Why won't a transformer work on a DC supply?
A transformer works by mutual induction: the changing current in the primary makes a changing magnetic flux that induces a voltage in the secondary.
A transformer works by mutual induction: the changing current in the primary makes a changing magnetic flux that induces a voltage in the secondary. DC gives a constant current and therefore a constant, unchanging flux, so dΦ/dt = 0 and no voltage is induced in the secondary. Only a changing (AC) current keeps the flux changing, which is why transformers need AC.
Key point
A transformer needs a changing flux; steady DC gives dΦ/dt = 0, so no induced emf.
Common mistake
assuming a transformer steps DC voltage the way it does AC.
Memory tip
no change, no induction — transformers are AC-only.
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