Rheostats - Variable Resistor and Potentiometers
How Rheostats are Made
Rheostats are variable resistors 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. Rheostats, in large, have been replaced by potentiometers, triacs and SCRs.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.
Construction of Rheostats
Many rheostats are wire-wound and have a lengthy conductive material (usually a wire) wrapped into a spiral. The resistance element found in rheostats 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 in rheostats; 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 coils found inside rheostats are sealed within the casing to shield 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. Rheostats are most often designed with a ceramic center.
When choosing between rheostats 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 rheostats. 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, depending on different types of rheostats. The smaller sized rheostats have screw-driver slots that make for easy adjustments.
Rheostats generally allow a switch to be attached in order to open the circuit or to access a self-regulating circuit. Rheostats can be equipped with a fixed or an adjustable stop. Rheostats have a stop that can be set to limit the angle of rotation to any desired part of the total possible rotation. Normally, these specific types of rheostats are used in operations where keeping a set amount of resistance within the circuit is very important.
How Rheostats Work
Rheostats and their Applications
Some light dimmers use rheostats to limit the current going through a light bulb so as to vary the illumination. The higher amount of a rheostats resistance, the dimmer the light bulb will glow. Some lights will not work effectively with dimmers or rheostats, including fluorescent and gas discharge lamps. These lights use resistive, inductive or electronic ballasts which 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. Rheostats 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.
Rheostats are still a common and fundamental electronic component used to control the outflow of current in a circuit. However, rheostats have 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