From http://www.electronicshub.org/resistor-power-rating/
Resistor Power Rating
Introduction
Resistors can be rated based on two values. First they can be based on the resistance of the resistor. Second they can be based on the power in Watts that a resistor can dissipate safely.
The power rating of a resistor can be defined as the loss of electrical energy in the form of heat in a resistor when a current flows through it in the presence of a voltage.
Resistors can be used in any circuit based on the requirement in any combination of current and voltage. These different combinations of currents and voltages are selected such that the dissipating power rating of a resistor does not exceed the resistor power rating, which indicates the amount of power a resistor can convert into heat without causing any damage to it. The resistor power rating can also indicate the amount of power a resistor can absorb without causing any damages.
The Resistor Power Rating is also called as Resistor Wattage Rating. The Resistor Power Rating is defined as “the amount of heat a resistor can dissipate for an indefinite time period without affecting or degrading its performance.”
The resistor power rating is measured in Watts which are the units of power.
Since the power dissipation is dependent on the size of the object, the resistor power rating can vary from a value as small as one tenth of a Watt to a large value as hundreds of Watts based on the size, temperature and construction procedure of resistor.
Generally the temperature used to define the power rating is ambient temperature or room temperature. Generally, most of the resistors have their maximum power rating at an ambient temperature of 70oC or less.
Power Dissipation
Resistors are basic electrical components which obey Ohm’s Law. When a voltage V is applied between the leads of the resistor of resistance R, a current I flows through it. This current I is given by
I = V / R
The movement of electrons is the cause for this current and they are accelerated by the electric field due to the applied potential. These accelerated electrons which possess kinetic energy, try to move towards the positive side of the material and in this process they collide with atoms and lose their energy. And the result of this collision is conversion of electrical energy to heat.
The rate of loss of energy or power dissipation can be calculated from the formula P = I × V.
From Ohm’s Law, we have V = I × R
Therefore P = I2 × R
And P = V2 / R
Power Resistors
Some resistors are designed for larger power dissipation. These are called Power Resistors. Resistors with power rating of at least 5W come under power resistors. The material used for construction of power resistors must be of high thermal conductive in nature. Power resistors often come with heat sink which helps them in dissipation of heat.
Wire wound power resistors are common, but they can also be found in other types. If Nichrome alloy based wire wound resistors are used with proper non-conductive enamel paint, they can withstand temperatures up to 4500C.
Another type of resistors used to withstand large amounts of currents is Grid Resistors. Grid Resistors can withstand a current up to 500 Amps and can have a resistance values as low as 0.04 Ω. The construction of Grid Resistors includes two electrodes with large metal stripes connected between them in the form of a matrix. Grid resistors are used as grounding resistors, brake resistors and harmonic filters for electric substations.
Another type of power resistors are Water Resistors. The construction includes tubes which carry saline solution with electrodes connected at both ends of the tube. The concentration of saline solution or salt water will determine the resistance. Because of the presence of water in the tubes, water resistors provide large heat capacity, which in turn result in high power dissipation.
Another type of power resistors are Water Resistors. The construction includes tubes which carry saline solution with electrodes connected at both ends of the tube. The concentration of saline solution or salt water will determine the resistance. Because of the presence of water in the tubes, water resistors provide large heat capacity, which in turn result in high power dissipation.
Power resistors can also be made in the form of Surface Mounted Devices. Because of their small size, the power dissipation capacity of SMD resistors is less than grid type resistors and water resistors. Usually the power dissipated by SMD resistors is in the order of few Watts.
The range of power dissipated by different types of power resistors are as follows
- SMD Resistors 5 W or less
- Helical Wound 50 W or less
- Edge Wound 3.5 KW or less
- Grid Resistors 100 KW or less
- Water Resistors 500 KW or less
Power rating Examples
1. For example, to choose an appropriate power rated resistor of resistance 800 Ω and a supply voltage of 12 V. The available power ratings are 0.25 W, 0.5W and 1 W.
Power P in the resistor is equal to P = V2 / R
Therefore P = (12)2 / 800 = 0.18 W.
Hence a resistor of Power rating 0.25 should be used.
Applications of Power Resistors
Every resistor is rated with its maximum power rating. This power rating indicates the maximum power a resistor can dissipate without causing any damages to itself or the circuit. Some applications require less power dissipation and others require large power dissipation. Power resistors are used in applications where we need to dissipate large power. Some of the applications of Power resistors are
1. Engine brakes in heavy locomotives and trams use power resistors. Locomotives move at high speed and possess high kinetic energy. While stopping these high speed locomotives, their kinetic energy is converted to heat.
Depending on the velocity of the locomotives, the amount of heat generated can be in the order of few Kilo Watts. Classic disc brakes can’t be used as they wear out easily. Hence regenerative brakes or high power resistors in the form of Grid resistors are used in locomotives.
2. Power resistors are used as grounding resistors to limit fault currents, high voltages and act as protective relays. These resistors can be rated up to 8 Kilo Amps.
3. Power resistors are used as load resistors in turbines and Uninterruptable Power Supplies. They can be designed to provide adjustable resistance and can dissipate a power of up to 6 Mega Watts. Because of this high power dissipation, load resistors are equipped with an efficient cooling system to control the temperature and prevent the devices from burning out.
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