Power rating

 
Power rating

What is the power rating of a resistor? The power rating of a resistor defines the maximum energy a resistor can (safely) dissipate. As is stated by Joule’s first law, the generated electrical power is related to the voltage and current: When the electrical power equals the dissipated heat (by radiation, convection and conduction), the temperature of the resistor will stabilize. The temperature is not equal across the resistor. The resistor body is slightly hotter than the terminals, with the highest temperature at the center of the body. The higher the rate of heat dissipation to the environment, the lower the temperature rise will be. Larger resistors with a bigger surface area can generally dissipate heat at a higher rate. If the (average) power dissipation is larger than the power rating, the resistor may be damaged. This can have several consequences. The resistance value can shift permanently, the lifetime can significantly be reduced or the component is completely damaged resulting in an open circuit. In extreme cases the excessive power can even cause a fire. Special flameproof resistors are available, that cause a circuit brake before the temperature reaches a dangerous state. Power rating definition The power rating of a resistor defines the maximum energy a resistor can (safely) dissipate. Resistor derating The nominal power rating is defined for a certain ambient temperature in free air. Note that the amount of energy that a resistor in practise can dissipate without causing damage, is strongly dependent on the operating conditions and therefore not equal to the nominal power rating. For example, a higher ambient temperature can significantly reduce the power rating. This effect is referred to as derating. It should be taking into account by the designer. Often the power rating is chosen largely above the electric power. Typically resistors are [… read more]

Heater resistor

 
Heater resistor

What are heater resistors? Heater resistors are used whenever an electronic device needs to generate heat for some reason. They are designed as a special type of power resistor to provide a reliable and controllable source of heat. A heating resistor can produce convective heat, meaning it heats up the surrounding air, or radiant heat, meaning it heats other objects directly through a phenomenon called infrared radiation. Radiant heating requires the heater resistor to be placed within line of sight of the object that is to be heated, while convective heating sometimes utilizes fans to blow air over heater resistors in order to increase the heating effectiveness. Heater resistor definition Heater resistors are a special type of power resistors whose main purpose is to convert electrical energy into heat. Radiant heating Wirewound radiant heater Wire-wound radiant heaters are essentially wire-wound power resistors. The heated object receives heat by absorbing infrared rays emitted by the glowing-hot wire. Some of the energy is given off as light in the visible spectrum as well. A reflector is often added behind the resistance heating element in order to direct as much heat as possible in the desired direction. The wire can be exposed or enclosed in a tube to protect it from damage. This is especially useful if there is a risk of water drops falling on the element, which could cause thermal stress damage. Wire-wound radiant heaters are often used in bathrooms or outdoor uses where the intent is to heat a person without having to heat up the surrounding air first. Halogen radiant heater Halogen radiant heaters, often called quartz heaters, are similar in design to halogen light bulbs. They are most often made of a quartz tube with a tungsten resistive filament inside of it. The air from the tube is [… read more]