I've already written about some of the interesting components I found inside mobile phone adapters, including the inrush current limiting resistor — often a fusible resistor — and the PC817C optocoupler. In this entry, I'm focusing on another commonly used part: the S8050 NPN BJT transistor. It's frequently found in charger circuits, and I wanted to explore its role and why it's used.
So first what is S8050 Transistor?
The S8050 is a general-purpose NPN bipolar junction transistor (BJT), and it is commonly found in mobile phone adapters (chargers) and other small electronic devices.
📌 S8050 Key Features:
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Type: NPN transistor
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Max Collector Current (Ic): 700 mA
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Max Collector-Emitter Voltage (Vceo): 25V
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Gain (hFE): ~100–300
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Package: TO-92 (through-hole) or SOT-23 (SMD version, like S8050S)
🛠 What is the S8050 used for in mobile adapters?
1. Switching Small Loads
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Turns on/off small parts of the circuit, like an optocoupler (e.g., PC817), indicator LED, or control signal line.
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Used to drive relays or control signals to other ICs.
2. Voltage Regulation Feedback
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Often works with 13001 BJT transistor and PC817 optocoupler in the feedback loop of a Switched Mode Power Supply (SMPS) to stabilize output voltage.
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S8050 may amplify error signals or buffer signals between 13001 transistor and PC817 optocoupler.
3. Startup or Soft-Start Circuit
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Sometimes used in a startup circuit to gradually power up the SMPS controller or delay turning it on to avoid inrush current issues.
4. Amplification (rare in adapters)
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Could be part of a basic amplifier, but in chargers, it’s usually for switching.
🔍 Real Example in Adapters:
After little bit of research, I learned that the S8050 transistor is a BJT transistor, often used in combination with a high-voltage 13001 BJT transistor. Together, they form an oscillator circuit that converts the rectified DC from the mains into a high-frequency AC signal, usually in the range of 15kHz to 50kHz. Such power supplies are called switch mode power supply(SMPS). This high-frequency signal is then fed into a small transformer to step down and isolate the output voltage. It was interesting to see how such a simple and low-cost arrangement can handle efficient power conversion in these tiny adapters.
