The RJ Journal - Electronics

Whenever there is an electric current flowing through a resistive element, may it be a resistor, a diode, a transistor or any component, which produces a voltage over it (U = R * I) it is said to do an amount of work and this work is measured in watts (W). Reference to an electric power in a text or on a schematic diagram is done with the letter P.

Another way of putting it is that power represents the rate at which energy is converted from the electrical energy of the moving charges to another form of energy like heat, mechanical energy (movement), sound or energy stored in electric fields or magnetic fields. It can also be said that power is the rate at which electric energy is transmitted.

Power is the product of current and voltage.

P=U*I

With the help of the U = R * I formula the power formula can be rewritten as:

P=U²/R and P=R*I²

This formula works for both DC and AC currents if the RMS value of the AC current and voltage is used. Note that if the load is not a pure resistive load when using AC, You will get both real power, reactive power and a resulting apparent power because the voltage and the current is out of phase. By just using the RMS voltage and the RMS current values we get the apparent power. In order to know the real power we also need to take the phase shift into account. For AC power where the voltage and current is out of phase, the real power is given in watts and the apparent power is given in VA (volt amperes). The apparent power is always larger than or equal to the real power and the ratio between them is called the power factor, which is either expressed as a value between 0 and 1 or as a value between 0% and 100%. There is a lot more to say about this but it will not, at the present time, be covered by this tutorial.

Power and energy

Energy is power over time. When you pay your electric bill, you pay for the used energy which has a unit of Ws (watt seconds). On your electric bill it will probably be given as kWh (kilo watt hour), though. If you have a 100W bulb and you let it burn for one hour, it will cost you 0.1kWh.

Power dissipation

When current is flowing through a component and is causing a voltage drop over this component due to it's resistance, it is said that this component is dissipating power which in turn produces heat. The more power the more heat. As we saw in the previous chapter, a resistor has a maximum power dissipation value (at a given maximum temperature). This is true for all components that produces a voltage drop when there is a current flowing through it. The maximum power dissipation parameter is sometimes also called just maximum power.

If a component that has a high power dissipation parameter (say above around 1W, depending on the component) and it is going to be used at or near this power, heat must often be conducted away from the component to keep it below its maximum operating temperature. This is done with a heat sink or a fan (or both).

Note also that some components does have a maximum current rating in addition to the maximum power dissipation rating. In some situations the actual power dissipation can be below the maximum power dissipation for the component but the current can still be above the maximum current for the component. This is specially true for components used at a low voltage (P = U * I) or where the voltage drop over the component can be low at high currents, such as in MOS transistors or where there can be a momentary inrush current (the instantaneous power dissipation is still high but it is during a very short time which gives a low temperature rise).

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