# Why is the power conserved in a phone charger?

Most phone chargers draw 220V 50mA (for example) from the socket to inversely proportional draw 5V and 2.2A (in its converter) to charge the phone at 2.2 A because power is conserved.

Why is power conserved? Why does the increase of voltage not increase current?

I don't understand because in a generator or a battery, when the voltage increases, the current also increases according of the resistance in the circuit because U=RI.

Why does the current increase in a generator when the voltage increases, but it's not the case in a transformer? Why is the energy conserved?

An example of charger specfications here to proove that watts are conserved between input and output sides:

• I edited my post with a picture to understand my question Commented Jun 26 at 9:41
• The markings on the device are maximums, they don't prove anything about efficiency. They suggest the device could take 240*0.5 = 120 W in and only deliver 5*3.4 = 17 W out. Commented Jun 26 at 12:58
• SMPSs can provide current multiplication. They still conserve power, but aren't bound by linear principles like ohms law. And the AC ratings cover inrush current too, not constant usage. Commented Jun 26 at 16:33

I don't understand because in a generator or a battery, when the voltage increases, the current also increases according of the resistance in the circuit because U=RI.

Why does the current increase in a generator when the voltage increases, but it's not the case in a transformer? Why is the energy conserved?

Your reasoning here doesn't seem to be correct.

One thing that Ohm's law ($$\V = I R\$$) tells us is that if we have a resistor with a certain voltage across it, and then we increase the voltage across the resistor, then the current through the resistor will increase too.

With a phone charger, on the other hand, that just isn't what the situation is. The input side of a phone charger doesn't behave like a resistor, so Ohm's law isn't applicable to it. The output side usually isn't connected to a resistor, so Ohm's law isn't applicable there either.

Theoretically, it would be possible to design a phone charger so that its input would behave like a resistor, and you could connect the output to a resistor, too. However, even if you did that, the input resistance and the output resistance still wouldn't be the same, so Ohm's law still would not tell us that the decrease in voltage should produce a decrease in current.

Why is power conserved? Why does the increase of voltage not increase current?

Because we choose components, topologies and, designs that attempt to conserve power as much as we can. It's a vital aim in any power supply project. We don't want to burn energy therefore, we must make a design that achieves the highest/best power efficiency.

why is power conserved?

The Nature just works that way. That means: our Universe happens to have this power conservation property. It follows from more basic laws, but that's how it is. Power cannot be created from "nothing".

For there to be higher power on the output than on the mains input, something else would need to provide the missing power, so that the sum of power out equals the sum of power in.

In practice: mains power in = USB power out + heat power out + radiated RF power out (tiny) + radiated sonic power out (minuscule, but if you can hear it, it loses some power to "audio").

why the increase of voltage does not increase current?

Conversely: why would it increase current? Where does this expectation come from? Perhaps when you think about what made you expect an increase in current, you'll figure out the underlying more basic misunderstanding you got.

in a generator or a battery, when the voltage increase, the current also increases according of the resistance in the circuit

No. That has nothing to do with a generator or a battery. If and only if the load is resistive, will increase in voltage increase current.

The power converters are not resistive. That is the whole point of why we bother with all the switching circuitry and transformers. Otherwise, you could just have a linear regulator from mains. It would waste tremendous amounts of power, but it would work.

why in a generator when the voltage increase, thus the current increase

That's generally false. It is only true when the load is resistive.

but it's not the case in a transformer, why ?

Because a transformer is not a resistive load.

Generally speaking, as long as energy/power are conserved, the load can have an arbitrary relationship between voltage and current. You can have loads that regulate current according to Ohm's law (resistors), loads that keep constant current, loads that have nonlinear resistance (light bulbs), loads that keep constant voltage (shunt regulators), and many others - as many as you can think of really.

why is the energy conserved?

It's a law of Nature. That's how our universe works. It's a fundamental principle really. There may be some other universes where energy is not conserved. We have some trouble imagining the rest of consistent physical laws in them, to say the least.

You are right, U=R*I is always true for a pure resistive circuit (where only DC is involved!)

When a transformer (or switching regulator) is present, you have 2 circuits, one on the primary side and one one the secondary side of the transformer.

Both circuits have very different R, so the U=R*I relation is completely different. You have high U and low I on the primary, thus high R. And you have low U and high I on the secondary, thus low R.

U*I (=P in Watts) is the same on both sides.

• U=RI is always true for a circuit* That is generally false. U=RI is a *special case of a resistive load. Few circuits are purely resistive. Commented Jun 26 at 9:51
• I agree. I will change my answer. Commented Jun 26 at 9:59