Charger AMT TRAVEL ADAPTER for cell phone. Encyclopedia of radio electronics and electrical engineering

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

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AMT TRAVEL ADAPTER chargers operate in a wide range of mains voltages of 100...240 V, provide a stabilized output voltage of 4,8...8,5 V with a maximum load current of 0,6 A. They are assembled according to the circuit shown in the figure, simple in design and interesting in principle of operation. The mains voltage rectified by the VD1-VD4 diode bridge and smoothed by the capacitor C1 feeds a single-cycle flyback voltage converter, the basis of which is a self-oscillator based on a VT1 transistor and a T1 transformer with three windings: I - network, II - feedback, III - output. Windings II and III are the same, the network winding I contains 10 times more turns.

When the power is turned on in the base circuit of the transistor VT1, a current Ib1 flows through the resistor R1, approximately equal to 300 V / R1 \u0,4d 30 mA. This current, amplified by the transistor, begins to flow through the I winding of the transformer. Under the action of strong positive feedback, the voltage of the winding II (about 1 V) is applied to the transistor VT2 through the circuit C3R2R5R1 in the opening polarity. A regenerative process develops, the transistor VT6 enters saturation, and as a result, the full rectified mains voltage is applied to the winding I. The current of this winding increases, the magnetic circuit of the transformer accumulates energy. At this time, the diodes VD8-VD30 are closed by reverse voltage. The voltage on windings II and III reaches 2 V. Capacitor C2 quickly charges, after which current Ib3 flows into the base of the transistor through resistor R30 (10 V / 3 kOhm = 1 mA), which is added to Ib1. The total base current Ib \u2d Ib3,4 + IbXNUMX is approximately equal to XNUMX mA.

The collector current of the saturated transistor increases to h21e*Ib. where h21e is the base current transfer coefficient, after which the transistor comes out of saturation, the voltage on the transformer windings decreases. The reverse regenerative process develops, the transistor closes, the voltage on the windings changes polarity. Diodes VD6-VD8 open. The damping circuit VD7C3R4 reduces the self-induction EMF surge of the transformer T1 and, accordingly, limits the voltage at the collector of the transistor VT1. Through the Schottky diode VD8, the capacitor C5 is charged, to which the load is connected. LED HL1 - output voltage indicator, resistor R7 sets the current through it. Through the diode VD6, the capacitor C4 is charged to the amplitude of the reverse voltage, which is greater than the stabilization voltage of the zener diode VD5, as a result of which it opens, the voltage at the base of the transistor VT1 becomes negative and prevents it from opening. This is the pause time between pulses, it is much longer than the duration of the pulse. The current I61 through the resistor R1 does not flow into the base of the transistor VT1, but into the open zener diode VD5 and the capacitor C4, discharging it. The voltage on this capacitor decreases, on the basis of the transistor VT1 increases. When it reaches about 0,7 V, the pause will end, the transistor will open and a new generation cycle will begin. By selecting the values ​​​​of the elements R3, C4 and VD5, you can adjust the generation frequency and the output voltage of the device. Zener diode VD5 - low-power with a stabilization voltage of 3,3 ... 8,2 V. LED HL1 - any low-power.

The considered chargers are characterized by very low reliability. The switching transistor VT1 from the Q13001 series often fails, causing damage to the elements associated with it. During the repair process, it was replaced by the KT940A transistor, which was chosen because it is widespread. It should be noted that it works without a voltage margin, since its maximum allowable collector-emitter voltage is 300 V. Despite this, the repaired devices did not fail.

Author: V.Zorin, Yurga, Kemerovo Region; Publication: radioradar.net

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