Resistors

RESISTORSby

Lorenz Karlo AlfuenteEmmanuel Celso

MestidioJP Alegario

SSC II - Orion

Resistor•  is a component of an electrical circuit that resists the

flow of electrical current. A resistor has two terminals across which electricity must pass, and is designed to drop the voltage of the current as it flows from one terminal to the next. A resistor is primarily used to create and maintain a known safe current within an electrical component.

• Resistance is measured in ohms, after Ohm's law. This rule states that electrical resistance is equal to the drop in voltage across the terminals of the resistor divided by the current being applied to the resistor.

• NO RESISTOR, LARGE FLOW OF CURRENT

Ohmmeter• An ohmmeter is

an electrical instrument that measures electrical resistance, the opposition to an electric current. Micro-ohmmeters (microhmmeter or microohmmeter) make low resistance measurements. Megohmmeters (aka megaohmmeter or in the case of a trademarked device Megger) measure large values of resistance. The unit of measurement for resistance is ohms (Ω).

• Resistors are used with transducers to make sensor subsystems. Transducers are electronic components which convert energy from one form into another, where one of the forms of energy is electrical. A light dependent resistor or LDR is an example of an input transducer. Changes in the brightness of the light shining onto the surface of the LDR result in changes in its resistance. As will be explained later, an input transducer is most often connected along with a resistor to make a circuit called a potential divider. In this case, the output of the potential divider will be a voltage signal which reflects changes in illumination.

• Resistors are used for regulating current and they resist the current flow and the extent to which they do this is measured in ohms (Ω). Resistors are found in almost every electronic circuit. The most common type of resistor consists of a small ceramic (clay) tube covered partially by a conducting carbon film. The composition of the carbon determines how much current can pass through.

RESISTOR IN CIRCUIT = SMALLER FLOW OF CURRENT

CONSTRUCTION / TYPES

Carbon Film Resistor• A carbon film is deposited on an insulating

substrate, and a helix cut in it to create a long, narrow resistive path. Varying shapes, coupled with the resistivity of amorphous carbon, can provide a variety of resistances. Carbon film resistors feature a power rating range of 0.125 W to 5 W at 70 °C. Resistances available range from 1 ohm to 10 meg ohm.

Wirewound Resistor• Wire wound resistors are made by winding thin

wire onto a ceramic rod. They can be made extremely accurately for use in multimeters, oscilloscopes and other measuring equipment. Some types of wire wound resistors can pass large currents without overheating and are used in power supplies and other high current circuits.

Thick and Thin Film Resistor• Thin film resistors are made

by sputtering (a method of vacuum deposition) the resistive material onto an insulating substrate. The film is then etched in a similar manner to the old (subtractive) process for making printed circuit boards; that is, the surface is coated with a photo-sensitive material, then covered by a pattern film, irradiated with ultraviolet light, and then the exposed photo-sensitive coating is developed, and underlying thin film is etched away.

• Thick film resistors are manufactured using screen and stencil printing processes

Metal Film Resistor• A common type of axial resistor today is

referred to as a metal-film resistor. Metal electrode leadless face resistors (MELF) often use the same technology, but are a cylindrically shaped resistor designed for surface mounting. Metal film resistors are usually coated with nickel chromium (NiCr), but might be coated with any of the cermet materials listed above for thin film resistors. Unlike thin film resistors, the material may be applied using different techniques than sputtering (though that is one such technique). Also, unlike thin-film resistors, the resistance value is determined by cutting a helix through the coating rather than by etching (this is similar to the way carbon resistors are made.)

Metal Oxide Film• Metal-oxide film resistors are

made of metal oxides such as tin oxide. This results in a higher operating temperature and greater stability/reliability than Metal film. They are used in applications with high endurance demands.

Foil Resistor• The primary resistance element

of a foil resistor is a special alloy foil several micrometres thick. Since their introduction in the 1960s, foil resistors have had the best precision and stability of any resistor available. One of the important parameters influencing stability is the temperature coefficient of resistance (TCR). The TCR of foil resistors is extremely low, and has been further improved over the years. 

Ammeter Shunt Resistors• An ammeter shunt is a special type of

current-sensing resistor, having four terminals and a value in milliohms or even micro-ohms. Current-measuring instruments, by themselves, can usually accept only limited currents. To measure high currents, the current passes through the shunt, where the voltage drop is measured and interpreted as current. A typical shunt consists of two solid metal blocks, sometimes brass, mounted on to an insulating base. Between the blocks, and soldered or brazed to them, are one or more strips of low temperature coefficient of resistance (TCR) manganin alloy. 

Grid Resistors• In heavy-duty industrial high-current

applications, a grid resistor is a large convection-cooled lattice of stamped metal alloy strips connected in rows between two electrodes. Such industrial grade resistors can be as large as a refrigerator; some designs can handle over 500 amperes of current, with a range of resistances extending lower than 0.04 ohms. They are used in applications such as dynamic braking and load banking for locomotives and trams, neutral grounding for industrial AC distribution, control loads for cranes and heavy equipment, load testing of generators and harmonic filtering for electric substations.[

COLOR CODING

• Resistors are too small to have numbers printed on them and so they are marked with a number of colored bands. Each color stands for a number. Three color bands shows the resistors value in ohms and the fourth shows tolerance. Resistors can never be made to a precise value and the tolerance band (the fourth band) tells us, using a percentage, how close the resistor is to its coded value.

RESISTOR’S VALUE TOLERANCE

• The first band on a resistor is interpreted as the FIRST DIGIT of the resistor value. For the resistor shown below, the first band is yellow, so the first digit is 4

• The second band gives the SECOND DIGIT. This is a violet band, making the second digit 7. The third band is called the MULTIPLIER and is not interpreted in quite the same way. The multiplier tells you how many noughts you should write after the digits you already have. A red band tells you to add 2 noughts. The value of this resistor is therefore 4 7 0 0 ohms, that is, 4 700 , or 4.7 . Work through this example again to confirm that you understand how to apply the colour code given by the first three bands.

• The remaining band is called the TOLERANCE band. This indicates the percentage accuracy of the resistor value. Most carbon film resistors have a gold-coloured tolerance band, indicating that the actual resistance value is with + or - 5% of the nominal value.

E6 AND E12 SERIES

• Real resistor values (the E6 and E12 series)• To produce a sensible range of resistor values you need to increase the size of the

'step' as the value increases. The standard resistor values are based on this idea and they form a series which follows the same pattern for every multiple of ten.

• The E6 series (6 values for each multiple of ten, for resistors with 20% tolerance) 10, 15, 22, 33, 47, 68, ... then it continues 100, 150, 220, 330, 470, 680, 1000 etc. Notice how the step size increases as the value increases. For this series the step (to the next value) is roughly half the value.

• The E12 series (12 values for each multiple of ten, for resistors with 10% tolerance) 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, ... then it continues 100, 120, 150 etc. Notice how this is the E6 series with an extra value in the gaps.

• The E12 series is the one most frequently used for resistors. It allows you to choose a value within 10% of the precise value you need. This is sufficiently accurate for almost all projects and it is sensible because most resistors are only accurate to ±10% (called their 'tolerance'). For example a resistor marked 390 could vary by ±10% × 390 = ±39, so it could be any value between 351 and 429.

POWER RATING

• When current flows through a resistance, electrical energy is converted into heat. This is obvious in an electric torch where the lamp filament heats up and glows white hot. Although the result may be less evident or imperceptible, exactly the same process of energy conversion goes on when current flows through any electronic component.

• The power output of a lamp, resistor, or other component, is defined as the rate of change of electrical energy to heat, light, or some other form of energy. Power is measured in watts, W, or milliwatts, mW, and can be calculated from:

P= VIwhere P is power.

• What is the power output of a resistor when the voltage across it is 6 V, and the current flowing through it is 100 mA?

0.6 W of heat are generated in this resistor. To prevent overheating, it must be possible for heat to be lost, or dissipated, to the surroundings at the same rate

• A resistor's ability to lose heat depends to a large extent upon its surface area. A small resistor with a limited surface area cannot dissipate (=lose) heat quickly and is likely to overheat if large currents are passed. Larger resistors dissipate heat more effectively.

Resistors of different sizes

• The standard size of carbon film resistor used in most circuits has a power rating of 0.5 W. This means that a resistor of this size can lose heat at a maximum rate of 0.5 W. Some resistors are designed to pass very large currents and are cased in aluminum with fins to increase surface area and promote heat loss.

• Input and signal processing subsystems in electronic circuits rarely involve large currents, but power rating should be considered when circuits drive output transducers, such as lamps, LEDs, and loudspeakers.

END.

Thank you for listening!Presented by: Lorenz Alfuente, Emmanuel Mestidio and JP Alegario

Credits to: http://www.doctronics.co.uk/resistor.htmhttp://en.wikipedia.org/wiki/Resistorhttp://

www.kpsec.freeuk.com/components/resist.htmhttp://www.wisegeek.com/what-is-a-resistor.htmhttp://www.technologystudent.com/elec1/resist1.h

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