When an electrical current passes through a resistor due to the presence of a voltage across it, electrical energy is lost by the resistor in the form of heat and the greater this current flow the hotter the resistor will get. This is known as the **Resistor Power Rating**.

Resistors are rated by the value of their resistance and the electrical power given in watts, (W) that they can safely dissipate based mainly upon their size. Every resistor has a maximum power rating which is determined by its physical size as generally, the greater its surface area the more power it can dissipate safely into the ambient air or into a heatsink.

A resistor can be used at any combination of voltage (within reason) and current so long as its “Dissipating Power Rating” is not exceeded with the resistor power rating indicating how much power the resistor can convert into heat or absorb without any damage to itself. The **Resistor Power Rating** is sometimes called the *Resistors Wattage Rating* and is defined as *the amount of heat that a resistive element can dissipate for an indefinite period of time without degrading its performance.*

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The power rating of resistors can vary a lot from less than one tenth of a watt to many hundreds of watts depending upon its size, construction and ambient operating temperature. Most resistors have their maximum resistive power rating given for an ambient temperature of +70^{o}C or below.

Electrical power is the rate in time at which energy is used or consumed (converted into heat). The standard unit of electrical power is the **Watt**, symbol **W** and a resistors power rating is also given in Watts. As with other electrical quantities, prefixes are attached to the word “Watt” when expressing very large or very small amounts of resistor power. Some of the more common of these are:

Unit | Symbol | Value | Abbreviation |

milliwatt | mW | 1/1,000th watt | 10^{-3} W |

kilowatt | kW | 1,000 watts | 10^{3} W |

megawatt | MW | 1,000,000 watts | 10^{6} W |

We know from Ohm’s Law that when a current flows through a resistance, a voltage is dropped across it producing a product which relates to power.

In other words, if a resistance is subjected to a voltage, or if it conducts a current, then it will always consume electrical power and we can superimpose these three quantities of power, voltage and current into a triangle called a **Power Triangle** with the power, which would be dissipated as heat in the resistor at the top, with the current consumed and the voltage across it at the bottom as shown.

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The above power triangle is great for calculating the power dissipated in a resistor if we know the values of the voltage across it and the current flowing through it. But we can also calculate the power dissipated by a resistance by using Ohm’s Law.

Ohms law allows us to calculate the power dissipation given the resistance value of the resistor. By using Ohms Law it is possible to obtain two alternative variations of the above expression for the resistor power if we know the values of only two, the voltage, the current or the resistance as follows:

[ P = V x I ] Power = Volts x Amps

[ P = I^{2} x R ] Power = Current^{2} x Ohms

[ P = V^{2} ÷ R ] Power = Volts^{2} ÷ Ohms

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

- Where:
- V is the voltage across the resistor in Volts
- I is in current flowing through the resistor in Amperes
- R is the resistance of the resistor in Ohm’s (Ω)

As the dissipated resistor power rating is linked to their physical size, a 1/4 (0.250)W resistor is physically smaller than a 1W resistor, and resistors that are of the same ohmic value are also available in different power or wattage ratings. Carbon resistors, for example, are commonly made in wattage ratings 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 its wattage rating. However, it is always better to select a particular size resistor that is capable of dissipating two or more times the calculated power. When resistors with higher wattage ratings are required, wirewound resistors are generally used to dissipate the excessive heat.

Type | Power Rating | Stability |

Metal Film | Very low at less than 3W | High 1% |

Carbon | Low at less than 5W | Low 20% |

Wirewound | High up to 500W | High 1% |

Wirewound power resistors come in a variety of designs and types, from the standard smaller heatsink mounted aluminium body 25W types as we have seen previously, to the larger tubular 1000W ceramic or porcelain power resistors used for heating elements.

Typical Power Resistor

The resistance value of wirewound resistors is very low (low ohmic values) compared to the carbon or metal film types. The resistive range of a power resistor ranges from less than 1Ω (R005) up to only 100kΩ’s as larger resistance values would require fine gauge wire that would easily fail.

Low ohmic, low power value resistors are generally used for current sensing applications were, using ohm’s law the current flowing through the resistance gives rise to a voltage drop across it.

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

The larger wirewound power resistors are made of corrosion resistant wire wound onto a porcelain or ceramic core type former and are generally used to dissipate high inrush currents such as those generated in motor control, electromagnet or elevator/crane control and motor braking circuits.

Generally these types of resistors have standard power ratings up to 500W and are connected together to form resistance banks.

Another useful feature of wirewound power resistors is in the use of heating elements like the ones used for electric fires, toaster, 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 used is generally made of Nickel-Chrome (Nichrome) allowing temperatures up to 1200^{o}C.

All resistors whether carbon, metal film or wirewound obey Ohm´s Law when calculating their maximum power (wattage) value. It is also worth noting that when two resistors are connected in parallel then their overall power rating is increased. If both resistors are of the same value and of the same power rating, then the total power rating is doubled.

What is the maximum power rating in watts of a fixed resistor which has a voltage of 12 volts across its terminals and a current of 50 milliamperes flowing through it.

Given that we know the values of the voltage and current above, we can substitute these values into the following equation: P = V x I.

Calculate the maximum safe current that can pass through a 1.8KΩ resistor rated at 0.5W.

Again, as we know the resistors power rating and its resistance, we can now substitute these values into the standard power equation of: P = I^{2}R.

All resistors have a **Maximum Dissipated Power Rating**, which is the maximum amount of power it can safely dissipate without damage to itself. Resistors which exceed their maximum power rating tend to go up in smoke, usually quite quickly, and damage the circuit they are connected to. If a resistor is to be used near to its maximum power rating then some form of heatsink or cooling is required.

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Resistor power rating is an important parameter to consider when choosing a resistor for a particular application. The job of a resistor is to resist current flow through a circuit and it does this by dissipating the unwanted power as heat. Selecting a small wattage value resistor when high power dissipation is expected will cause the resistor to over heat, destroying both the resistor and the circuit.

Thus far we have considered resistors connected to a steady DC supply, but in the next tutorial about Resistors, we will look at the behaviour of resistors that are connected to a sinusoidal AC supply, and show that the voltage, current and therefore the power consumed by a resistor used in an AC circuit are all in-phase with each other.

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I believe that in a light string ie Christmas lights, say string of 20, wired in parallel,

that if some individual lights (resistors)are not working, a greater amount of Voltage will go to the remaining lights making them more likely to fail. Is this true and if so can you explain it to me.

Yes they are wired in a Parallel configuration most of the time (sometimes they are wired series parallel. Aka 5 lights are in series, and that is in parallel with 4 other sets of 5.) No, the voltage will not change if one set goes out, because the voltage drop in parallel is a constant 120VAC across each set of parallel lights. The current would be the factor that changed, but you would not see an increase that would hurt the rest of the bulbs. If you measured the total current of the system before the light burned out you would see 10A (that’s hypothetical). If a light burned up, you would then see a drop in total current to say 9A (also hypothetical). And the current would continue to drop based on how many lights burned out. The other lights would not draw more current if one was burned out, the total system current would simply go down (actually saving you money). All that to say, the performance of the rest of the light bulbs is not effected by a single bulb burning out. Hope this makes sense.

Christmas lights are connected in series, not parallel and as such the current is the same through each lamp. When each lamp is working and illuminated it creates a voltage drop across it, so if we had 20 lamps connected to 220VAC supply, each lamp would drop 11 volts. However, if one lamp becomes faulty and open-circuits, no current can flow so the lamps connected to the phase side of the faulty lamp will have full mains voltage, 220VAC while those on the neutral side of the faulty lamp will have zero volts.

Since P=I^2 R, and equal to V^2/R, then what is the relation between Power and Resistance, direct or indirect?

Best

I have a heating pad that can emit heat up to 260 C with a power rating of 100W with a supply voltage of 230V. How can I get the heating power of this pad. I need to know this for heat transfer through a plate, Q/A = K ((T(Hot) – T(Cold))/Thickness). It the “Q” I’m looking for

I need help with this question.

Three parallel resistors are each dissipating three watts of power. What is the power dissipation of the circuit. Please explain it to me in detail.

9 watts, 3 x 3 watts.

This helped me with my clock

Thank you sir.Your answer is correct.

it was fun reading, but i like to know about R-C circuits

12x50ma is incorrect. this should read 12×0.05 or 12 x (50/1000)

the answer is correct but the formula is incorrect and confusing to first timers.

Mili means x10^-3 it isn’t wrong.

It is not incorrect, in fact it is far more confusing 12x(50/1000)… Exponential notation was invented exactly to avoid writing like that, and the meaning of 50mA or 50e-3 A is something every first timer should learn before doing anything else.

Thanks for this but the equation is correct. 12 volts times 50mA equals 0.6 watts.

Room heaters have low resistance and you can calculate it by taking rated volate 220v and power rating of room heater ..simply P=v^2/r ..so u can calculate resistance nd that value wil be low

Resistive room heaters transfer heat into a room by convection, conduction and/or radiation as the heating element becomes hot. Metallic elements are made from metals and alloys that change their resistance from cold to hot. Because these heating elements have a positive temperature coefficient their resistance increases with higher temperatures. This is why their resistance value when measured with an ohmmeter when cold is completely different to the calculated value using the heaters wattage value when the element is hot as its cold resistance can be about 1/10th that of its hot resistance.