.

After the repair, this charger has been working reliably for more than a year without comment. Given that the converter circuit used is widely used in many chargers, the described repair and reliability improvement method can be recommended for other similar devices.

Author: Sergey Dyakevich

See other articles Section Power Supplies.

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