If you have ever worked with mobile phone adapter you may have noticed a big resistor usually in the range of 5ohm to 15ohm located at the input ac side. Many times this is the resistor which will get damaged due to overheating. One can check if there is any continuity across the resistor with a digital meter. Picture below shows the mobile phone adapter circuit board inside the casing.
This resistor is called Inrush Current Limiting Resistor and its purpose and function is explained below.
When you first plug in the adapter, large capacitors on the DC side (after rectification) charge rapidly, drawing a huge surge current.
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This 10Ω resistor limits that inrush current to protect components like:
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The bridge rectifier
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The filter capacitors
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The switching MOSFETs or ICs
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🧱 It’s usually a:
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Wirewound resistor (cylindrical, ceramic)
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Or a cement resistor (rectangular white block)
🔥 Why It’s Big:
Because it may dissipate a lot of power during the surge, it's designed to:
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Handle high voltage
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Withstand high current briefly
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Not catch fire if something goes wrong
⚠️ Important Tip:
In many designs, this resistor is sometimes bypassed after startup using a relay or NTC thermistor, so that efficiency isn’t lost during normal operation. If it's always in the circuit, it must be robust. These methods are covered in how to control Inrush Current in Switchmode Power Supplies (SMPS).
See the next blog post Inrush current limiting resistor in Mobile Phone Adapter to learn how about this resistor and how incorrect use could damage your adapter circuit. Also know that this resistor is also called fuse-able resistor as it acts as a fuse to prevent damage to the rest of the circuit from overloading.
