# Transformer secondary demands more power than primary can provide?

Discussion in 'General Electronics Chat' started by electronice123, Jan 18, 2011.

1. ### electronice123 Thread Starter Senior Member

Oct 10, 2008
309
0
I have been trying to understand this for a while and still can't seem to...

If a transformer primary has an impedance of 3 Ohms and 12 V is applied to it the current will be 4 amps. So the power in the primary side will be 48 Watts.

So what if the secondary side is 120V with a impedance of 200 Ohms. 120/200=.6A. Then the power would be 120X.6=72 Watts?

What would happen to the transformer if it were operated this way?

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
293
Transformers are more complex than that, although you can't control the power with the load on the secondary alone. A transformer that can handle 48 watts is not capable of more power. You can supply less than the 48 watts, but not more.

3. ### electronice123 Thread Starter Senior Member

Oct 10, 2008
309
0
So I guess the secondary side of the circuit would require an additional 100 ohms of impedance or more making it's power equal to the power provided by the primary, or slightly less due to core losses....

I'm trying to understand how transforformer's limit current as well....Is the only method of current limiting with the use of impedance?

4. ### Hagen Active Member

May 8, 2010
30
1
Here briefly is how I understand transformers. AC voltage is applied to the primary. An alternating current starts to flow in the primary. An alternating magnetic field is set up, which in turn generates a counter emf in the primary, which almost but not quite cancels the applied voltage. This greatly limits current flow to the point that only a small magnetizing current flows. This is how current is limited in the primary.
The aforementioned alternating magnetic field is also inducing a voltage into the secondary winding. When a load is placed on the secondary, the current that is then flowing in the secondary sets up a magnetic field that opposes the field set up by the primary, effectively reducing the counter emf in the primary and causing more primary current. That is why loading the secondary causes an increase in primary current.
Make sense?