Resistor Noise

 
Resistor Noise

What is resistor noise? Noise is an unwanted phenomenon for resistors. For some applications the noise properties are important. Examples are high gain amplifiers, charge amplifiers and low-level signals. Resistor noise is often specified as microvolts noise per volt of applied voltage, for a 1 MHz bandwidth. Thermal noise is the predominant source of noise for resistors.  It is dependent on three variables: resistance, temperature and bandwidth. The relation between these three parameters is describes by the formula: Where E is the RMS noise signal in volts, R is the resistance in ohms, k is Boltzmann’s constant, T is the temperature in Kelvin and dF is the bandwidth in Hz. The equation shows that the noise level can be decreased by reducing the resistance, the temperature or the bandwidth. Knowing Boltzmann’s constant, the formula is simplified to: Where E is now the noise voltage in nanovolts, R in kΩ, and dF in kHz. Thermal and current noise There are two types of noise: the thermal noise and the current noise. To understand their principle, they will be discussed in more detail. In all materials, the electrons permanently move. As temperature increases, the movements increase. The vibrations of the electrons cause an electric signal (AC) across the terminals of the component. Because the vibrations are completely random, the electrical signal is noise. This is called thermal noise or Johnson noise. It is the main contributor to noise for resistors. Thermal noise is constant over a wide frequency range. Current noise however, declines when frequency is increased. The thermal noise increases with a larger resistance value, while the current noise decreases. Noise standards The way to measure resistor current noise is defined in norm IEC 60195. This makes the comparison of different manufacturers possible. The current noise of a resistor is described [… read more]

Temperature Coefficient of Resistance

 
Temperature Coefficient of Resistance

Resistance changes with temperature The temperature coefficient of resistance, or TCR, is one of the main used parameters to characterize a resistor. The TCR defines the change in resistance as a function of the ambient temperature. The common way to express the TCR is in ppm/°C, which stands for parts per million per centigrade degree. The temperature coefficient of resistance is calculated as follows:     Where TCR is in ppm/°C, R1 is in ohms at room temperature, R2 is resistance at operating temperature in ohms, T1 is the room temperature in °C and T2 is the operating temperature in °C. Often instead of TCR, α is used. Positive or Negative Temperature Coefficient of Resistance? Resistors are available with a TCR that is negative, positive, or stable over a certain temperature range. Choosing the right resistor could prevent the need for temperature compensation. In some applications it is desired to have a large TCR, for example to measure temperature. Resistors for these applications are called thermistors, and can have a positive (PTC) or negative temperature coefficient (NTC). Measuring methods for the TCR The temperature coefficient of resistance for a resistor is determined by measuring the resistances values over an appropriate temperature range. The TCR is calculated as the average slope of the resistance value over this interval. This is accurate for linear relations, since the TCR is constant at every temperature. However, many materials have a non linear coefficient. For Nichrome for example, a popular alloy for resistors, the relation between temperature and TCR is not linear. Because the TCR is calculated as average slope, it is therefore very important to specify the TCR as well as the temperature interval. The way to measure TCR is standardized in MIL-STD-202 Method 304. With this method, TCR is calculated for the range between [… read more]

Kirchhoff law

 
Kirchhoff law

Kirchhoff laws are essential for resistor network theory. They were formulated by the German scientist Gustav Kirchhoff in 1845. The laws describe the conservation of energy and charge in electrical networks. They are also called Kirchhoff’s circuit laws. Kirchhoff contributed also to other fields of science, therefore the generic term Kirchhoff law can have different meanings. Both circuit laws, the Kirchhoff Current Law (KCL) and the Kirchhoff Voltage Law (KVL), will be explained in detail. Kirchhoff Current Law (KCL) The Kirchhoff Current Law (KCL) states that the sum of all currents leaving a node in any electrical network is always equal to zero. It is based on the principle of conservation of electric charge. The law is also referred to as Kirchhoff’s first law. In formula form this is given by: The KCL is easier to understand with an example. Look at an arbitrary “node A” from a resistor network. Three branches are connected to this node. Two of the currents are known: I1 is 2 amperes and I2 is 4 amperes. The current law states that the sum of I1, I2 and I3 must be zero: Kirchhoff Voltage Law (KVL) The second law is also called Kirchhoff’s voltage law (KVL). It states that the sum of the voltage rises and voltage drops over all elements in a closed loop is equal to zero. In formula form: Let’s take an example to explain the second law. Consider a part of a resistor network with an internal closed loop, as shown in the picture below. We want to know the voltage drop between node B and C (VBC). The sum of voltage drops in the loop ABCD must be zero, so we can write: The two circuit laws are explained in the video below.  Kirchhoff law example The Kirchhoff laws form [… read more]

Potentiometer taper

 
Potentiometer taper

What is potentiometer taper? Potentiometer taper is the relation between the position and the resistance of a pot. In the majority of variable resistors available this is a linear relationship, meaning that the relative position is equal to the resistance ratio. For example when the potmeter is at the middle position, the output voltage is half of the full voltage over the potentiometer. For some applications and especially audio volume control, non-linear, logarithmic tapers are used. Definition Taper is the relation between the position of the potentiometer and the resistance ratio. Types The simple linear taper is the most common form, when we plot the position against the resistance ratio we can visualize the different position-resistance relations. The graph below shows the most used tapers. The first and last few percents of travel are often only mechanical with no change in resistance. The region between 5 and 95% where the electrical resistance changes is called the electrical travel. The available travel for rotary pots is often denoted in degrees, a mechanical travel of 300° combined with a electrical travel of 270° is common. Audio taper The most used non-linear taper is the logarithmic (log) or audio taper. This is mainly used for audio volume control, to obtain a more natural ‘linear’ perception in sound intensity change when you adjust the volume. Because the human ear is sensitive to sound intensity in a logarithmic fashion, at low sound intensities a small change in intensity is perceived as a big change in loudness, while at high intensities a large change is required for the same change in perceived loudness. To compensate for the ears logarithmic behavior, audio taper pots were developed. While it is called logarithmic, it is actually an exponential curve (the opposite of the logarithmic behavior of the human ear). [… 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]

Wirewound resistor

 
Wirewound resistor

What is a wire-wound resistor? A wire wound resistor is an electrical passive component that limits current. The resistive element exists out of an insulated metallic wire that is winded around a core of non-conductive material. The wire material has a high resistivity, and is usually made of an alloy such as Nickel-chromium (Nichrome) or a copper-nickel-manganese alloy called Manganin. Common core materials include ceramic, plastic and glass. Wire wound resistors are the oldest type of resistors that are still manufactured today. They can be produced very accurate, and have excellent properties for low resistance values and high power ratings. Definition of a wirewound resistor A wire wound resistor is a resistor where a wire with a high resistivity is wrapped around an insulating core to provide the resistance. Construction Wire wound resistor construction varies widely. The manufacturing and choice of materials used is dependent on the way the resistor will be used in a circuit. All are made by winding a wire around a core. The resistance value is dependent on the resistivity of the wire, the cross section and the length. Since these parameters can be accurately controlled, a high precision can be achieved. For high tolerance requirements, the resistance value is measured to determine exactly the cut to length of the wire. To create a high resistance, the wire diameter needs to be very small and the length very long. Therefore wire wound resistors are mainly produced for lower resistance values. For low power ratings, very thin wire is used. The handling of the wire is for this matter critical. Any damage may sever contact. After winding the wire is well protected from access of moisture to prevent electrolytic corrosion. Next to precision, there are also wire wound resistors with high power rating for 50W or more. [… read more]