Transformer secondary winding

Thread Starter

ben sorenson

Joined Feb 28, 2022
I am wondering what happens in a circuit with a transformer when the secondary is not connected to anything. Does it affect the primary circuit in any way?


Joined Jul 18, 2013
Then you would have very High current and overheating of the winding.
Visualize what happens when this is done on purpose to a transformer e.g. a spot welder operation or a Weller Gun-style soldering iron.
Result is a very HOT one!
Last edited:


Joined Aug 21, 2017
Transformer fully loaded have pure active resistance. Transformer in idle have fully inductive reactance. Thus, in the idle the good transformer consumes about 1 % of nominal current, average about 10% and very very very bad Japanese transformer may consume about 50% - never produce such, and try never to use that. The 10% is what ought to be not overstepped. Just add the primary turns if current is too large and may wound off a part if too small current in idle. Sai is true for any kind of transformers.

There yet is one another component, the leaking magnetic field and it bound loss current. To measure that precize, must be measured current with idle secondary and then with short-circued secondary (may use small voltages to not cook it). Then put a figures in certain formula and voila! This figure is important exclusively in a case of high frequency high power ferrite transformers.


Joined Aug 21, 2008
People tend to get worried about magnetizing current, but it reactive current and consumes very little power, and since it is in quadrature with the load current it does not increase RMS current by much. (Actually that is the same argument stated two different ways.)