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The following article was published in our article directory on July 26, 2012.
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Rheostats at a glance

Article Category: Computers and Technology

Author Name: Galco Industrial Electronics

Rheostats

Rheostats are variable or adjustable resistors used to divide voltage in applications that require the adjustment of resistance in an electrical circuit. Rheostats, like potentiometers, are generally used in applications that demand variable control for higher amounts of voltage.
As a result of the reduction of size and power consumption in many of today's electrical devices, rheostats, are not as frequently found in commercial and industrial products and have been replaced by potentiometers, triacs and SCRs. However, rheostats are still commonly used for applications requiring a large amount of current or high voltage. Common utilization of rheostats include light dimmers, motor speed regulators, arc lamps, pumps, fans, respirators, x-ray devices and medical apparatus.

Rheostat Construction.
Many rheostats are wire-wound and have a lengthy conductive material (usually a wire) wrapped into a spiral. Its resistance element can be a ribbon or metal wire, a conducting liquid, or carbon, completely dependent on the application. For average current flow, the metallic type is most common; for very small current flow, the carbon form is used; and for large amounts of current the electrolytic type, whereby electrodes are placed in a conducting fluid, is most suited. The contacts and the coil are sealed within the casing to sheild them from dirt, grime, dust and other impurities. These often lead to an open circuit and form moisture ,which ultimately, causes a short circuit. Rheostats have 3 terminals that are made up of a resistive wire wrapped to create a toroid coil with a wiper that glides along the surface of the coil. It is most often designed with a ceramic center.
When choosing a rheostat for a certain function, the current rating is usually a more crucial factor than the wattage rating. If using a rheostat for motor control it is essential to understand that all types of direct current motors could be speed-controlled, however, a small amount of AC motors are controllable with a rheostat. Therefore, it is essential to obtain the appropriate type of AC motor when speed control is needed. Most rheostats have either a round or flat shaft that enables you to attach a knob to the rheostat. The smaller sized rheostats have screw-driver slots that make for easy adjustments.
Switches can be attached to a rheostat in order to open the circuit or to access a self-regulating circuit. Rheostats can be equipped with a fixed or an adjustable stop. The stop can be set to limit the angle of rotation to any desired part of the total possible rotation. Normally, this specific type of rheostat is used in operations where keeping a set amount of resistance within the circuit is very important.

How Rheostats Work.
The basic fundamentals that apply to the effect of a rheostat in a circuit is Ohm's law, which explains that, current is inversely proportional to resistance for a given voltage. This means the current diminishes as the resistance escalates or current increases as the resistance decreases. Current enters the rheostat by means of one of its terminals, flows through the wire coil and contact, and exits through the other terminal. Rheostats do not have polarity and behave the same when the terminals are reverted.

Applications.
Some light dimmers use rheostats to limit the current going through a light bulb so as to vary the illumination. The more the resistance of the rheostat, the dimmer the light bulb will glow. Some lights will not work effectively with dimmers, including fluorescent and gas discharge lamps. These lights use resistive, inductive or electronic ballasts which restrict and maintain a constant current for the lighting circuit. Rheostats may have no effect on varying the brightness of the lamps and may even harm the ballasts.
Motor controllers also use rheostats to control a motor's velocity by limiting the flow of current. They are used in many small devices such as blenders, mixers, fans, and power tools. Rheostats are also used as test devices to provide a reference resistance value. While rheostats can be used to manage electric ovens and cook-tops, thermostats or temperature controllers are favored because they have additional controls and temperature feedback capabilities that enable a more consistent temperature regulation.
The rheostat is still a common and fundamental electronic component used to control the outflow of current in a circuit. However, it has largely been superseded by solid state devices such as triacs and silicon controlled rectifiers (SCR). A triac is more energy competent than a rheostat and is more reliable because of the absence of mechanical components. Rheostats commonly fail because their contacts become unclean or the coil wire corrodes and breaks, while solid state devices are reasonably immune to these external conditions.

Rheostat vs Potentiometers.
Both rheostats and potentiometers offer the same function in an electronic circuit, with the primary variance in terminology being used to describe the current capacity and composition of the devices. Rheostats are typically rated at 5 watts or more and are wire-wound, while potentiometers are usually less than 5 watts and are of carbon composition. Through their diversified structures, potentiometers and rheostats are utilized in different applications where voltage, current, range, tolerance, operating AC and DC voltage and environment are key factors. Potentiometers and rheostats also differ in regards to performance specifications, mounting, packaging and attributes.

Keywords: rheostats, potentiometers, variable resistors, adjustable resistors, electronic components

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