What is Power Resistor : Working & Its Applications

In the electrical field, power resistors play a key role as compared to the normal resistor, because normal resistors are arranged on PCBs directly to modify electronic and electrical circuits. These resistors handle a huge amount of power in high-powered systems which range from the continuous flow of current to a direct power surge. So this article discusses an overview of a power resistor – working & its applications.

What is a Power Resistor?

Definition of Power Resistor is the resistor that is designed to resist & dissipate a large amount of power. These resistors are mainly designed to dissipate huge power by keeping their size small if possible. The power resistor symbol is shown below.

Specifications

The specifications of the power resistor mainly include resistor material, mounting style, and lead type.

• The mounting style of the resistor mainly includes chassis mounted, bolted, through-hole mounted or surface mounted.
• The chassis-mounted resistor can be connected to a metal surface for dissipation of maximum heat. Other common types of mounting styles are SMT or Surface mount technology & THT( through-hole technology).
• Lead types resistors mainly include gull-wing, axial, J-leads, screw terminals, radial leads & tab terminals. Power resistors with no leads are also available.
• The materials used in these resistors mainly include; ceramic, carbon film, metal alloy, metal oxide, metal film, thin film, thick film, and wire-wound.

How Does Power Resistor Works?

The working of a power resistor can be done by dissipating energy by controlling the flow of current & voltage. Here the resistor power rating mainly describes how much power this resistor can handle securely before it gets to start to experience permanent damage. In most electronic applications, low-power resistors are used which have 1/8th watt or below.

The dissipation of power through this resistor can be found with the help of Joule’s first law known as the power resistor formula like Power = Voltage x Current or P = VxI. This dissipated power can be changed into heat to increase the temperature of this resistor. So the resistor temperature will be increased until it reaches its specific point where the heat can be dissipated throughout the air.

A device may require this resistor based on the required wattage to avoid overheating. By maintaining the temperature of this resistor, higher currents can be handled without any damage.

When this resistor is operated above its range of operating power and temperature then it may result in severe consequences like shifts within resistance value, operating life can be reduced, electrical fires, or open circuits. So these failures can be avoided by derating the power resistor depending on expected operating conditions.

Power Resistor Vs Resistor

The difference between a power resistor and a resistor is discussed below.

Types of Power Resistors

There are different types of power resistors which are discussed below.

• Grid.
• Water.
• Chip or SMD.
• Wirewound.

Grid Resistors

Grid resistors are one kind of power resistor which are designed by collecting a matrix of resistive metal strips in between two electrodes. These resistors are bulky & they have a high current rating like 500 A. Generally, these types of resistors are used as load banks within trains, brake resistors, for harmonic filtering within electric substations & electric generators which can have resistance values from 0.04 ohms to a few megaohms. The typical power dissipation of this resistor is approximately 100 kW.

Water Resistors

These resistors are designed with tubes that are filled with copper sulfate solutions or saline connected in between two electrodes. When water is a good electricity conductor, then it can be used as resistive material by changing the concentration of suitable salts within the water.

Water is also a good heat conductor which allows water resistors through high power dissipation capacity & high thermal conductivity. Water resistors with high power utilize copper sulfate solutions in place of salt solutions. These types of resistors are applicable in electrical applications which need typical power dissipation above 500 mW. They do not dissolve the rated power immediately, but they dissolve large amounts of power over a specified period

Chip Resistor or SMD Resistor

Chip resistors or SMD resistors are similar to integrated circuits. The materials used to make these resistors are pressed carbon, metal & ceramics, or metal foil. SMD resistors are surface-mounted chip resistors that are available in smaller forms.

These resistors include metal oxide film which is deposited on a ceramic substrate. The length & thickness of the film will determine the resistance. As compared to grid or water resistors, the power dissipation ratings of this resistor are very less & can dissipate a few watts of power.

Wirewound Resistor

These resistors are designed by winding a metal wire around a solid form, frequently made of fiberglass, plastic, or ceramic. At one end of a winding, metal caps are connected whereas, at another end, metallic leads are connected. The end product is frequently covered with enamel or non-conductive paint to provide some safety from the surroundings.

Please refer to this link to know more about the wire-wound resistor.

Power Rating of the Power Resistor

The Power rating or wattage rating of the resistor can be defined as the amount of heat dissipated by a resistive element in an unspecified period of time without changing its performance. The power rating is one of the specifications of a resistor that defines the highest quantity of power that the resistor can resist

The power rating of the resistor can be changed based on the construction, size & ambient temperature from <1/10 watt to 100 watts. Most of the resistors have a maximum power rating at +70 degrees C of an ambient temperature.

The power rating can be calculated by multiplying the difference of voltage across two points through the current flowing between them & is calculated in units of a W or watt. If the power ratings of resistors is above 1W are generally called power resistors. So these resistors can handle a huge amount of power before they blast. The examples are 3W, 5W, and 25W, 5W including resistance values of 0.1Ω, 2Ω, 3Ω & 22kΩ. Small power resistors are frequently used to detect current.

How to Calculate Power Rating?

We know that electrical power can be given as P=V x I

Where, ‘V’ is voltage and‘I’ is current

From Ohm’s Law, we know the formula for V = I × R

Where ‘R’ is resistance

So, P = I^2 × R & P = V^2 / R

The power dissipation within a power resistor can be calculated by using any of the below formulas.

Power P = V × I or P = I^2 × R or P = V^2 / R

Advantages

The advantages of a power resistor include the following.

• High blocking accuracy.
• Less noise during operation.
• Stability& reliability.
• low-temperature coefficient.
• It can resist high temperatures.
• It can work at an ambient temperature like 170 degrees Celsius.

Disadvantages

The disadvantages of a power resistor include the following.

• These resistors are large in size & weight.
• These are expensive
• Wire wound power resistors are not applicable beyond 200 kHz due to the interwinding capacitance & inductance.

Applications

The applications of power resistors include the following.

• Power resistors are applicable where large amounts of energy are converted into heat very safely through electrical energy as a medium.
• These types of resistors are extremely resilient & mainly applicable for high-temperature ranges.
• These resistors restrict high voltage as well as power values through high precision & narrow tolerances.
• These resistors exhibit an outstanding pulse behavior & less current noise through high reliability based on the resistive element.
• These components can be used as protective devices, controllable power dissipation devices, etc.
• These are used to dissipate generated power throughout engine braking within vehicles using electrical motors.
• These are applicable in trams & locomotives that change the vehicle’s kinetic energy (K.E) to heat securely.
• These types of resistors are applicable in power generation & distribution, control systems, power systems, and high-voltage applications.

Thus, this is an overview of a power resistor – working, advantages, disadvantages & Its applications. High power resistors have huge demand in different applications like dynamic breakers, power supplies, heaters, amplifiers, and power conversion. These resistors are normally rated at a minimum of 1W or above. Here is a question for you, what is the power rating of the wire-wound resistor?