I started my journey to learning how to make power supply from the mobile adapter circuit. While learning what components are used has led me to PC817C optocoupler IC found inside these mobile phone voltage regulator circuit. The PC817C is an opto-isolator or optocoupler, and it's commonly used in mobile phone adapters (chargers) for electrical isolation and feedback control.
📌 What is a PC817C?
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It’s a 4-pin optocoupler that contains:
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An infrared LED on the input side.
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A phototransistor on the output side.
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When the input LED turns on, it activates the transistor through light (not a direct electrical connection).
🛠Why is it used in mobile phone adapters?
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Isolation between high-voltage AC side and low-voltage DC side:
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Protects the user and the phone from dangerous high voltages.
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Ensures that noise or surges on the AC side don’t pass into the DC output.
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Feedback in SMPS (Switched-Mode Power Supply):
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Mobile chargers use SMPS circuits.
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PC817C sends feedback from the output side (low-voltage) to the primary side controller to regulate the output voltage.
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Often works with an error amplifier and a reference voltage (like TL431) to maintain stable 5V output.
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🔄 Typical Usage Circuit:
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PC817C + TL431 are often seen together.
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When output voltage changes, TL431 adjusts the current through the PC817's LED.
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This in turn changes the feedback signal to the controller IC on the primary side, adjusting switching duty cycle.
Summary:
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✅ Optical Isolation: Keeps the high-voltage and low-voltage sides safe and separate.
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✅ Voltage Regulation: Helps the SMPS controller keep the output voltage stable.
One thing to note is that the PC817C optocoupler itself does not protect against inrush current — that's not its job. For inrush current protection a resistor or NTC thermistor are commonly used. For this, see the previous guide on Inrush current limiting resistor in Mobile Phone Adaptor.
Let me know if you want to see a real circuit diagram or simulation example!
