Wire has certain resistance which is of finite value. This value can vary a lot – it may be small or it may be large depending on the material of the wire used. The resistance is directly proportional to the length of the wire – these are the main 2 principles used in the construction of the wire wound resistor.
Now, for metal or conductor the resistance directly proportional to the temperature, so when the temperature is increased, its resistance also increases. We do not want the resistance to be varying with the temperature and it is not possible to construct a material which has no variation with temperature. However, that can be minimized by the use of the material which has a low-temperature coefficient. For the construction of wire wound resistor, we generally use material which has high resistivity.
Need for Wire Wound Resistor
We know that the carbon resistors are cheap as well as they have small size. But why it is so that we use wire wound resistors also?? The answer to the question is very simple. The carbon resistor has very small power rating. Also, it has very sensitive to temperature variations. They are not reliable, very sensitive to noise. So in the case of a wire wound resistors, those disadvantages are minimized so we use wire wound resistors. But cost will increase as the resistor is costlier compared to carbon resistors.
Construction of Wire Wound Resistor
Wire wound resistors are constructed of the material which has high resistivity and also low temperature coefficient. Generally we use alloys to construct the resistor. Constantan or manganin wire may be used. But the other allows may be used also which satisfies the desired criteria. Insulation is provided so that it can provide safety. The terminals are made up of tinned alloy. The construction varies according to the accuracy desired, but for highly accurate system, the cost of the resistor will also increase. For the resistor to be highly accurate, special alloys are used for providing long term stability and low temperature coefficient. Tolerance as low as ± .0005 % can also be achieved.
The wire wound resistor has a resistance which varies from 1 ohm to 1 Mega Ohm and power dissipation may vary from .5 watt to 250 watt. The tolerance level may be there from 5 % to 10 %.
Application of Wire Wound Resistor
The wire wound resistor has a very wide range of application, especially when we need variable or adjustable resistor, or if we need a fixed resistor which has to dissipate larger power, this may be used. We can construct variable resistor, such as potentiometers, rheostat.
Rheostat is a three terminal device which has two fixed contacts and one sliding contact which is used to adjust the resistance value. If we connect only two fixed point then rheostat acts as a potential divider. When all three are used, then it can vary the resistance with the help of a sliding contact.
In case of a potentiometer we have wires and a rotating shaft so that the resistance can be adjusted.
Advantages of Wire Wound Resistor
- It can be used in higher power circuits compared to carbon resistors.
- It is not that sensitive to noise compared to carbon resistor.
- It is thermally stable.
Disadvantages of Wire Wound Resistor
- It is costlier compared to carbon size
- It has large size
Conclusion of Wire Wound Resistor
It can be said that this resistance are costlier so they may be used wherever it is desired to use. These resistors cannot be used in electronic circuits also. But it is a very important type of resistor we have and mostly used in high power application whenever we cannot use carbon resistors and also they provide excellent thermal stability also.
