How does a Mobile Charger Circuit Actually Work?

Every day, we use our smartphones. However, have you ever wondered how the mobile charger on your smartphone actually functions? You are aware that a charger changes AC power into DC power, but the process is more complicated than that. It converts AC to DC first, then back to AC, and then back to DC again. We’ll examine how the 5V mobile charger circuit accomplishes this today, along with the rationale behind the intermediate steps.


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This circuit is a typical charger that changes 220V AC to 5V DC. Let’s investigate inside. All of the electronic parts that were employed in it are now visible. Optocouplers, resistors, transformers, diodes, capacitors, and transistors are all present. Resistors are also located beneath the PCB. It activates when the electricity is turned on. To gain a better understanding, let us reorganise the circuit.

Circuit Schematic

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Ferrite Transformer
PC817C Optocoupler
1N4007 PN Bridge Rectifier Diode (x4)
1N5819 Schottky Diode
Resistor (2MΩ, 560Ω, 1KΩ, 10Ω, 120Ω, 100Ω)
2.6Ω/1W Fuse Resistor
S8050 NPN Transistor
13001 NPN Transistor
2.2uF/450V Polyester Film Capacitor
4.7nF/100V Polyester Film Capacitor
470uF/25V Electrolyte Capacitor
22uF/25V Electrolyte Capacitor
100nF Ceramic Capacitor
4.2V Zener Diode

Circuit Connection

All of the parts and connections are now visible. The black wire is neutral, and the red wire is the phase wire. We start with a resistor. It is 2.6Ω, as shown by the reference table and colour bands. This fusible resistor guards against overload damage. Next, there is a 2.2uF/450V filter capacitor and a bridge rectifier composed of four 1N4007 PN junction diodes.


This circuit is an oscillator. DC is subsequently converted back to high-frequency AC at 15–50 KHz. The component values are visible to us. The pin arrangements for these two NPN transistors, S8050 and 13001, are shown below.


There’s a little diode after that part. Although it has the appearance of a Zener diode, this 1N4148 fast switching diode features a 22uF/63V capacitor.


This creates the circuit for the phototransistor in the optocoupler; it is an AC-to-DC converter. We use it to transmit signals without physical contact. An infrared led is located on the right, while a phototransistor is located on the left. The phototransistor’s base becomes illuminated when the LED turns on, activating it. This 100nF capacitor is used for precautionary measures. Its purpose is to prevent electromagnetic interference by connecting primary and secondary grounds.

This is the transformer; the core is surrounded by three windings: primary, secondary, and auxiliary. Here, it is employed to reduce the voltage. The oscillator circuit is powered by the auxiliary winding.

Next, we have an LED for indication and a Schottky diode 1N5819 to convert AC to DC with a capacitor of 470uF/16V. A 4.2V Zener diode and an optocoupler PC817C make up the feedback circuit as well.

Working Principle of Mobile Charger Circuit

Let’s activate it so we can watch it in action. Positive voltage is carried by the red wires, and negative voltage, or ground, is carried by the black wires.


The input voltage is 220V AC at 50Hz. This rectifier, a bridge rectifier, transforms variable AC into variable DC. As we can see, the varying DC almost completely purges the capacitor and becomes pure DC. As we can see, the circuit contains DC. In order to turn on the Q1 transistor, this current now flows from the 2M ohm resistor to its base. This transistor partially turns on due to resistance, not turning on completely. A modest current flowed from the transformer’s primary winding as a result of the transistor’s partial on. The auxiliary winding experiences a low voltage as a result.

The capacitor is now fully charged by the induced voltage, which activates the transistor. Now that it is fully on, the transistor permits current to pass through it. This now shunts the base of the Q1 transistor, shutting it off and turning on the Q2 transistor. The current flowing to the Q2 is interrupted when the Q1 shuts off. The cycle now repeats as the current moves to the Q1’s base.

At 15 to 50 kHz, which is a thousand times quicker than the rectifier circuit, this scenario takes place. As a result, the rectifier circuit would appear to be stopped. Simultaneously, the auxiliary voltage charges the capacitor, activates the diode, and supplies power to the optocoupler. The auxiliary coil’s AC signal is converted to DC for the optocoupler by this diode and capacitor.

The secondary winding experiences an induced current as well. A filter capacitor and a Schottky diode convert this to DC. The LED indicates this. Thus, what happens if the voltage exceeds 5 volts? Thus, a feedback circuit is present. The Zener diode activates at 4.2V, allowing current to reach the optocoupler. Additionally, it reduces the voltage by 4.2V, which prevents the optocoupler’s transmitter LED from turning on. For the Transmitter LED to illuminate, 0.8V is needed. The optocoupler’s LED turns on when the voltage rises to greater than 5V.

The phototransistor of the optocoupler is activated by the infrared light from the transmitter LED, allowing current to flow to the transistor T2. This activates transistor T2, shunting the first and cutting off the primary winding’s current flow. Additionally, the optocoupler and Zener diode are turned off when the secondary side of the transformer’s voltage falls below 5V. To function normally, the circuit must be continuous.

Why We Should Not Directly Convert AC to DC Than This?

The typical power source, which runs at 50 or 60 hertz, is to blame for this. Both the transformer and the capacitors are substantial in size. This type of tiny charger is not designed to accommodate them. Therefore, the 50 or 60 Hz frequency is transformed to 50 kilohertz in the 5V mobile charger. As a result, the circuit requires a smaller transformer and capacitor. Therefore, in order to change the frequency of AC, we must first convert it to DC and then back to AC.

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