I understand the concept of a transformer, but it doesn't reduce power, only voltage or current. A resistor will reduce the power getting to other components but will turn the excess energy to heat. So how do appliances reduce the amount of power drawn from the mains?
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Let's say you have a basic appliance with a resistance of 100 ohms. That's not a resistor you added to it, that's just the intrinsic resistance in it.
Hook it up directly to 120V mains power and yeah, you'd have a pretty big power waster. P = V^2 / R = 144W.
But if you put a transformer in it that reduces the voltage down to 10V, then it just uses 1W.
You may now be confused because you thought a transformer doesn't reduce power. In a sense you are correct, and good thing it doesn't. If you put in 144W into the transformer and got out 1W, you didn't really save any in your electricity bill, did you?
What you have to realize is that putting the transformer in the system changes how much power goes into the transformer as well as out. Think again about the 100 ohm device. It's got a 0.1A current in it. How much current is going into the transformer? If the transformer reduces the voltage by a factor of 12, it increases the current by a factor of 12. That means the current going into the transformer is 0.1/12 = 0.0083A. Without the transformer, the current would have been 1.2A, a lot more. Notice if you apply the power formula at the input of the transformer
P = i x V = 0.0083 x 120 = 1W
The little transformer can't flow more current than that through its input windings, because to flow more current it has to build up a big magnetic field. By building a transformer into a small appliance, you will use less current, therefore less power.
Hook it up directly to 120V mains power and yeah, you'd have a pretty big power waster. P = V^2 / R = 144W.
But if you put a transformer in it that reduces the voltage down to 10V, then it just uses 1W.
You may now be confused because you thought a transformer doesn't reduce power. In a sense you are correct, and good thing it doesn't. If you put in 144W into the transformer and got out 1W, you didn't really save any in your electricity bill, did you?
What you have to realize is that putting the transformer in the system changes how much power goes into the transformer as well as out. Think again about the 100 ohm device. It's got a 0.1A current in it. How much current is going into the transformer? If the transformer reduces the voltage by a factor of 12, it increases the current by a factor of 12. That means the current going into the transformer is 0.1/12 = 0.0083A. Without the transformer, the current would have been 1.2A, a lot more. Notice if you apply the power formula at the input of the transformer
P = i x V = 0.0083 x 120 = 1W
The little transformer can't flow more current than that through its input windings, because to flow more current it has to build up a big magnetic field. By building a transformer into a small appliance, you will use less current, therefore less power.