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Resistor Basics and Electrical Resistance

1. Definition:

A resistor is a circuit element designed to reduce or limit current flow in a circuit. All conductors of electricity offer opposition to the movement of electrons and, therefore, possess resistance. A resistor offers a certain amount of opposition and is used for this purpose.

2. Rating of Resistors:

a. Resistance value in OHMS
b. Value tolerance in percent
c. Power handling capability in WATTS

The resistance value may vary from a fraction of an ohm to several million ohms. The tolerance ranges from 0.1% to 20%. The power rating of various resistors varies from 1/4 watt to several hundred watts and is determined by the physical size of a particular type.

3. Construction:

a. Composition
1. fixed value
2. variable

b. Wire wound
1. fixed value
2. variable

Composition resistors are made of very fine carbon powder, and formed with a bonding agent into the desired shape, with connecting wire attached. The fixed types are then enclosed in a bakelite casing. Variable composition types have a thin layer of the carbon compound applied to a strip of insulating material and a sliding or wiping contact is made to travel along the surface to permit variations in the amount of resistance inserted in the circuit.

Wire wound types are constructed of very fine, high resistance wire, often enameled, wound around an insulating core. If close spacing is required, enameled wire must be used to provide insulation between turns. A sliding contact is used to obtain variation in resistance values. Enamel must be removed from the outer surface of the turns to permit contact with the sliding contact.

4. Temperature Effect:

Since the flow of electrons through any medium will develop heat, all resistors will be subject to changes in temperature. The effect of these temperature variations will depend upon the type of construction and is known as the Temperature Coefficient.

Temperature coefficient indicates how temperature changes affect the resistor's value and may be either positive or negative. In general, composition resistors have a negative temperature coefficient and metallic or wire wound resistors have a positive coefficient. This means that composition resistors will decrease in resistance with an increase in temperature, while metallic types will increase in resistance with an increase in the temperature. A LOW temperature coefficient indicates that the change in resistance per degree of temperature change is slight. High quality resistors have a low temperature coefficient; some even zero. These, of course, are most desirable, especially in precision work.

5. Color Code:

Most composition resistors have their ohmic value indicated by a series of colored bands around the body of the unit. The colors used are those designated by the Electronic Industries Association (E.I.A.), and have been universally adopted. You must become familiar with this code.

Specific Information:

1. Composition Resistors:

This type is most commonly found in electronic circuits. The element's resistive material is molded into a small rod or deposited upon an insulating core. Wire leads are coaxially attached to each end of the element and an outside covering of natural bakelite is applied for insulation. The resistance value is marked on the body, using the E.I.A. color code. Resistance values range from a fraction of an ohm to several million ohms. Exact values are difficult to manufacture and usually are not required, therefore, tolerance limits of 5% and 10% are often used. In each tolerance group only certain preferred values are made so no overlapping of values is possible due to normal manufacturing variation. Exact value precision resistors are available for applications where extreme accuracy is required. Common power ratings are in the range from 1/4 watt to 2 watts, physical size increasing with required wattage. Actual power dissipation in service should not exceed 50% of the rating for good stability. Since power is dissipated in the form of heat this is a good rule to follow, because excessive heat will result in a decrease in resistance. Overheating may cause permanent damage to the resistor. Care must be exercised in soldering these units in place to prevent their being over heated. Excessive heating will cause discoloration of the resistor body and the color code stripes. For precision applications, a low wattage I% composition type made of pure carbon deposited in a spiral groove on a ceramic rod is also made. Resistance values are generally marked on the body in English. The physical size of these precision resistors may vary between manufacturers and may often be misleading as they are somewhat larger for a given rating than the common composition type. Their cost is several times that of the ordinary composition resistor.

2. Wire Wound Resistors:

This type is generally used when a considerable amount of power is to be dissipated, as for example, in power supplies as voltage dividers and bleeder resistors. They are constructed by winding the required number of turns of high resistance wire upon an insulating core of the desired shape and dimensions and then covering the exterior with an insulating material such as vitreous enamel, cement, asbestos or molded bakelite. The core material may be bakelite, mica, ceramic, glass or other high quality insulating materials. Some types, such as the Candohm are enclosed in tight fitting metal containers which may be riveted to the equipment chassis to facilitate the dissipation of heat. Some wire wound resistors are provided with a non insulated section along one side to permit a sliding contact to be used to select any desired value of resistance. Other types are permanently tapped at the required points. A resistor of this type may have several connection points along its length. Resistance values in ohms are usually printed on the body. Wire wound resistors are made in values ranging from a fraction of an ohm to several thousand ohms, but due to their relatively large physical size, cost and other manufacturing problems, high values are not made. Power ratings up to several hundred watts are possible, the size increasing in proportion to the increase in power rating. Wire wound resistors have a positive temperature coefficient, which means that the resistance increases as the resistor becomes heated. This change in resistance is quite small; but care should be exercised to keep any resistor as cool as possible for best stability of resistance. They should be mounted in a well ventilated position and should be at least capable of twice the power dissipation required. In other words, if the calculations show that 5 watts will be dissipated, the resistor used should be rated at no less than 10 watts. This rule should be followed invariably, although overheating is more likely to result in permanent damage to the composition resistor than to a wire wound unit.

3. Potentiometers and Rheostats:

These units are adjustable, or variable resistors, constructed so that the resistance may be varied continuously or in pre determined steps without opening the circuit. The resistive element may be either the wire wound or composition type, depending upon the purpose for which it is intended. This element is enclosed in a circular housing, and a contacting arm attached to a rotatable shaft is arranged so that it can be moved over the surface of the element to select the desired resistance value. The potentiometer has three terminals, one at each end of the element and the third for connection to the rotating arm. This unit is used when variable voltage changes are required in a circuit. Volume controls, tone controls, contrast and brightness controls are examples of application of the potentiometer. Potentiometers which use composition elements frequently have "tapered" resistance values. Tapering means that resistance variations per degree of rotation of the arm may be greater at some points than at others. Tapered units have the resistance material lumped at low resistance positions and spread out in high resistance areas. Different tapers are available for several particular requirements. The reasons for this will become apparent later in your studies. Resistance values of composition potentiometers may be as high as 10 megohms. Safe power dissipation ratings are generally low, so the current flow must be kept at the lowest possible value.

Rheostats have only two terminals for connection, one at one end of the element, the other to the control arm. They are used to control current flow in circuits and due to the higher amounts of current and resultant higher power, are of wire wound construction. Resistance values are seldom in excess of a few thousand ohms. Power ratings are available to meet any reasonable requirement. Care must be exercised in the use of rheostats that their power rating is not exceeded just as in the case of other types of wire wound resistors.

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