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Since the voltage in a coil is proportional to the number of turns, the ratio of voltages is equal to the ratio of turns.

So if you need a big increase in voltage, what stops you from having one turn on the primary (instead of a bunch) and then many more on the secondary. Or is this actually a thing?

We wouldn't need as big transformers if so, assuming the current reduction is irrelevant. All I can think of is some kind of power loss or current not being induced.

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Imagine that the secondary isn't connected to anything and isn't pulling any power.

What keeps the primary current from being infinite? It's just a wire, connected across a voltage source, right?

The answer is the inductance of the primary: It impedes the AC current from the AC voltage source. Even when the current in the secondary is zero, there's an inductive current in the primary. You want to keep this small, so you want the inductance of the primary to be large (or at least as large as economically possible) so you use multiple turns in the primary to get the inductance up.

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  • $\begingroup$ I would also add that depending on the transformer geometry, having more turns on coils can help reduce leakage flux and thus increase the coupling between the two coils. The coupling affects the secondary to primary voltage ratio. $\endgroup$ – Puk Jul 17 '19 at 18:51
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The transformer with too small inductance can not work normally, the primary is basically equal to shortcut circuit if the number of turns is too small.

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