Electronics Wizards: If a Transformer turns 120v to 12v with 80watts how much energy?

[240meowth]

Hello, new here.

Okay, so this question has been blowing my mind for a long long time, and never knew who/where to ask I figure we have enough gurus here to give me some kind of insight.

If a transformer or adapter converts 120V AC to 12V DC rated at 80 watts and you only plug in a 5 watt bulb in, does the extra 75 watts go to waste or what?

[BoredEE]

The device is capable of outputting 80Watts but it doesn't mean it always does. It will output 12VDC up to 80W. If you only use 5Watts on the output assuming 100% efficiency(not possible!) you will only use 5W on the input. Newton's 3rd law, conservation of energy.

To go from 120AC to 12VDC, the transformer will step down the voltage. Then there will be some rectification then some sort of regulation.

For more information google "constant voltage source" "constant current source" and "constant power source."

The 120AC to 12DC supply is a constant voltage source.

What you described would be a constant power source.

Note: supply and source are interchangeable terms.

[S14DB]

You'll want a Switching power supply, Not a constant power source. It will regulate power from 0~max depending on load.

Note transformers are only 75~85% efficient. So, for 5w 12v it would use up to 7w of power(120V).

[240meowth]

now does 80w at 120v the same as 80w at 12vDC? as far as what the electric company charges

[S14DB]

HowStuffWorks "What are amps, watts, volts and ohms?" (http://science.howstuffworks.com/environmental/energy/question501.htm)