The transformer equation:

The secondary p.d. of a transformer depends on the primary p.d. and the number of turns on each coil. We can use the following equation to calculate any one of these factors if we know the other ones.

**p.d. across primary, Vp/p.d. across secondary, Vs = number of turns on primary, Np/number of turns on secondary, Ns**

**- For a step up transformer,** the number of secondary turns (Ns) is greater than the number of primary turns (Np). Therefore the Vs is greater than the Vp.

- **For a step down transformer,** the number of secondary turns (Ns) is less than the number of primary turns (Np). Therefore the Vs is less than Vp.

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