# How to work out the turn ratio on a transformer

• speedy46
In summary, the turns ratio on a transformer can be calculated by dividing the primary voltage by the secondary voltage. In real transformers, the turns ratio may be slightly higher than this calculated value. To improve accuracy, a primary voltage that is only 10% of the rated voltage can be used. There are two types of transformers, step-up and step-down, with different turns ratios depending on the type. The output voltage of a transformer can be calculated using the turns ratio and input voltage. However, if the turns ratio is unknown, it must be measured using the primary and secondary voltages.

#### speedy46

I would like to know how to work out the turn ratio on a transformer from having just the input and output voltages.

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For an ideal transformer (that is, no losses in the windings or in the core), the turns ratio is just N = Vp/Vs where
N = turns ratio
Vp = primary Voltage (rms)
Vs = secondary Voltage (rms)

In real transformers, the turns ratio will be slightly higher (typically about 2%) than what is calculated by this formula. One way to improve the accuracy of the calculation is to use a primary Voltage that is only about 10% of the rated Voltage.

For example = 244 / 9.88 = 24.6

So the turn ratio is 24 - 6

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There are two types of transformers,step-up transformer(sut) and step-down transformer(sdt),the turns ratio for (sut)is bigger than 1 and for (sdt) is less than 1.

speedy46 said:
For example = 244 / 9.88 = 24.6

So the turn ratio is 24 - 6

what? isn't it 246 to 10? or 24.6 to 1?

ws0619 said:
There are two types of transformers,step-up transformer(sut) and step-down transformer(sdt),the turns ratio for (sut)is bigger than 1 and for (sdt) is less than 1.

There are quite a few more types of transformers than just step-up and step-down (e.g. autotransformers, buck-boost, isolation).

CS

Thank you

How do you calculate the out put voltage of a transformer when you only know the input voltage

speedy46 said:
Thank you

How do you calculate the out put voltage of a transformer when you only know the input voltage

its based on the ratio as stated

Is there a formula for working this out please

The formula is in post #2. But, you can only solve for one unknown when you have only one equation. If you don't know the turns ratio, you must measure the primary and secondary Voltages.

thank you