Power and Energy in Resistors

The relationship between power and energy in resistors is directly related to the heat energy, which is the product of voltage and current.

Electric Power is absorbed by a resistance, as it is the product of voltage and current with some resistances that convert this power into heat.

When an electric current passes through a resistance due to the voltage on it, electrical energy is lost by the resistance in the form of heat, and the larger this current, the warmer the resistance.This event is known as the Resistance Power Rating.

Resistors are rated with the electrical power given in watts (W), which they can safely distribute according to their value and mainly their size.Each resistance has a maximum power rating, generally determined by its physical size, the larger the surface area, the more power it can safely dispense if it is connected to ambient air or a cooler.

A resistance can be used in any combination of voltage (in cause) and current, unless the resistance power rating and "Emitting Power Rating" are exceeded, indicating how much power it can convert or absorb into heat without causing any damage to itself.

The degree of power of resistors can vary from one-tenth of a watt to at least hundreds of watts, depending on its size, structure and ambient operating temperature.Most resistance has the maximum resistance power given for an ambient temperature of +70 o C or below.

Electrical power is the proportion during which energy is used or consumed (converted to heat).The standard electrical power unit is Watt, the W symbol, and a resistance power rating is given as Watt.As with other electrical sizes, prefixes are added to the word "Watt" when expressing resistance power in very large or very small quantities.Some of the more common are:

Electric Power Units

UnitSymbolValueAbbreviation
milliwattMw1/1,000 watts10 -3 W
kilowattKw1,000 watts10 3 W
megawattMW1,000,000 watts10 6 W

Resistance Power (P)

We know from the Ohm Act that when a current passes through a resistance, a voltage that produces a power-related product drops.

In other words, if a resistance is exposed to a voltage, or transmits a current, then it always consumes electrical power.

Resistance Power Triangle

Power and Energy in Resistors
Resistance Power Triangle
Power and Energy in Resistors
Resistance Power Triangle

The power triangle above is great for calculating the power spent in a resistance, if we know the values of the voltage on it and the current flowing through it.But we can also calculate the power expended by resistance using the Ohm Act.

Ohm law allows us to calculate the loss of power given the resistance value of resistance.Using the Ohm Law, it is possible to obtain two alternative variations of the above statement for resistance power, if we know the values of only two, voltage, current or resistance as follows:

[ P = V x I ] Power = Volt x Amperage

[ P = I 2 x R ] Power = Current 2 x Ohm

[ P = V 2 ÷ R ] Power = Volt 2 ÷ Ohm

The electrical power loss of any resistance in a DC circuit can be calculated using one of the following three standard formulas:

Power and Energy in Resistors
  • Here:
  • V is the voltage on the resistance in Volts
  • I is the current flowing through the resistance in amps
  • R is the resistance of resistance in Ohm (Ω).

Since the degree of resistance distributed depends on their physical size, a resistance of 1/4 (0.250)W is physically smaller than a resistance of 1W, and resistors with the same omic value are available at different power or wattage values.For example, carbon resistances are usually made at watts of 1/8 (0.125)W, 1/4 (0.250)W, 1/2 (0.5)W, 1W and 2 Watts.

Generally speaking, the larger their physical size, the higher the wattage value.However, it is always better to choose a resistance of a certain size, which can distribute two or more times the calculated power.When resistances with a higher wattage value are required, wire-winded resistors are often used to dissipate excess heat.

TypePower ratingStability
Metal filmVery low below 3 WattsHigh %1
CarbonLow below 5 WattsLow 20%
WirewoundUp to 500 Watts highHigh %1

Power Resistors

Wirewound power resistors are not like standard smaller resistors, as we have seen before. Stone resistors covered with a refrigerant aluminum body can come in different designs such as aluminum and porcelain according to the need.

Power and Energy in Resistors
Wirewound Resistance

The resistance value of stone resistors is very low (low omic values) compared to carbon or metal film types.The resistance range of a power resistance varies from 1Ω (R005) to only 100kΩ, since larger resistance values require finely measured wire, which can easily fail.

Low omic, low power value resistors are often used for current sensing applications, using ohm law the current flowing through the resistance causes a voltage drop on it.

This voltage can be measured to determine the value of the current flowing in the circuit.This type of resistance is used in test measuring equipment and controlled power supplies.

Larger stone resistors are made of corrosion-resistant wire wrapped around a porcelain or ceramic core type shaper and are often used to disperse high currents in engine control, electromagnet or elevator/crane control and engine brake circuits.

Usually such resistors have standard power ratings of up to 500 Watts and are often connected to form structures called "resistance banks".

Another useful feature of Wirewound power resistors is its use as heating elements for electric toasters, irons, etc. In this type of application, the wattage value of the resistance is used to produce heat, and the type of alloy resistance wire is used. It is usually made of Nickel-Chrom, which allows temperatures up to 1200 o C.

Carbon, metal film or stone resistors complie with the Ohm Act when calculating maximum power (watt) values.It is also worth noting that when the two resistors are connected in parallel, the total power ratios increase.If both resistors are of the same value and the same power value, the total power value is doubled.

Power and Energy in Resistors Question Sample 1

What is the maximum power rating in watts of a constant resistance with a voltage of 12 volts between its terminals and a current of 50 milliamper through it?

Since we know the values of the voltage and current above, we can change these values in the following equation: P = V*I .

Power and Energy in Resistors

Power and Energy question sample in resistances 2

Calculate the maximum safe current that can pass through 1.8KΩ resistance at 0.5 Watts.

Power and Energy in Resistors

All resistors have a maximum distributed power rating, which is the maximum amount of power they can safely distribute without harming themselves.Resistors that exceed the maximum degree of power usually burn quite quickly and tend to damage the circuit they are connected to.If resistance close to the maximum power value is to be used, a type of cooler or cooling is required.

Resistance power rating is an important parameter to consider when choosing a resistance for a particular application.The task of a resistance is to resist the flow of current passing through a circuit, and it does so by distributing unwanted power as heat.Choosing a small wattage value resistance when high power loss is expected causes the resistance to overheat, destroying both the resistance and the circuitry.

So far we have considered resistances connected to a fixed DC source, but in the next tutorial about Resistors, we will look at the behavior of resistors connected to a sinusoidal AC source and show that there is voltage, current and therefore the power consumed.