# Power Estimation

Assumption: I have 10V 1Amps(10 watts) input power supply for my system. Using this supply I design the power supply of 5V 1Amps(5 watts), 3.3V 1Amps(3.3 watts), 1.8V 900mA(1.6 watts). So, I get 10 watts of total power and I use it for different SoC's for powering them up.

But the question what I have in mind here is how from 1 Amps of current in power supply and we can generate 2.9Amps (1Amps+1Amps+900mAmps) of current on different voltages. Is it correct?

So, to satisfy the universal law of constant energy(power), I should assume that voltage is converted into current? Is it correct. What am I missing here?

• "How from 1A of current gives 2.9 amps"--> Consider in terms of power. if Current is increased at the output from input then voltage has to decrease..Voltage will not be converted to current and current also not converted to voltage Jul 30 '13 at 5:15

Energy is conserved. Charge is conserved. And, with a few simplifying assumptions, current is equal everywhere in a series circuit, and the voltages across parallel components is equal.

While still conforming to these rules, there are electrical machines that trade current and voltage between an input and an output. The transformer is one such machine, but there are others. For an ideal transformer, power in equals power out, but the ratio of voltage between sides is equal to the transformer's turns ratio, and the current ratio similarly different such that energy is conserved. For non-ideal transformers (and indeed most real power converting machines), some of the input power goes into losses in the converter, and usually manifests as heat.

You could consider this as converting voltage into current, or not, depending on how you define converting. Does a car transmission convert torque into angular velocity? If so, a transformer does convert voltage into current.

There are many mechanical machines analogous to a transformer. Gears, levers, pulley systems, turbines, etc. Anything that affords mechanical advantage is a good example. All machines are subject to the law of conservation of energy. As long as they also have no means to store energy, that means power in equals power out. Mechanical power is the product of force and velocity:

$$P = Fv$$

which is why if you want to use one of these mechanical advantage machines to apply more force to lift a thing, you will also lift it more slowly. Or, if you want to lift a thing quickly, you must also apply more force.

Perhaps then it's no mistake that electrical power is the product of voltage and current:

$$P = VI$$

and voltage is how hard something is pushing on charge, and current is how fast that charge is moving.

No, voltage is not converted to current. Voltage is a measure of potential energy, current is the number of electrons that flow past a given point in a given amount of time. The requirement is that energy is conserved and you have done the math to see that this is in fact the case (roughly). It's not necessarily the case that the electrons that enter the power supply at 10V are the same ones that leave the power supply at lower voltages. All of the electrons that make up these currents are already in the circuit before you apply any power. It's the energy you add at the input that causes the electrons in the output to start moving and make a "current". In a good DC-DC converter, about 90% of the energy available from the input power source can be converted to energy that is delivered to the outputs.

A normal transformer will step up current while stepping down voltage in the same ratio: a transformer that can deliver 10 amps at 12 volts will only draw 1 amp at 120 volts.

A switching power supply works on a similar principle - the input voltage is switched off and on at a high frequency, then applied to a transformer that steps that voltage down (and the current up) as required. By using different taps on the transformer secondary winding, or multiple secondary windings, one switching supply can have multiple output voltages.

These supplies are not 100% efficient, so you may have to supply 12 watts to get a total of 10 watts output, with two watts being lost heating the power supply.