Some important resistor characteristics
In my previous post I described already resistance value and tolerance and the color code. In this post I will describe to more important resistor properties: power rating and temperature coefficient of resistance.
Power rating The definition of power rating is the quantity of electrical power that can be turned into thermal energy for an indefinite time without damage to the resistor. When any resistor releases power as heat, it ends up being hotter than its surroundings so that heat can flow from the resistor to its surroundings. Heat flows more readily between two objects when their temperature difference is greater. That means, the lower the ambient temperature is, the more easily it cools the resistor and so it can dissipate more energy. Because it's not viable to constantly keep resistors at outside temperatures, the maximum power rating of a resistor is generally given at nominal temperature of 70°C. We can continue to use the resistors above that temperature, even up to 100°C or above, but we need to reduce the resistor power rating after that point to stop overheating. Alternatively, we have to come up with a way to reduce the temperature. Typical applications of high power resistors include things like motor braking resistors, electric powered heating units or grounding resistors. These kinds of resistors possess standard power ratings as much as a kW. Independent of the resistor material, all resistors comply with Ohms law and Joules law and this needs to be used when the desired power dissipation of the resistor must be calculated. It is also worth noting that when a number of resistors will be connected in parallel or series then their overall power rating is increased since the current and so power is distributed over the several resistors.
Temperature Coefficient All resistor kinds exhibit a difference in R-value when the temperature alters. Herein we're focusing on short-term changes only, presuming that the resistance value will go back to normal at the normal temperature. Temperature has two impacts on resistance. One effect is because of changes in the the resistive material's size, whose length and cross-sectional area will both raise. On the other hand, the value of resistivity with regard to all of the material adjusts and this difference in value of resistivity is much greater than the first effect, where the dimensions are transformed and this vital aspect is what we focus on in the rest of this informative article. A positive result temperature coefficient causes the amount of resistance to increase with temperature rises (PTC resistor). The other way around is called a NTC resistor. Some resistors are designed specifically to guarantee a bigger resistor temperature coefficient. These are known as thermistors and are used to measure temperature or even to control over-current. Metal oxide semiconductors are normally intended for these thermistors.