What needs to be taken into account to determine the required current flow or power (VI) in the primary winding of a transformer.

Say you just bought a transformer from a hobby store and you need to determine if the primary winding needed a load resistor, or what the current requirement is?

What steps and or calculations do you have to do?

 

Transformers don't need resistors.
Generally they consume 10 watts per pound just idling at load.

 

Ideally the primary and secondary will dissipate the same power, so you can figure how much power you will be using from the secondary and do the conversion based on the ratio of the transformer.

 

you can use the approximate equivalent circuit of the transformer to calculate the Primary current.


Alt.:-

The rated current, and VI rating must be mentioned on the name plate..

The ratio of the Load you are using and the VI rating multiplied by the rated current(on the secondary side) would give you the approx load current(i.e secondary)

from that you can get the primary current by multiplying Load current with the turns ratio