Metal oxide film resistor

 
Metal oxide film resistor

What are metal oxide film resistors? Metal-oxide film resistors are fixed form, axial resistors.  They are made of ceramic rod that is coated with a thin film of metal oxides, such as tin oxide. Metal oxide film resistors must not be confused with metal oxide varistors, made of zinc oxide or silicon carbide. Properties Metal oxide film resistors exceed the performance of metal film and carbon film for the following properties: power rating, voltage rating, overload capabilities, surges and high temperatures. Designers choose often the metal oxide film resistor for high endurance applications. For an overview of resistor types with their properties, look here. Stability properties are less good than for the metal film resistor. The metal oxide film resistors have poor properties for low values and tolerance. The temperature coefficient is around 300 ppm/°C, which is higher than for metal film types. The resistance material for metal oxide resistors is tin oxide that is contaminated with antimony oxide, this is to increase the resistivity. Metal oxide resistors can withstand higher temperatures than carbon or metal film resistors. The noise properties are similar to carbon resistors. Maximum operating temperature comparison Material Carbon film Metal film Metal oxide Temperature 200 °C / 390 °F 250-300 °C / 480-570 °F 450 °C / 840 °F Typical Applications Many properties of metal oxide film resistors are similar to metal film resistors. For basic use, metal film and metal oxide film are currently the predominant resistor types. Compared to carbon film, the prices are just as low. Only for dissipation values above 1 watt combined with reasonable stability, the carbon film resistors are still more cost efficient. Construction The metal oxide film is mostly produced with chemical deposition methods. Almost always a ceramic carrier is used as substrate. The deposition process involves the reaction [… read more]

Potentiometer

 
Potentiometer

What is a potentiometer? A potentiometer is a manually adjustable variable resistor with 3 terminals. Two terminals are connected to both ends of a resistive element, and the third terminal connects to a sliding contact, called a wiper, moving over the resistive element. The position of the wiper determines the output voltage of the potentiometer. The potentiometer essentially functions as a variable voltage divider. The resistive element can be seen as two resistors in series(potentiometer resistance), where the wiper position determines the resistance ratio of the first resistor to the second resistor. A potentiometer is also commonly known as a potmeter or pot. The most common form of potmeter is the single turn rotary potmeter. This type of pot is often used in audio volume control (logarithmic taper) as well as many other applications. Different materials are used to construct potentiometers, including carbon composition, cermet, wirewound, conductive plastic or metal film. Potentiometer definition A potentiometer is a manually adjustable, variable resistor with three terminals. Two terminals are connected to a resistive element, the third terminal is connected to an adjustable wiper. The position of the wiper determines the output voltage. Types of potentiometers A wide variety of potmeters exist. Manually adjustable potmeters can be divided in rotary or linear movement types. The tables below list the available types and their applications. Besides manually adjustable pots, also electronically controlled potentiometers exist, often called digital potmeters. Rotary potentiometers The most common type of potentiometer where the wiper moves along a circular path. Type Description Applications Single-turn pot Single rotation of approximately 270 degrees or 3/4 of a full turn Most common pot, used in applications where a single turn provides enough control resolution. Multi-turn pot Multiple rotations (mostly 5, 10 or 20), for increased precision. They are constructed either with a wiper [… read more]

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]

Fixed resistor

 
Fixed resistor

What is a fixed resistor? Fixed value resistors have a defined ohmic resistance and are not adjustable. Fixed resistors are the most commonly used resistors and in general one of the most used electronic components. Fixed resistors are available in axial leaded and surface mount packages as well as more customized packages depending on their application. While axial leaded resistors used to be the most used resistors, nowadays the advantages of surface mount devices make the SMD resistors the most popular. Fixed resistor definition A resistor having a fixed, defined electrical resistance which is not adjustable. In an ideal world a perfect resistor would have a constant ohmic resistance under all circumstances. This resistance would be independent of for example frequency, voltage or temperature. In practice no resistor is perfect and all resistors have a certain stray capacitance and inductance, resulting in an impedance value different from the nominal resistance value. The resistor materials have a certain temperature coefficient, resulting in a temperature dependency of the resistor value. The different resistor types and materials determine the dependency of the resistance value on these external factors. Depending on e.g. the required accuracy, power dissipation and noise requirements, the type and material of resistor are selected. Some common types of fixed resistors are displayed below. For a complete overview of the basic properties and applications of standard resistors, take a look at the article ‘What is a resistor‘. Identifying fixed value resistors In the following table an overview of general purpose resistors is given. The types listed here are among the most used resistors in general. The carbon film is the most common axial leaded resistor which is used for applications where a very good tolerance and temperature coefficient are not necessary. The metal film is the general axial leaded resistor of [… 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]