Why do voltage fluctuations damage only adapters but not devices?

Same thing happened to me.

If the AC adapter were just a transformer, it would have damaged your equipment. But, the AC adapter contains circuitry that converts AC to DC electronically (usually between 5-20 V DC) as these equipments operate on DC power. When there was a voltage surge, the circuit that converts AC to DC was "burnt" (It became so hot that the circuit broke and current could no longer flow) before there was a chance for the circuit to convert that voltage to DC. Moreover, there are voltage limiters on the DC side in such adapters so the voltage cannot go beyond a certain value.

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