Question about transformers and amperage?

Question

Ok, so I understand what a transformer does. I have a step down transformer that changes the 120 VAC to 12 VAC. I also know that power in must equal power out. Which means the current is going to be ten times more on the secondary winding.

But here's where I'm confused; does this mean that 10 times more current will run through the circuit? Let's say that you've got the transformer, then off of that you have a 10 ohm resistor. So at 12 V this should draw 1.2 amps (if it were plugged into the wall). But if the current is supposed to be 10 times that, is it 12 amps running through it?

Or does 1.2 amps still run through it, but 12 amps is drawn on the primary winding?

I'm so confused!!!

Answer 1

If the secondary is drawing 1.2 amps, which it will in your example, the primary will draw only .12 amps. Although 12V will remain constant, the combination will end up matching the load.

You know this by the way that power is consumed when you tap directly into the wall (without the transformer). If you have a 120V supply and hook up various appliances to it, you will draw only so much amperage as that appliance consumes. Your house is fused so that amperage is limited, but it isn't fused to reduce the amperage to everything you run off outlets.

In other words, you don't have to reduce the available amperage going to an adapter that provides 12V and 13 milliamps to your cell phone charger. The adapter is only going to draw as much amperage as the load requires.

Answer 2

The amount of current in the secondary circuit affects the magnetic fields of the transformer.
This "feedback" affects the inductive reactance (apparent resistance to AC) of the primary windings.
The more current you draw out of the secondary; the more current will be drawn into the primary.
If you are drawing 1.2 amps from the secondary; you will be drawing 0.12 amps into the primary.
Remember: watts (power) = volts * amps
and: power in = power out (+ inefficiencies).
Wattage is energy per unit time.
One watt = one joule per second.
It is the product of voltage and amperage which is relevant to the power, not the ratio.
It is the ratio of the voltage to amperage that is transformed by the inductive coupling of the secondary and primary windings (hence the name).

Answer 3

It means the transformer secondary voltage is 12 volts and the current rating is 1 amp. If you put a higher current load on the transformer it won't "pop", but it will overheat and the insulation will degrade faster than if the current was kept below 1 amp. If the current is allowed to go too high, the transformer will fail in a short time.