Resistor Tutorial, series, parallel, surface mount, metal, power oxide

Notes on Resistor

A resistor is an electronics component which resists the flow of electric current. By resisting the flow of current it creates a potential difference (which is the difference between the two points). Resistance is measured in Ohm. According to Ohm's law the potential difference between two points is directly related to the resistance between the two points. In other words, V=IR, where V is the potential difference, R is the resistance between two points and I is the amount of current that passes through the points. In an ideal resistor the resistance remains constant at all times. However in a practical resistor the resistance value may change, and the resistor might be physically damaged under too much current or potential difference. A resistor also generates heat since it is working against the current. Resistors can be divided into two main categories: Fixed and Variable. A fixed resistor has a constant resistance. A fixed resistor might be cylindrical with the resistive material at the center or on the surface of the resistor. Power Resistors usually are wire-wound where they effectively dissipate heat. A smaller circuit such as that of computer have surface-mount technology where the electronics component are mounted directly onto a board. The resistors used for these are surface mount resistors.

A variable resistor's resistance can be changed by turning a shaft or sliding a control. A potentiometer or a rheostat is a resistor that has variable resistance. One of the places you might notice a variable resistor is in audio amplifiers where by turning the shaft you are changing the resistance of the circuit.

The resistors come in different sizes and power (watt) ratings. Resistors are rated in watts (power handling capability). This is because resistors generate heat. The higher this rating the more heat they can dissipate.

Some of the resistors that are widely available are:

Surface Mount
Carbon Film
Metal Film / Metal
Power Oxide
Wirewound

Resistors can be combined in a series or in parallel in circuit. When they are aligned such we can calculate the total resistance in the circuit using various methods.
If the resistors are in series then the total resistance is R = R1+R2+....+Rn.
If the resistor are in parallel then the total resistance is

Surface Mount: These resistors are printed with numerical values in a code related to that used on axial resistors. Standard-tolerance SMT resistors are marked with a three-digit code, in which the first two digits are the first two significant digits of the value and the third digit is the power of ten. For example, "574" represents "57" (the first two digits) multiplied by ten to the power "4" (the third digit), i.e. 57*10000=570000 ohms. Precision SMT resistors are marked with a four-digit code in which the first three digits are the first three significant digits of the value and the fourth digit is the power of ten. Fixed resistors are usually manufactured from these resistors.



Carbon film resistors: They are usually constructed by depositing a thin film of carbon on a small ceramic rod. This is the most general purpose, cheap resistor. Usually the tolerance of the resistance value is ±5%. Power ratings of 1/8W, 1/4W and 1/2W are frequently used. Carbon film resistors have a disadvantage; they tend to be electrically noisy. Metal film resistors are recommended for use in analog circuits.



Metal Film / Metal Resistor: They are usually constructed by depositing a thin film of metal on a small ceramic rod. The resistive coating is spiralled onto the rod systematically until the desired resistance is reached between the two ends of the rod. They have higher precision than the carbon film resistors. Metal leads and end caps are added at both ends of the rod. The resistor is covered with an insulating coating and finally painted with coloured bands to indicate the resistor value. The metal film resistor is used for bridge circuits, filter circuits, and low-noise analog signal circuits.



Power Oxide Resistors: The power oxide resistors are manufactured in a similar way to the metal film resistor. However oxide of metal is used to wrap the ceramic rod instead of the metal. For example tim-oxide resistors are made by depositing tin oxide onto a high purity ceramic rod. End caps are force fitted and termination wires are welded to the end caps.



Wirewound Resistors: They are constructed by wrapping the resistive element (in form of wire) is wound onto an insulating form. The wire is usually nichrome. Because the resistance can be controlled by the amound of wire that is wrapped around the rod, these can be manufactured to precise values.




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