Rheostat

 
Rheostat

What is a rheostat? A rheostat is a variable resistor which is used to control current. They are able to vary the resistance in a circuit without interruption. The construction is very similar to the construction of a potentiometers. It uses only two connections, even when 3 terminals (as in a potentiometer) are present. The first connection is made to one end of the resistive element and the other connection to the wiper (sliding contact).  In contrast to potentiometers, rheostats have to carry a significant current. Therefore they are mostly constructed as wire wound resistors. Resistive wire is wound around an insulating ceramic core and the wiper slides over the windings. Rheostats were often used as power control devices, for example to control light intensity (dimmer), speed of motors, heaters and ovens. Nowadays they are not used for this function anymore. This is because of their relatively low efficiency. In power control applications they are replaced by switching electronics. As a variable resistance they are often used for tuning and calibration in circuits. In these cases they are adjusted only during fabrication or circuit tuning (preset resistor). In such cases trimpots are often used, wired as a rheostat. But dedicated 2 terminal preset resistors also exist. Rheostat definition  A rheostat is a variable resistor which is used to control the current flowing in a circuit. Types of rheostats Several types of rheostats exist. The rotary type is the most used in power control applications. Most of the time these rheostats are using an open construction, but enclosed types are also available. Just as with potentiometers, multi-gang types are also available. They are used to control multiple applications in parallel or to increase the power rating or adjusting range. Optionally rheostats can be equipped with a mechanical stop to limit the minimum or maximum resistance. For [… read more]

Variable resistor

 
Variable resistor

What is a variable resistor? A variable resistor is a resistor of which the electric resistance value can be adjusted. A variable resistor is in essence an electro-mechanical transducer and normally works by sliding a contact (wiper) over a resistive element. When a variable resistor is used as a potential divider by using 3 terminals it is called a potentiometer. When only two terminals are used, it functions as a variable resistance and is called a rheostat. Electronically controlled variable resistors exist, which can be controlled electronically instead of by mechanical action. These resistors are called digital potentiometers. Variable resistor definition A resistor of which the ohmic resistance value can be adjusted. Either mechanically (potentiometer, rheostat) or electronically (digital potentiometer). Types of variable resistors Potentiometer The potentiometer is the most common variable resistor. It functions as a potential divider and is used to generate a voltage signal depending on the position of the potentiometer. This signal can be used for a very wide variety of applications including: Amplifier gain control(audio volume), measurement of distance or angles, tuning of circuits and much more. When variable resistors are used to tune or calibrate a circuit or application, trimmer potentiometers or trimpots are used, this are mostly small potentiometers mounted on the circuit board, which can be adjusted using a screwdriver. Rheostat Rheostats are very similar in construction to potentiometers, but are not used as a potential divider, but as a variable resistance. They use only 2 terminals instead of the 3 terminals potentiometers use. One connection is made at one end of the resistive element, the other at the wiper of the variable resistor. In the past rheostats were  used as power control devices in series with the load, such as a light bulb. Nowadays rheostats are not used as power control anymore [… read more]

Resistor types

 
Resistor types

There are a lot of different resistor types, all with their own applications, characteristics and construction. This page lists the different resistor types which are described on the resistor guide. Fixed resistors The fixed resistor type is the most common resistor, when people talk about a resistor they most probably mean a fixed resistor. The picture to the right shows an axial carbon film resistor, the most common type. Fixed resistors have a constant resistance value. Different resistor materials are used for fixed resistors. For all resistor types the used materials has influence on the resistor properties like the tolerance, cost and noise. Fixed resistors are available in axial and SMD packages. Variable resistors There are several variable resistor types, their general property is that the resistance value is adjustable. Most variable resistors are adjusted by mechanical movement (linear or rotary). When they are used as a variable voltage divider, they are called potentiometers. When they are used as a variable resistance to control the current in a circuit, they are called rheostats. Digital potentiometers are controlled electronically instead of by mechanical action. In general variable resistors have a higher tolerance of 20% compared to fixed resistors where 5% is the most common. Thermistors Thermistors are resistors of which the resistance changes significantly when temperature changes. Different thermistor types exist, the two most common types are the NTC and PTC thermistor. NTC thermistors decrease in resistance when the temperature rises, while PTC thermistors increase in resistance when the temperature rises. Thermistors are often used as temperature sensors or thermal protection devices. Varistors Varistors have a non-linear resistance which is dependent on the voltage over the varistor. Above a certain threshold voltage the resistance of a varistor drops very fast. Different varistor types exist, but nowadays the MOV varistor is the [… read more]