# How can AC source with different voltage provide same power

As some of us maybe aware, USA mains connection is 110 V while those in Asian countries are 220 V. How is it still possible for the Adapter to provide the necessary power to the device, say a laptop, despite having a lower voltage of 110 V at some places.

I understand that the Adapter is built to handle different input AC voltage, but is a 110 V source sufficient to provide the necessary power which a 220 V can.

Perhaps, it doesn't matter for light power consumers such as laptop, but won't High Power demanding home appliances such as Iron, may lack necessary input voltage to operate at maximum level.

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Power = Voltage × Current –  endolith Dec 28 '11 at 22:16

I think that most people in the world knows that usa has app. 110 V while most(not just asia) of the rest of the world has app. 220 V. see map here http://users.telenet.be/worldstandards/electricity.htm

I believe that your question is asked with a limited back ground knowledge in electricity - and I will try to answer. Please correct me if you really want a technical explanation of how an adapter works.

It takes two things to deliver power to an apparatus - voltage and current. If you think of electricity like a flow of electrons, a river if you will. The voltage will tell you how much pressure there is on the electricity. -and the current tells you how much it is flowing. An adaptor that is build for more than one voltage will automatically adjust the current to deliver the same power. - If the pressure(voltage) drops, the flow(current) will increase.

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Thanks for the reply. Actually my background in electricity isn't that limited and I also don't want to know the technical explanation of how adapters work. I was just wondering, if having a lower source voltage would limit the net power output an adapter could provide. Say instead of 110 V, if the Mains line was just 5 V, would it be possible to design an adapter to provide power to existing laptops. –  Shamim Hafiz Dec 27 '11 at 19:09
I believe that it is possible to design high power apparatus and electronics to low voltage supply. One example of this is large non superconducting magnets in physics experiments. Often the cables are finger thick hollow copper with cooling water inside, and the current is controlled by huge racks of many coupled transistors. - It would be a bit stupid to have a house size transformer to power a laptop :o) –  Hans-Peter E. Kristiansen Dec 27 '11 at 20:11
Thanks for the information. I guess we can draw the necessary power, but as you mentioned the hardware design would have to change(house size transformer), so it does matter to some extent after all. –  Shamim Hafiz Dec 28 '11 at 7:01

For the most simple electrical devices such as an incandescent lightbulb or a water kettle, which are little more and a resistance, they will draw a quarter as much power if the voltage is halved, since $V^2/R$.

This will probably mean that their operation is unsatisfactory: much dimmer or much slower to heat up. Things are slightly more complicated for an iron which is typically regulated using a thermostat.

This is why suppliers rate their appliances. The reverse of doubling the voltage can be dangerous as the power may be four times than the equipment is designed to take. You should either use equipment designed and labelled for the local voltage or use a transformer.

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Yes, I am aware of using transformer to adjust voltage. However, a laptop power adapter can be connected to the entire range of 110-240. How are they able to provide the necessary power to the laptop with different input voltage? Does the adapter act as a DC Source, which is able to draw the necessary power from Mains irrespective of input voltage? –  Shamim Hafiz Dec 26 '11 at 16:31
@shamim: I would guess that laptop PSUs using a switched mode power supply or possibly a linear regulator. These can cope with different input voltages and still produce the same output voltage. Wikipedia has good descriptions of both types of power supply. –  John Rennie Dec 26 '11 at 16:40

It depends greatly on what you are using. A 15 Amp circuit in Asia, running at a full 15 Amps through some appliance is using 3300 watts, while in the US is using half of that. So, high-power appliances (toasters, microwaves) can't simply be plugged in to the wrong voltage.

That said, most small appliances do not draw the entire 15 amps that a circuit can handle. A DC power adapter, in particular (like for a laptop or cellphone) is converting higher voltage (and current) AC to low voltage and low current DC. For example, my cellphone charger outputs 5V at 0.7A which corresponds to 3.5 watts, well under the the maximum power a circuit can provide. So, even the 1650W available in 110V-land is more than enough to power the adapter.

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