RESISTORS_ramesh.pdf

Resistors: Classification and characteristics EC04 403 Electronic Circuits

<Ramesh K.> <MEAEC> 1

I. RESISTORS The resistor is an electrical device whose primary function is to introduce resistance to the flow of electric current. The magnitude of opposition to the flow of current is called the resistance of the resistor. A larger resistance value indicates a greater opposition to current flow. The resistance is measured in ohms.

An ohm is the resistance that arises when a current of one ampere is passed through a resistor subjected to one volt across its terminals.

The various uses of resistors include setting biases, controlling gain, fixing time constants, matching and loading circuits, voltage division, and heat generation. The following sections discuss resistor characteristics and various resistor types. 1.1 Resistor Characteristics and Specifications Voltage and Current Characteristics of Resistors The resistance of a resistor is

directly proportional to the resistivity of the material and the length of the resistor, and

Inversely proportional to the cross-sectional area perpendicular to the direction of current flow. The resistance R of a resistor is given by

where r is the resistivity of the resistor material (W· cm),l is the length of the resistor along direction of current flow (cm), and A is the cross-sectional area perpendicular to current flow (cm2) . Resistivity is an inherent property of materials. Good resistor materials typically have resistivities between 2´10-6 and 200´10-6 W·cm. The resistance of a resistor can be defined in terms of the voltage drop across the resistor and current through the resistor related by Ohm’s law,

Where R is the resistance (W), Vis the voltage across the resistor (V), and I is the current through the resistor (A). Whenever a current is passed through a resistor, a voltage is dropped across the ends of the resistor. Figure 1.3 depicts the symbol of the resistor with the Ohm’s law relation. All resistors dissipate power when a voltage is applied. The power dissipated by the resistor is represented by

Where P is the power dissipated (W), V is the voltage across the resistor (V), and R is the resistance (W). An ideal resistor dissipates electric energy without storing electric or magnetic energy. Symbol: Fixed Variable:

R R



Specifications

Ohmic Value: It is the resistance value of the resistor, expressed in ohms, Kohms, or Mohms.

Tolerance: Tolerance indicates the accuracy. It is generally expressed in the + % deviation from the resistance value.

Power Rating: It is the maximum power in watts that the resistor can safely dissipate.

Temperature Coefficient: Resistance of the material will change with temperature. Temperature coefficient indicates the rate at which the resistance of the resistor changes with temperature effect. It is usually expressed in parts per million per unit temperature change (ppm/o

C)

Voltage Coefficient: Values of resistors will have a slight change with the applied voltage across it. It is generally found as a decrease with increase in voltage. Voltage coefficient of a resistor respresents the possible variations in resistance value due to voltage changes. It is defined as the change of resistance of resistor per ohm per volt and is expressed in ppm%

where R1 =R @V1 volts and R2 =R@V2 volts.

Voltage rating: This is the max voltage that can be applied to a resistor without any damage.

Noise: The thermal agitation produced in the resistor while current passes through it. This type of noise is generally referred as White Noise. The noise is measured in µV.

Stability: Stability represents the change in resistance value, which occurs, under specified physical and electrical operating conditions. Expressed in %.

Reliability: It gives the % of failure per 100 hours of use; 1.2 Color Coding of Resistors Resistors are generally identified by color coding or direct digital marking. The color code is given in Table 1.1. The color code is commonly used in composition resistors and film resistors. The color code essentially consists of four bands of different colors. The first band is the most significant figure, the second band is the second significant figure, the third band is the multiplier or the number of zeros that have to be added after the first two significant figures, and the fourth band is the tolerance on the resistance value. If the fourth band is not present, the resistor tolerance is the standard 20% above and below the rated value. When the color code is used on fixed wire-wound resistors, the first band is applied in double width. Two types of color coding system: Four band and Five band Systems. The five band system is similar to four band system except that the first three colour bands represents the first three significant digits of the resistance value. Examples: a. The value and tolerance of a resistor, R, with following colour bands Brown, black, red and gold From the table below; Brown (1st digit)=1, black(2nd digit)=0, red(multiplier)=2; gold(tolerance)=+5%

Hence R=10 x 102 ±5%=1x103Ω±5%=1kΩ±5% b. Brown, black, gold, gold R=10x100.1+5%=1 Ω+5%



1.2 Resistance Types: Resistors can be broadly categorized as fixed, variable, and special-purpose. Each of these resistor types is discussed in detail with typical ranges of their characteristics.

1.2.1 Fixed Resistors The fixed resistors are those whose value cannot be varied after manufacture. Fixed resistors are classified into composition resistors, wire-wound resistors, and film resistors. Table 1.2 outlines the characteristics of some typical fixed resistors.

a. Composition resistors

Carbon Resistors are the most common type of Composition Resistors as they are a cheap general purpose resistor. Their resistive element is manufactured from a mixture of finely ground carbon dust or graphite (similar to pencil lead) and a non-conducting ceramic (clay) powder to bind it all together. The ratio of carbon to ceramic determines the overall resistive value of the mixture and the higher this ratio is the lower the resistance. The mixture is then moulded into a cylindrical shape and metal wires or leads are attached to each end to provide the electrical connection before being coated with an outer insulating material and colour coded markings. Composition resistors are economical and exhibit low noise levels for resistances above 1 MW. Composition resistors are usually rated for temperatures in the neighborhood of 70°C for power ranging from 1/8 to 2 W. Composition resistors have end-to-end shunted



capacitance that may be noticed at frequencies in the neighborhood of 100 kHz, especially for resistance values above 0.3 MW.

b. Film Resistors

The generic term "Film Resistor" consist of Metal Film, Carbon Film and Metal Oxide Film resistor types, which are generally made by depositing pure metals, such as nickel, or an oxide film, such as tin-oxide, onto an insulating ceramic rod or substrate. The resistive value of the resistor is controlled by increasing the desired thickness of the film and then by laser cutting a spiral helix groove type pattern into this film. This has the effect of increasing the conductive or resistive path, a bit like taking a long length of straight wire and forming it into a coil.

This method of manufacture allows for much closer tolerance resistors (1% or less) as compared to the simpler carbon composition types. The tolerance of a resistor is the difference between the preferred value (i.e, 100 ohms) and its actual manufactured value i.e, 103.6 ohms, and is expressed as a percentage, for example 5%, 10% etc, and in our example the actual tolerance is 3.6%. Film type resistors also achieve a much higher maximum ohmic value compared to other types and values in excess

of10MΩ (10 Million Ω´s) are available.

Metal Film Resistors have much better temperature stability than their carbon equivalents, lower noise and are generally better for high frequency or radio frequency applications. Metal Oxide Resistors have better high surge current capability with a much higher temperature rating than the equivalent metal film resistors.

Another type of film resistor commonly known as a Thick Film Resistor is manufactured by depositing a much thicker conductive paste of CERamic and METal, called Cermet, onto an alumina ceramic substrate. Cermet resistors have similar properties to metal film resistors and are generally used for making small surface mount chip type resistors, multi-resistor networks in one package for pcb's and high frequency resistors. They have good temperature stability, low noise, and good voltage ratings but low surge current properties.

Metal Film Resistors are prefixed with

a "MFR" notation (eg MFR100kΩ) and a CF for Carbon Film types. Metal film resistors are available in E24 (±5% & ±2% tolerances), E96 (±1% tolerance) and E192 (±0.5%, ±0.25% & ±0.1% tolerances) packages with power ratings of 0.05 (1/20th) of a Watt up to 1/2 Watt. Generally speaking Film resistors are precision low power components.



c. Wirewound Resistors

Another type of resistor, called a Wirewound Resistor, is made by winding a thin metal alloy wire (Nichrome) or similar wire onto an insulating ceramic former in the form of a spiral helix similar to the Film Resistors. These types of resistors are generally only available in very low ohmic high precision values

(from 0.01 to 100kΩ) due to the gauge of the wire and number of turns possible on the former making them ideal for use in measuring circuits and Whetstone bridge type applications. They are also able to handle much higher electrical currents than other resistors of the same ohmic value with power ratings in excess of 300 Watts. These high power resistors are moulded or pressed into an aluminum heat sink body with fins attached to increase their overall surface area to promote heat loss. These types of resistors are called "Chassis Mounted Resistors". They are designed to be physically mounted onto heat sinks or metal plates to further dissipate the generated heat increasing their current carrying capabilities even further.

Wirewound resistor types are prefixed with a "WH" or "W" notation (eg WH10Ω) and are available in theWH Aluminium Cladded package (±1%, ±2%, ±5% & ±10% tolerance) or the W Vitreous Enamelled package (±1%, ±2% & ±5% tolerance) with power ratings from 1W to 300W or more.

1.2.2 Variable Resistors

Variable resistors are resistors whose resistance can be changed to any value between zero and a certain maximum value. Ad in case of fixed resistors variable resistors are also of carbon composition type and wire wound type. These include potentiometers, preset potentiometer and rheostat.

a. Carbon Potentiometer The potentiometer is a special form of variable resistor with three terminals. Two terminals are connected to the opposite sides of the resistive element, and the third connects to a sliding contact that can be adjusted as a voltage divider.

Potentiometers are usually circular in form with the movable contact attached to a shaft that rotates. Potentiometers are manufactured as carbon composition, metallic film, and wire-wound resistors available in single-turn or multi-turn units. The movable contact does not go all the way toward the end of the resistive element, and a small resistance called the hop-off resistance is present to prevent accidental burning of the resistive element.

Symbol



b. Rheostat The rheostat is a current-setting device in which one terminal is connected to the resistive element and the second terminal is connected to a movable contact to place a selected section of the resistive element into the circuit. Typically, rheostats are wire-wound resistors used as speed controls for motors, ovens, and heater controls and in applications where adjustments on the voltage and current levels are required, such as voltage dividers and bleeder circuits.

Symbol of rheostat

c. Presets These are miniature versions of the standard variable resistor. They are designed to be mounted directly onto the circuit board and adjusted only when the circuit is built. For example to set the frequency of an alarm tone or the sensitivity of a light-sensitive circuit. A small screwdriver or similar tool is required to adjust presets. Presets are much cheaper than standard variable resistors so they are sometimes used in projects where a standard variable resistor would normally be used.

1.2.3 Special-Purpose Resistors Integrated Circuit Resistors: Integrated circuit resistors are classified into two general categories: semiconductor resistors and deposited film resistors. Semiconductor resistors use the bulk resistivity of doped semiconductor regions to obtain the desired resistance value. Deposited film resistors are formed by depositing resistance films on an insulating substrate which are etched and patterned to form the desired resistive network. Depending on the thickness and dimensions of the deposited films, the resistors are classified into thick-film and thin-film resistors. Varistors: Varistors are voltage-dependent resistors that show a high degree of nonlinearity between their resistance value and applied voltage. They are composed of a nonhomogeneous material that provides a rectifying action. Varistors are used for protection of electronic circuits, semiconductor components, collectors of motors, and relay contacts against overvoltage. The relationship between the voltage and current of a varistor is given by

where V is the voltage (V), I is the current (A), and k and b are constants that depend on the materials and manufacturing process. The electrical characteristics of a varistor are specified by its b and k values.

Thermistors: Thermistors are resistors that change their resistance exponentially with changes in temperature. If the resistance decreases with increase in temperature, the resistor is called a negative temperature coefficient (NTC) resistor. If the resistance increases with temperature, the resistor is called a positive temperature coefficient (PTC) resistor.

Symbol

Documents

RESISTORS_ramesh.pdf