Saturday, March 10, 2007


Resistor is an electronic device that have function to reduce the electric current. The magnitude to the opposition to flow current is called resistance in the resistor. Resistance is measured in ohm. 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.
There are three type of resistor, fixed and variable, and special purpose resistor. Resistor also classified according to the material which they are made for. Usually resistor is made of carbon film or metal film. There are other thing that need to consider when selecting a resistor, resistor tolerance and power rating. Resistor tolerance specified in % and maximum power rating of resistor is specified in watt.

Fixed resistor
Composition Resistors
Composition resistors are composed of carbon particles mixed with a binder. This mixture is molded into a cylindrical shape and hardened by baking. Leads are attached axially to each end, and the assembly is encapsulated in a protective encapsulation coating. Color bands on the outer surface indicate the resistance value and tolerance. 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.

Metal film resistors
Metal film resistors are used when a higher tolerance (more accurate value) is needed. They are much more accurate in value than carbon film resistors. They have about ±0.05% tolerance. They have about ±0.05% tolerance. Resistors that are about ±1% are more than sufficient. Metal-film resistors are commonly made of nichrome, tin-oxide, or tantalum nitride, either hermetically sealed or using molded-phenolic cases. The metal film resistor is used for bridge circuits, filter circuits, and low-noise analog signal circuits.

Wire-Wound Resistors
Wire-wound resistors are made by winding wire of nickel-chromium alloy on a ceramic tube covering with a vitreous coating. The spiral winding has inductive and capacitive characteristics that make it unsuitable for operation above 50 kHz. The frequency limit can be raised by non inductive winding so that the magnetic fields produced by the two parts of the winding cancel.

Variable Resistors
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.

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.

Special-Purpose Resistors
LDR (Light Dependent Resistor)
LDR is a device whose resistance changes in response to the amount of light falling on it. An LDR's resistance value in the presence strong light is just a few ohms, but in the absence of light the value can be many tens of mega ohms. It is important to note that they are not very linear in their response. The base material from which LDRs made is cadmium sulfide or lead sulphated.

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.

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