# Why in a inverter DC to AC 12V et 220V when I increase the voltage, the current decrease to keep the same power? [closed]

There is a connection between current and voltage.

Amps (A) is voltage (V) divided by resistance (R): A = V/R

And yes, the more voltage, and thus the more current it draws in an electric component, the faster it goes (for a fan for example) or the more the light is strong (for a led for example).

I have a step up inverter that steps up voltage...

For example this product

So it would be logical that more voltage pass on the out side of the socket of the transformer, the more current will be transmitted in the out side socket of the transformer...

But here we speak about the keep on the power, but why here its thus the inverse?

What's the relationship with my sentence: "the more voltage, and thus the more current it draws, the faster it goes.", in this case of a transformer? Why a transformer that steps up voltage delivers less current? Why do we keep the same power in this case?

Is it differs from a battery that "create" voltage and current from "zero"?

Why in this case there is conservation of energy? Because when current flows through a component like a led or a fan, more voltage pass though it, more current pass also.

• A transformer isn't a resistor. Also, there are no DC to AC transformers. Do you mean an inverter? Commented Jun 20 at 11:13
• ok, but I don't understand why it keeps the same power all along it ? Commented Jun 20 at 11:14
• @winny yes for example this product Commented Jun 20 at 11:17
• Please edit your question accordingly. Have you looked up conservation of energy? Commented Jun 20 at 11:18
• @winny, Yes but why in this case there is conservation of energy ? Because when current flows through a component like a led or a fan, more voltage pass though it, more current pass also.... Commented Jun 20 at 11:20

Unlike simple passive loads, an inverter uses active electronics to switch connections on and off as needed to provide its designed output.

When the input voltage is higher, these connections don't need to be switched on for as long to produce the required amount of output energy per cycle, so the average input current decreases.

A transformer or DC->AC inverter passes Power, not just Voltage or just Current.

Power is Voltage times Current, so if the transformer or inverter increases the voltage, it must also decrease the current to maintain the same power.

Similarly, if a transformer reduces the voltage, it will increase the current to pass the same power.

Amps (A) is voltage (V) divided by resistance (R): A = V/R.

Correct - for a resistor.

I have a step up inverter that steps up voltage... So it would be logical that more voltage pass on the out side of the socket of the transformer, the more current will be transmitted in the out side socket of the transformer.

You need the transformer or inverter because the load's resistance is too high for a 12 V power supply. When you step up the voltage it can drive the required current through the load.

Consider a 12 V, 60 W bulb. We can calculate its resistance when hot: From $$\ P = \frac {V^2} R \$$ we get $$\ R = \frac {V^2} P = \frac {12^2}{60} = 2.4 \ \Omega \$$.

Now consider a 120 V, 60 W bulb. We can calculate its resistance when hot: We get $$\ R = \frac {V^2} P = \frac {120^2}{60} = 240 \ \Omega \$$.

So to drive enough current through the 120 W bulb you need to step up the voltage.

If you were to connect your 12 V lamp to the inverter or step-up transformer then yes, a much higher current would flow and the bulb would burn out.

• Shouldn't that say "Now consider a 120 V, 60 W bulb"? Commented Jun 20 at 15:35
• @Finbarr, sometimes I'm not convinced about the benefits of cut'n'paste. I remember the MS-DOS days when there was no such thing! Fixed, thanks. Commented Jun 20 at 21:53