Repair charger for MPEG4-player. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Repair charger for MPEG4-player. Encyclopedia of radio electronics and electrical engineering

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

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After two months of operation, the "unnamed" charger for a pocket MPEG4/MP3/WMA player failed. Of course, there was no diagram of it, so I had to draw it up according to the circuit board. The numbering of active elements on it (Fig. 1) is conditional, the rest correspond to the inscriptions on the printed circuit board.

MPEG4 charger repair

The voltage converter unit is implemented on a low-power high-voltage transistor VT1 of the MJE13001 type, the output voltage stabilization unit is made on a VT2 transistor and an optocoupler VU1. In addition, the transistor VT2 protects VT1 from overload. Transistor VT3 is designed to indicate the end of battery charging.

When examining the product, it turned out that the transistor VT1 "went to a break", and VT2 was broken. Resistor R1 also burned out. Troubleshooting took less than 15 minutes. But with proper repair of any radio-electronic product, usually it is not enough just to troubleshoot, you still need to find out the causes of their occurrence so that this does not happen again. As it turned out, during the operation of the charger, even with the load turned off and the case open, the VT1 transistor, made in the TO-92 case, heated up to a temperature of about 90 ° C. Since there were no more powerful transistors nearby to replace the MJE13001, I decided to glue a small heat sink to it.

A photograph of the charger is shown in fig. 2.

MPEG4 charger repair

A duralumin heatsink measuring 37x15x1 mm is glued to the transistor body with Radial tell-conductive glue. The same glue can be used to glue the radiator to the circuit board. With the heat sink, the temperature of the transistor case decreased to 45...50°C. The reason for the initially strong heating of the VT1 transistor may lie in the "simplification" when assembling its damper circuit. The drawing and topology of the printed circuit board give reason to believe that instead of the resistor R10 with a resistance of 100 kOhm, two capacitors and a diode should be in the collector circuit of the transistor VT1.

This charger at idle consumes about 220 mA from the 3.5 V mains. and at a load current of 200 mA - about 18 mA.

After simple calculations, it can be seen that its efficiency is about 25%. A properly designed low-power linear power supply with a step-down transformer is able to provide at least 50% efficiency, so if you are tired of low-power switching power supply failures. designed to power portable equipment, it may be advisable to replace the switching power supply with a traditional one, which, losing to the switching power supply in terms of weight and dimensions, will turn out to be much more reliable.

Author: A. Butov, village of Kurba, Yaroslavl region.

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