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Question
A transformer is designed to convert an AC voltage of 220 V to an AC voltage of 12 V. If the input terminals are connected to a DC voltage of 220 V, the transformer usually burns. Explain.
Solution
A transformer is ideally an inductive coil. For an inductor connected across a DC voltage,
`V - L (di)/(dt) = 0`
`⇒ V = L(di)/(dt)`
⇒ \[\int\limits\] `di = V/L` \[\int\limits\] dt
`⇒ i = (Vt)/L`
For a DC source, the current across the inductor will increase with time and can reach a very large value, which can burn the transformer.
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