## Resistor Power Rating

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 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.*

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:

### Electrical Power Units

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 |

## Resistor Power (P)

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.

### The Resistor Power Triangle

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% |

## Power Resistors

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.

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.

## Resistor Power Rating Example No1

What is the maximum power rating in Watts of a resistor which has a voltage of 12V across it and a current of 50mA flowing through it.

Given that we know the voltage and current, we can substitute the values into P = V x I.

## Resistor Power Rating Example No2

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

Given that we know the resistor power rating and resistance, we can substitute the values into 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.

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.

## Mark Hodgins

Hi Wayne,, I will try to keep this short,, would you be able to recommend a resistor wattage based on an application?

I am working on a motorcycle and replacing older style 3 ohm ignition coil “packs” with newer 1.5 ohm ignition coils that are integrated into the spark plug boot. These ratings are measured on the 12 volt supply side of the coil.

So in order to prevent damaging the ignition module, I need create 3 ohms of resistance to prevent damaging the aftermarket module that gives the 12 volt supply. So logic would would tell me a 1.5 ohm resistor will create my needed resistance,, however i don’t know what the minimum wattage I can use? 10? thanks! I only know the voltage is 12vdc or less, i would think a very low amp or current, as they are very thin wires that plug into the coil, that, and the idea is for the coil to do the hard work anyway, right?. thanks! Mark

## Wayne Storr

Hello Mark, assuming all things equal, your 3 ohm coil connected to a 12 volt battery would draw 4 amps, (12÷3 = 4). If you use a 1.5 ohm resistor in series with your new coil, the total resistance would still be 3 ohms, drawing 4 amps. The I^2R loss for the resistor would therefore be: 4x4x1.5 = 24 watts. Then you would require a 1.5 ohm power resistor, similar to above, with a minimum power rating of 25 watts (nearest preferred wattage value).

## Bruno Pera

Hi! I’m wondering if i can make a HomeMade power supply to my laptop (20VDC-2A) out of an hold printer power supply (out : 40VDC) just making a current divisor with 2 10 ohms resistors. Should they be wirewound, since it will dissipate 20*2=40W?

Sorry for my rusty English =P

Cheers.

## Wayne Storr

Hello Bruno, Yes you could be you would not get 20V across the lower resistor due to the loading effect of your power supply. Also only the upper resistor needs to be 40W rated as its the one taking all the power supply current. It would be better to use a fixed voltage regulator circuit (http://www.electronics-tutorials.ws/blog/variable-voltage-power-supply.html) to produce the 20V d.c.

## Steve Kennedy

This may seem like a dumb question (probably because it is!), as it reflects an appreciation of electronics still very much in its infancy. One book describes a resistor like ‘brakes for electric current’, and speaks of something like friction inhibiting the flow of current. In that analogy, the power dissipated by a resistor must be akin to the drop in wattage from the supply side of the resistor to the wattage of the output side. Car brakes heat up more when bringing a car travelling at 100kph down to 10kph, than brakes would to bring a car from 20kph down to 10kph. So a 12VDC 2A supply reduced to a 100mA output using a 120 ohm resistor, means a reduction of 22.8W (24W – 1.2W). However, all the calculations of power ratings for resistors only take into account the output side of the resistor. So, in the above example, I’d only use a 120 ohm resistor with a power rating of 1.2W or higher, and forget about the 22.8W. Is the analogy confusing then? Or have I miss some fundamental concept altogether?

And I guess a superordinate question is this: Does it really matter what ampage my DC power produces as long as it is higher than the circuit load?

## Wayne Storr

Your power supply is rated at 12VDC, 2 Amps, that is 2 Amps maximum. The load connected to it will only draw from the supply the amount of current it needs so if your load is rated at 100mA then that’s what it will take from the supply. Even if you had a power supply rated at 12VDC, 5 Amps, the load would still only consume 100mA, then effectively in this case, 4.9 Amps of rated current is unused. Then NO to your question, it does not matter what output amperage your power supply is (within reason) as long as it is higher than the circuit load.

## Steve Kennedy

Thanks, Wayne, for your reply. Do you have any advice or comments on the first question in my post?

## Wayne Storr

About the brakes of a car, no.