resistors: types and uses

RESISTORS:TYPES AND ITS

USES

RESISTOR:TYPES AND

USES

PRESENTED BY:BAIS, PERLITO JR. A.BEQUILLA, MARY ROSE INGILICA M.

BSED III-B

What is Resistor?

A resistor is a passive electronic component that we use in electronic circuits either to limit current through an active component or to reduce voltage in the circuit (by dropping voltage across the resistor).

Note:- Current through a resistor is directly proportional to the voltage applied across the resistor. This is derived from Ohms Law and is represented by equation I = V/R – where I=current through resistor (in amperes), V = voltage across resistor (in volts) and R = resistance of the resistor (in ohms)

Types Of Resistor

1. Fixed Resistor2. Variable Resistor3. Rheostat4. Tapped Resistor5. Thermistor6. Light Dependent Resistor (LDR)7. Voltage Dependent Resistor

1. Fixed ResistorA fixed resistor cannot be changed as it

is set at a specific value.

Classification of Fixed Resistor

Carbon composition type resistor

This is the most common type of low wattage resistor. The resistive material is of carbon-clay composition and the leads are made of tinned copper. These resistors are cheap and reliable and stability is high.

Wire wound resistors:These resistors are a length of

wire wound an insulating cylindrical core. Usually wires of material such as constantan (60% copper and 40% nickel) and manganin which have high resistivity and low temperature coefficients are employed. The completed wire wound resistor is coated with an insulating material such as baked enamel.

Metalized resistors

It is constructed using film deposition techniques of depositing a thick film of resistive material onto an insulating substrate. Only approximate values of resistance can be had by this method.

2. Variable resistor

- a device that is used to change the resistance according to our needs in an electronic circuit. It can be used as a three terminal as well as a two terminal device. Mostly they are used as a three terminal device. Variable resistors are mostly used for device calibration.

Variable resistor element with two connections

Variable resistor symbol for circuit diagrams

The variable resistor symbol used in circuit diagrams indicates its construction. Effectively it is a fixed resistor with a slider that can move along the length of the resistive element. In this way it forms a potentiometer as described before.

It depicts the current version use in circuit diagrams today and the traditional format that may be seen on older circuit diagrams.

Types of Variable Resistor: Preset Variable Resistor

This is micro version of variable resistor. Preset resistor is directly mounted on circuit board and adjusted only when the circuit is built. There is an adjustable screw attached to the resistor and a small screwdriver is required to adjust this screw for desired resistance value. These  are quite cheaper than standard variable resistor available in the market.

Wire wound variable resistors

- are manufactured using very fine resistance wire. This is wound around a former that is almost toroid. The most commonly used form of resistance wire used is a nickel chrome alloy. Which has some further additives to improve its electrical characteristics.

Wire wound variable resistors offer a high level of linearity and close tolerance. Some very close tolerance versions may be able to offer linearity tolerances of ±0.1%. These variable resistors are also stable over a wide temperature range.

Two main disadvantages with the wire wound variable resistor

The first is that often as the slider moves over the wires, the resistance changes have discrete steps. This may not be a problem in many applications, but it is a point to note.

The second is that they are not suitable even for low frequency RF applications as the resistance wire forms a coil and has significant inductance.

Cermet variable resistors

-are widely used, particularly for trimmer resistors. The name cermet is derived from the fact that the resistive element is made from CERamic and METal. The resistive element is made from a mixture of fine metal oxides or precious metal particles and glass in a viscous organic material. The resulting paste is applied to the substrate and fired to solidify the mixture.

Cermet variable resistors are ideal for trimmer resistors because they have a low to medium adjustment life, and they often have temperature coefficients of around ±100ppm/°C.

Carbon composition variable resistors

-For the carbon composition variable resistor, a mixture of carbon powder and a binder are molded under heat into the required shape. In some manufacturing processes the carbon composition element is molded at the same time as the plastic substrate.

Carbon composition variable resistors are some of the least expensive types and they are widely used in many areas - they are a good all round general purpose variable resistor. 

VARIABLE RESISTOR

TYPE

TYPICAL RESISTAN

CE RANGES

TYPICAL TOLERAN

CE

TYPICAL POWER

HANDLING

TYPICAL LIFE

(ROTATIONS)

TYPICAL TEMPERAT

URE COEFFICIE

NT(PPM/°K)

Wirewound 10Ω - 50kΩ

±5% up to 1 Watt

500 ±50

Cermet 50Ω - 2MΩ

±10% 500mW 200 100

Conductive Plastic

50Ω - 2MΩ

±10% 250mW 100 000 500

Carbon composition

50Ω - 2MΩ

±20% 250mW 1000 ~±10%/°K

Rheostat

3. Rheostat -is an adjustable or variable resistor. It is used to control the electrical resistance of a circuit without interrupting the flow of current. Rheostat has 3 terminals and usually consist of a resistive wire wrapped to form a toroid coil with a wiper that slides along the surface of the coil. It is most often designed with a ceramic core. Rheostats are used in applications that require high voltage and current.

As a result of the reduction of size and power consumption in many of today’s electrical devices rheostats, once are very common item in commercial and industrial products, have been replaced by potentiometers. The most common uses today for rheostats are as light dimmers and motor speed controls. They are often used in arc lamps, pumps, fans and blowers, respirators, dental and medical equipment and model trains.

Tapped Resistor

4. Tapped Resistor

A wire wound fixed resistor having one or more additional terminals along its length, generally for voltage-divider applications

THERMISTOR

5. Thermistor

-is a thermally sensitive resistor whose resistance value changes with changes in operating temperature. Because of the self-heating effect of current in a thermistor, the device changes resistance with changes in current. It exhibits either a positive temperature coefficient (PTC) or a negative temperature coefficient (NTC).

If a thermistor has a positive temperature coefficient, its resistance increases as the operating temperature increases. Conversely, if a thermistor has a negative temperature coefficient, its resistance decreases as the operating temperature increases.

Thermistors are frequently used in electronic circuits that handle temperature measurement, temperature control, and temperature compensation. In digital thermometers, it is used to measure oil and coolant temperatures, and also used in ovens and refrigerators, but they are also found in almost any application that requires heating or cooling protection circuits for safe operation.

The thermistor symbols are: 

The arrow by the T signifies that the resistance is variable based on temperature. The direction of the arrow or bar is not significant. 

How does the thermistor "read" temperature?

A thermistor does not actually "read" anything, instead the resistance of a thermistor changes with temperature. How much the resistance changes depends on the type of material used in the thermistor

Light Dependent Resistor (LDR)

An LDR is a component that has a (variable) resistance that changes with the light intensity that falls upon it. This allows them to be used in light sensing circuits.

Hence, they are light sensitive devices. They are also called as photo conductors, photo conductive cells or simply photocells. They are made up of semiconductor materials having high resistance.

There are many different symbols used to indicate a LDR, one of the most commonly used symbol is shown in the figure below. The arrow indicates light falling on it.

Voltage Dependent Resistor

7.Voltage Dependent Resistor

Varistor are voltage dependent, nonlinear device which have an electrical behavior similar to back-to-back zener diodes. It provide reliable and economical protection against high voltage transients and surges which may be produced, for example, by lighting, switching or electrical noise on AC or DC power lines. They have the advantage over transient suppressor diodes in as much as they can absorb much higher transient energies and can suppress positive and negative transients.

When a transient occurs, the varistor resistance changes from a very high stand-by value to a very low conducting value. The transient is thus absorbed and clamped to a safe level, protecting sensitive circuit components. Varistor are manufactured from a non-homogeneous material, giving a rectifying action at the contact points of two particles. Many series and parallel connections determine the voltage rating and the current capability of the varistor

A common element in electronic devices is a three-terminal resistor with a continuously adjustable tapping point controlled by rotation of a shaft or knob. These variable resistors are known as potentiometers when all three terminals are present, since they act as a continuously adjustable voltage divider. A common example is a volume control for a radio receiver.

Accurate, high-resolution panel-mounted potentiometers (or "pots") have resistance elements typically wire wound on a helical mandrel, although some include a conductive-plastic resistance coating over the wire to improve resolution. These typically offer ten turns of their shafts to cover their full range. They are usually set with dials that include a simple turns counter and a graduated dial. Electronic analog computers used them in quantity for setting coefficients, and delayed-sweep oscilloscopes of recent decades included one on their panels.

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