Trimpot

 
Trimpot

What is a trimpot? A trimpot or trimmer potentiometer is a small potentiometer which is used for adjustment, tuning and calibration in circuits. When they are used as a variable resistance (wired as a rheostat) they are called preset resistors. Trimpots or presets are normally mounted on printed circuit boards and adjusted by using a screwdriver. The material they use as a resistive track is varying, but the most common is either carbon composition or cermet. Trimpots are designed for occasional adjustment and can often achieve a high resolution when using multi-turn setting screws. When trimmer potentiometers are used as a replacement for normal potentiometers, care should be taken as their designed lifespan is often only 200 cycles. Trimpot definition Trimmer potentiometers and preset resistors are small variable resistors which are used in circuits for tuning and (re)calibration. Types of trimpots Several different versions of trimpots are available, using different mounting methods (through hole, smd) and adjusting orientations (top, side) as well as single and multi-turn variations. Single turn Single turn trimmers/presets are very common and used where a resolution of one turn is sufficient. They are the most cost effective variable resistors available. Multi turn For higher adjustment resolutions, multi-turn trimpots are used. The amount of turns varies between roughly 5-25, but 5, 12 or 25 turns are quite common. They are often constructed using a worm-gear (rotary track) or leadscrew (linear track) mechanism to achieve the high resolution. Because of their more complex construction and manufacturing, they are more costly than single turn preset resistors. The lead screw packages can have a higher power rating because of their increased surface area. Trimpot symbols The following IEC symbols are used for trimpots and preset resistors. Although this are the official symbols for occasionally adjusted resistors, the standard symbols for [… read more]

PTC thermistor

 
PTC thermistor

What are PTC thermistors? PTC stands for „Positive Temperature Coefficient“. PTC thermistors are resistors with a positive temperature coefficient, which means that the resistance increases with increasing temperature. PTC thermistors are divided into two groups, based on the materials used, their structure and the manufacturing process. The first group of PTC thermistors is comprised of silistors, which use silicon as the semiconductive material. They are used as PTC temperature sensors for their linear characteristic. The second group is the switching type PTC thermistor. This type of PTC thermistors is widely used in PTC heaters, sensors etc. Polymer PTC thermistors, made of a special plastic, are also in this second group, often used as resettable fuses. The switching type PTC thermistor has a highly nonlinear resistance-temperature curve. When the switching type PTC thermistor is heated, the resistance starts to decrease at first, until a certain critical temperature is reached. As the temperature is further increased above that critical value, the resistance increases dramatically. This article will focus on the switching type PTC thermistors. PTC thermistor definition A PTC thermistor is a thermally sensitive resistor whose resistance increases significantly with temperature. Characteristics of PTC thermistors Switching PTC thermistors are usually made of poly-crystalline ceramic materials that are highly resistive in their original state and are made semi-conductive by the addition of dopants. They are mostly used as PTC self-regulating heaters. The transition temperature of most switched PTC thermistors is between 60°C and 120°C. However, there are special application devices manufactured that can switch as low as 0°C or as high as 200°C. Silistors have a linear resistance-temperature characteristic, with a slope that is relatively small through most of their operational range. They may exhibit a negative temperature coefficient at temperatures above 150 °C. Silistors have temperature coefficients of resistance of about 0.7 to 0.8% °C. Transition temperature [… read more]

NTC thermistor

 
NTC thermistor

What are NTC thermistors? NTC stands for “Negative Temperature Coefficient”. NTC thermistors are resistors with a negative temperature coefficient, which means that the resistance decreases with increasing temperature. They are primarily used as resistive temperature sensors and current-limiting devices. The temperature sensitivity coefficient is about five times greater than that of silicon temperature sensors (silistors) and about ten times greater than those of resistance temperature detectors (RTDs). NTC sensors are typically used in a range from −55°C to 200°C. The non-linearity of the relationship between resistance and temperature exhibited by NTC resistors posed a great challenge when using analog circuits to accurately measure temperature, but rapid development of digital circuits solved that problem enabling computation of precise values by interpolating lookup tables or by solving equations which approximate a typical NTC curve. NTC thermistor definition An NTC thermistor is a thermally sensitive resistor whose resistance exhibits a large, precise and predictable decrease as the core temperature of the resistor increases over the operating temperature range.  Characteristics of NTC thermistors Unlike RTDs (Resistance Temperature Detectors), which are made from metals, NTC thermistors are generally made of ceramics or polymers. Different materials used result in different temperature responses, as well as other characteristics. Temperature response While most NTC thermistors are typically suitable for use within a temperature range between −55°C and 200°C, where they give their most precise readings, there are special families of NTC thermistors that can be used at temperatures approaching absolute zero (-273.15°C) as well as those specifically designed for use above 150°C. The temperature sensitivity of an NTC sensor is expressed as “percentage change per degree C”. Depending on the materials used and the specifics of the production process, the typical values of temperature sensitivities range from -3% to -6% per °C. As can be seen from the figure, the [… read more]

Magneto resistor

 
Magneto resistor

What is a magneto resistor Magneto resistors have a variable resistance which is dependent on the magnetic field strength. A Magneto resistor can be used to measure magnetic field presence, strength and direction. They are also known as magnetic dependent resistors (MDR). A magneto resistor is a subfamily of magnetic field sensors or magnetometers. Magneto resistor definition A magneto resistor is a resistor of which the electrical resistance changes when an external magnetic field is applied. Magneto resistor characteristics Magneto resistors make use of the magnetoresistance effect. This effect was first discovered in 1856 by William Thomson, also known as Lord Kelvin. The effect is noticed in ferromagnetic materials and dependent on the magnetic field strength and angle between the direction of electric current and the magnetic field. This effect is therefore known as anisotropic magnetoresistance (AMR). Other, more recently discovered magnetoresistance effects are the giant magnetoresistance effect (GMR), collosal magnetoresistance effect (CMR) and tunnel magnetoresistance effect (TMR). Because most conventional magneto resistors utilize the AMR effect, the other effects will not be discussed in this article. Permalloy, an alloy consisting of 81% nickel (Ni) and 19% iron (Fe) has a high anisotropic magneto resistance as well as a low magnetostriction (change in size due to magnetic fields) and therefore is a favorite material for magneto resistors. Magneto resistors are often constructed of long thin films of permalloy. To increase the sensitivity of a permalloy magneto resistor, shorting bars of aluminium or gold are placed on the thin permallow films under an angle of 45 degrees. This forces the current to flow in a direction of 45 degrees relative to the length of the film. This is called a barber pole configuration. a typical AMR magnetoresistive sensor is constructed of a combination of 4 permalloy thin film magnetoresistors, connected in [… read more]

Varistor

 
Varistor

What is a varistor? A varistor is a voltage dependent resistor (VDR). The resistance of a varistor is variable and depends on the voltage applied. The word is composed of parts of the words “variable resistor”. Their resistance decreases when the voltage increases. In case of excessive voltage increases, their resistance drops dramatically. This behavior makes them suitable to protect circuits during voltage surges. Causes of a surge can include lightning strikes and electrostatic discharges. The most common type of VDR is the metal oxide varistor or MOV. Definition Varistors are nonlinear two-element semiconductors that drop in resistance as voltage increases. Voltage dependent resistors are often used as surge suppressors for sensitive circuits. Packages Here are some examples of different packages which are often encountered. The block packages are used for higher power ratings. Characteristics A voltage dependent resistor has a nonlinear varying resistance, dependent on the voltage applied. The impedance is high under nominal load conditions, but will sharply decrease to a low value when a voltage threshold,  the breakdown voltage, is exceeded. They are often used to protect circuits against excessive transient voltages. When the circuit is exposed to a high voltage transient, the varistor starts to conduct and clamps the transient voltage to a safe level. The energy of the incoming surge is partially conducted and partially absorbed, protecting the circuit. The most common type is the MOV, or metal oxide varistor. They are constructed of a sintered matrix of zinc oxide (ZnO) grains. The grain boundaries provide P-N junction semiconductor characteristics, similar to a diode junction. The matrix of randomly oriented grains can be compared to a large network of diodes in series and parallel. When a low voltage is applied, only very little current flows, caused by the reverse leakage through the junctions. However when a [… read more]

Thermistor

 
Thermistor

What is a thermistor? A thermistor is a temperature sensitive resistor, they are often used as a temperature sensor. The term thermistor is a contraction of the words “thermal” and “resistor”.  All resistors have some dependency on temperature, which is described by their temperature coefficient. In most cases for (fixed or variable) resistors the temperature coefficient is minimized, but in the case of thermistors a high coefficient is achieved. Unlike most other resistors, thermistors usually have negative temperature coefficients (NTC) which means the resistance decreases as the temperature increases. These types are called NTC thermistors. Thermal resistors with a positive temperature coefficient are called PTC thermistors (Positive Temperature Coefficient). Thermistor definition A resistor whose resistance changes significantly with a change in temperature. Types and applications Thermistors are ceramic semiconductors. In most cases they are composed of metal oxides, which are dried and sintered to obtain the desired form factor. The types of oxides and additives determine their characteristic behavior. For NTC’s cobalt, nickel, iron, copper or manganese are common oxides. For PTC’s barium, strontium or lead titanates are commonly used. NTC thermistor The NTC type is used when a change in resistance over a wide temperature range is required. They are often used as temperature sensors in the range of -55°C to 200°C, although they can be produced to measure much lower of higher temperatures. Their popularity can be accounted to their quick response, reliability, robustness and low price. PTC thermistor The PTC type used when a sudden change in resistance at a certain temperature is required. They exhibit a sudden increase in resistance above a defined temperature, called the switch, transition of “Curie” temperature. The most common switching temperatures are in the range of 60°C to 120°C. They are often used for self-regulating heating elements and self-resetting over-current protection. [… read more]

Photo resistor

 
Photo resistor

What are photoresistors? Photo resistors, also known as light dependent resistors (LDR), are light sensitive devices most often used to indicate the presence or absence of light, or to measure the light intensity. In the dark, their resistance is very high, sometimes up to 1MΩ, but when the LDR sensor is exposed to light, the resistance drops dramatically, even down to a few ohms, depending on the light intensity. LDRs have a sensitivity that varies with the wavelength of the light applied and are nonlinear devices. They are used in many applications but are sometimes made obsolete by other devices such as photodiodes and phototransistors. Some countries have banned LDRs made of lead or cadmium over environmental safety concerns. Light dependent resistor definition Photo resistors are light sensitive resistors whose resistance decreases as the intensity of light they are exposed to increases. Characteristics Types of photo resistors and working mechanisms Based on the materials used, photo resistors can be divided into two types; intrinsic and extrinsic. Intrinsic photo resistors use undoped materials such as silicon or germanium. Photons that fall on the device excite electrons from the valence band to the conduction band, and the result of this process are more free electrons in the material, which can carry current, and therefore less resistance. Extrinsic photo resistors are made of materials doped with impurities, also called dopants. The dopants create a new energy band above the existing valence band, populated by electrons. These electrons need less energy to make the transition to the conduction band thanks to the smaller energy gap. The result is a device sensitive to different wavelengths of light. Regardless, both types will exhibit a decrease in resistance when illuminated. The higher the light intensity, the larger the resistance drop is. Therefore, the resistance of LDRs is [… read more]

Digital potentiometer

 
Digital potentiometer

What is a digital potentiometer? A digital potentiometer (also known as digital resistor) has the same function as a normal potentiometer but instead of mechanical action it uses digital signals and switches. This is done by making use of a ‘resistor ladder’, a string of small resistors in series. At every step of the ladder, an electronic switch is present. Only one switch is closed at the same time and in this way the closed switch determines the ‘wiper’ position and the resistance ratio. The amount of steps in the ladder determines the resolution of the digital pot. The diagram below shows the working principle of a digital potentiometer with 64 steps. Digital resistors can be controlled by using simple up/down signals or by serial protocols such as I²C or SPI. Digital potentiometer definition A digital potentiometer is a variable resistor which is controlled by digital signals instead of by mechanical movement. Properties of digital potentiometers Digital potentiometers are integrated circuit (ICs), some variants have a nonvolatile memory (EEPROM) which remembers the ‘wiper’ position. When there is no on-board memory, the initial position of the wiper is often the middle position. Because of their relatively small size compared to conventional potentiometers, multiple potentiometers can be packed on a chip and ICs with up to 6 channels are available. The amount of steps available determines the resolution of the digital potentiometer. The following table lists common step values available, including the bit count: Number of steps Bits 5 6 7 8 9 10 Steps 32 64 128 256 512 1024 Digital resistors are available in a range of values, but 10 kΩ is the most used. Other common values are 5, 50 and 100 kΩ. The standard tolerance is 20% but nowadays digital potentiometers with a tolerance down to 1% are [… 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]