How to calculate the resistance of the transformer


Excursus: low and high currents¶

The following applies to the electrical power converted by a consumer ; A certain electrical power is thus conceivable both as a product of a high voltage value with a low current intensity or, conversely, as a product of a high current intensity with a low voltage. In the first case, the electrical resistance of the consumer would have to be high, in the second, low, as the following example shows:


  • A consumer with an electrical power of should be built in such a way that it can achieve this output at a voltage of respectively should deliver. What resistance values respectively must the consumer have in these two cases?

    In the first case must be to achieve the achievement the following currents occur:

    According to Ohm's law, this results in the following resistance:

    In the second case applies to the current strength :

    This results in the resistance :

At first, both variants appear to be equivalent. However, there is a clear difference if the (low) electrical resistance of the cables is taken into account. Together with the actual consumer, these represent a series connection of resistors; the resistance values ​​of the line and the consumer must therefore be added.


As the above example shows, electricity is transported much more effectively at high voltages, taking into account the (low) electrical resistance of real lines, since heat losses are minimized here; In addition, when high voltages are used, the actual value of the line resistances, which can vary somewhat depending on the length of the connection cables and the quality of the conductive connections, hardly plays a role.

The relatively high voltages applied to the lines can then be adjusted to the desired value again within the respective electronic devices by means of transformers.