Resistors are conductors of electricity which offers resistance to the flow of current and resistance is the ability of a resistor to resist electric current.
When you were new to electronics, you might have came across a cylindrical shaped material with two terminals on a circuit board, printed on with color bands. You may have asked yourself, "what is this?" What you saw is a resistor.
Types of resistors.
Normally resistors are use for resisting current or voltage in circuits, they are basically two types of resistors;
- Fixed resistors
- Vairable resistors.
1.Fixed resistors:
their resistance value is always fixed.
They are used in circuits which requires a fixed resistance to the flow of current. Moreover, their resistance value can only be tailored in the industry.
Circuit Symbol of a fixed resistor.
The left symbol is for the IEC (international Electro-technical commission) standard, while the other one at the left is the American standard fixed resistor symbol.
There are many types of Fixed resistors, example are listed below.
- Wire wound resistor
- Metal film resistor
- Metal oxide resistors
- Foil resistors
- Carbon film resistors
- Metal glaze resistors
- Carbon composition resistor.
Wire wound resistor.
This type of resistor is made by winding nichrome or maganin wire on a metal core. The wire acts as resistor to the flow of current.
The wire resistance depends on it length.
Metal film resistor.
It resistive material is made from a metal film unlike a carbon film resistor. The metal film is made from
Nickel-chromium, tin or antimony.
They have low temperature coefficient resistance which is also known as TCR. TCR is the rate by which a resistor or substance resistance changes with increase or decrease in temperature.
Metal oxide resistors.
Their resistive material is made from oxide of metals such as tin oxide.
The only difference between it and the metal film type is just the oxide prescent in it. One advantage is that it can be used in high temperature. It is more cheaper than the metal film resistor.
Foil resistors.
Their resistive material is also made from metal foils. They are accurate and stable, and also have a low temperature Co-effecient rate.
Foil resistors produces low noise compared to other types.
Carbon film resistors.
Also their resistive material is made of carbon film coated on a ceramic subtrate. The carbon film is connected to two metal caps at the ends, the caps act as the terminals while the ceramic subtrate acts as an insulator.
It is the most common type used on circuit boards, it is preferred to carbon composition type because it produces low noise and more efficient.
Metal glaze resistors.
Here, glass power and metal particles are use for making the resistive material, it resistance can be tailored according to the mixture. It has low TCR.
2.Variable resistors:
Their resistance value can be adjusted or varied to a desired value.
Circuit Symbol of a variable resistor.
They are used in systems which requires a reduction or increase in current flow, e.g volume systems, joysticks and game pads. Examples are;
- Potentiometer
- Thermistors
- Humistors
- Force sensitive resistors
- Magneto resistors
- Rheostat
- Light dependent resistors
Potentiometer.
This type of resistor's resistance is vairable, it resistance can be varied by turning it axis leftward or rightward.
It consists of three terminals, the terminals at the sides has fixed resistance, while the middle one has a variable resistance, what do I mean? I mean, it has three pins as shown in the image, they are connected together to a resistive material, the first terminal to the last is fixed to the resistive material, but the middle one is made to slip over the resistive material. The resistor can function as fixed and vairable.
A potentiometer having a resistance of 100kΩ means it has a fixed resistance of 100kΩ when its first and last terminals are connected in circuits and works as variable when one end and the middle terminal is connected to a circuit, when used in this mode, its resistance can be varied from 0-100kΩ.
Thermistors.
Having a full meaning of "Thermal resistor" it is clearly understood that this type of component has a variable resistance depending on it temperature which can be from it surroundings or the heat it generates itself when excess current passes through it. We can say, thermistors are electrical current whose resistance depends on it temperature. It is made from Semiconductors or metallic oxides.
There are basically two types of thermistors;
- Negative type co-effecient (NTC).
- Positive type Co-effecient (PTC).
Negative temperature co-effecient.
It is a two terminal thermistor whose resistance decreases with increases in temperature. It allows the flow of current when when it temperature increases.
It is use in automatic air-conditioners for cooling a room when it gets hot and modern systems that requires operation when surrounding temperature increases.
Positive temperature co-effecient.
Its resistance increases with increase in temperature. When there is much heat it shuts down a circuit, thus it can also be use to protect circuits from excess heat which can damage it.
This property make it find applications in fire alarms, cellphone circuits and heat controlling devices.
Humistors.
It is also known as humidity sensors.
Humistor is coined from two words humidity and resistor.
Its resistance changes with slight changes in humidity of the surrounding air. Therefore it is use in astronomical and weather instruments for measuring air humidity.
Force sensitive resistors.
It resistance depends on the applied pressure. When much pressure is exerted to this resistor, its resistance reduces drastically, but when no or little pressure is applied it has high resistance.
It consists of a plastic foam which contains powdered graphite, placed in-between two metallic sheets. When force is applied on the foam the graphite conducts electricity when it powders are shrinked together. Some force sensitive resistors are made with conductive liquids.
Magneto resistors.
Their resistance depends on the strength of magnetic field around them.
An Increase in the magnetic field around them results in an increase in resistance, decrease in magnetic field around them results in decrease in resistance.
Rheostat.
Rheostat is coined from two Greek words, rheo and statis meaning a current controlling device. It is use for providing vairable resistance in experiments. It consists of three terminals, two terminals are attached to a resistive metallic wire wound on a plastic cylinder and a slider is mounted over it with a terminal for circuit connection.
It is much similar to a potentiometer, however it only two terminals are used, the slidable terminal and a fixed one. If the length is increased resistance increases and decrease in length increases conductivity.
Light dependent resistors.
Shorted into LDR and also known as photo resistor. Its resistance increases when the amount of light rays exposed or it receives reduces therefore, its conducts when exposed to light rays.
They are made from Semiconductors and are used in street lights and light sensors.