# Controlling power?

From Ohm's law, we can control the voltage and current of a circuit by changing its resistance? Say we have a power source of 10kW, we can increase V by increasing R, and we can reduce V by reducing R? and P = IV stays the same?

• Your 2nd sentence answers itself - if it's a power source then IV MUST remain constant. – Andy aka May 11 '14 at 10:44
• It might be a "power source" but it will either produce current or 'voltage' and in some way be limited by the power output ratings. Like you can have something output current at a particular rate, so you can generate a voltage at the load by putting a resistor there to 'ground'. The current will push through the resistor thusbmaking a voltage across build up at the resistor. The other way works too, the source might merely be abstash of electrons built up somewhere that is replenished inifitely fast and pushes out a current based on the resistivity of the load, or power rating.. – KyranF May 11 '14 at 11:19

From Ohm's law, we can control the voltage and current of a circuit by changing its resistance?

No, this doesn't follow from Ohm's law at all. For example:

simulate this circuit – Schematic created using CircuitLab

The voltage across the rheostat is fixed - the current through is variable but not the voltage across. In this circuit, by Ohm's law, the product of the current and resistance is constant

Now, consider another example:

simulate this circuit

The current through the rheostat is fixed - the voltage across is variable but not the current through. In this circuit, by Ohm's law, the ratio of the voltage and resistance is constant.

Say we have a power source of 10kW

Typically, the power rating of a source is the power the source can safely deliver. When you write "a power source of 10kW", I think of a source capable of delivering 10kW without self-destructing.

If instead you mean a source that delivers a constant power of 10kW to almost any load, then you should write "a 10kW constant power source".

Now, while a constant voltage source and constant current source, like in the above ideal circuit diagrams, are linear circuit elements, a constant power source is a non-linear circuit element since power is the product of the voltage across and current through.

If you have a constant 10kW power source connected to a variable resistor, the voltage across is proportional to the square root of the resistance:

$$v = \pm \sqrt{10kW \cdot R}$$

Equivalently, the current through is inversely proportional to the square root of the resistance:

$$i = \pm \sqrt{\frac{10kW}{R}}$$

• Can we control voltage & current from a constant power source? Maybe with the use of transformers? If we require V to be a certain value and I to be a certain value depending on the circuits at hand? – Pupil May 11 '14 at 20:58
• @Key, For a constant power source with a certain value, you can pick either the voltage to have a certain value or the current to have a certain value but not both since the product of the voltage and current is fixed. – Alfred Centauri May 11 '14 at 21:03
• Can you please explain this using the 10kW power source as an example? – Pupil May 11 '14 at 21:27
• @Key, the source supplies 10kW of power and suppose that you require the voltage to be 100V then the current must be 100A. You don't have any choice: $100V \cdot 100A = 10kW$ – Alfred Centauri May 11 '14 at 21:30
• I assumed with the use transformers its possible to have an array of V and A. Example: Using a transformer we can have 10A @ 1000V, or we could have 50A @ 200V? This would not be possible? – Pupil May 12 '14 at 0:45

Strictly speaking yes, if you have a power source, it will 'adjust' its voltage such that the current * the voltage is equal to its characteristic power.

In practice we most often have (approximations of) of voltage sources. Current sources are more rare, but they occur often inside analog chips.

Power sources do exist, often involving an inductor that is periodically 'charged' to some level and then discharged into the load.

It's possible to have a constant power source, for example a temperature controller with feed-forward control of the current based on mains voltage. It's not that common.

It's more common to have a constant power load. A switching power supply with a constant load on the output approximates a constant power load. Ideally it will have no losses, so power in = power out. Since the output load is fixed, the input power must be constant with input voltage, so the current will drop inversely with the input voltage to maintain constant power in. Of course there are limits- too low input voltage and the supply will no longer work (the current does not go to infinity) and too high input voltage and something will break.