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Insight - How Resistor works

Written By: 

Arpit Jain


Carbon Film Resistor
Resistors are one of the most crucial parts of any circuit. They vary from micro-ohms to mega-ohms in terms of their value. Also, they come in various forms like embedded as a chip in an IC, can be a bare metal or can be in form of a conductive plastic. This Insight will give a detailed exploration of carbon film resistor which has a ceramic core.
Outer Structure
Resistor looks like a solid capsule with leads at both ends which are used to connect it to the circuit.
On the capsule body, there are colored bands, typically four to six in number which tells the value of the resistance. 

Outer Structure of A Typical Resistor

Fig. 1: Outer Structure of A Typical Resistor

The resistance of the given resistor is 1000 ohm +/-5% tolerance.
It is interesting to know that the temperature coefficient is in ppm instead of being in °C . This can be explained using a simple example. Consider a resistor with a rating 30 ppm for its temperature coefficient. This means that for every 1°C rise in temperature, the resistance would not change more than 30/10000000 or 0.000030 ohms. 

Outer Coating

Outer Coating
The outer coating and color bands are composed of silicone conformal coating. This coating protects the resistor from high temperatures, moistures and thermal shocks, hence keeping the resistor intact and increasing working tendency at various ambient conditions.
The coating is very firmly adhered to the surface and cannot be easily removed using mechanical forces. Under the coating is the heat conducting ceramic covered with a carbon film that has a metallic alloy wiring engraved over it. This alloy wiring is the one that functions as per the specifications of the resistance.

Outer Structure of A Typical Resistor

Fig. 2: Image Showing Outer Coating of A Typical Resistor

Metal Cap terminals

Metal Cap Terminals
When the coating is scrapped off from the terminals, two metal caps are seen fixed over the ceramic element at its ends. These metal caps form the electrical connections of the resistor with the circuit and vice versa.
Metal Cap Terminals

Fig. 3: Metal Cap Terminals

The metal caps are tightly attached to the capsule and the silicone coating tightens the bond even more. The leads are welded to the caps and hence both of them aid in sustaining electrical connections with the circuit.

Ceramic Capsule

Ceramic Capsule
After the caps are removed, carbon film covering a ceramic cylinder can be seen as a solid cylinder that has metal alloy wire embedded on it. The carbon film is closely adhered to the ceramic cylinder, making the wires strongly positioned. At the ends, where the caps were attached, there are no wires, indicating that they connect to the metal caps which through the lead connections complete the electric circuit.

Ceramic Capsule Inside The Resistor

Fig. 4: Ceramic Capsule Inside The Resistor
When the capsule is opened further, a solid internal structure of white ceramic is found as can be seen in the next image.

Image Showing Internal Structure of Ceramic Capsule

Fig. 5: Image Showing Internal Structure of Ceramic Capsule

The ceramic works as a heat conductor and can handle thermal stress very well so that the desired resistance levels can be obtained and maintained across the resistor.  Heat dissipation capacity of the resistor will increase with the thickness of the ceramic capsule. Resistance is dependent on the length of the metal alloy wire. 




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