Resistor Power Rating
When an electrical current passes through a resistor, 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 power in watts 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 +70oC 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
Resistor Power (P)
We know from Ohm's Law
that when a voltage is dropped across a resistor, a current will be passed through the resistor producing a product of power.
In other words, if a resistor is subjected to a voltage, or if it conducts a current, then it will always consume power and
we can superimpose these three quantities of power, voltage and current into a triangle called a Power Triangle
with the power dissipated as heat in a resistor at the top and the current and the voltage 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 = I2 x R ] Power
= Current2 x Ohms
[ P = V2 ÷ R ] Power
= Volts2 ÷ Ohms
The electrical power dissipation of any resistor in a DC circuit can be calculated using one of the following
three standard formulas:
- 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.
|Metal Film||Very low at less than 3W
|Carbon||Low at less than 5W
|Wirewound||High up to 500W
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. 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.
Typical Power Resistor
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. 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 1200oC.
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
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.
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 = I2R.
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.