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A resistor is a two-terminal electronic component that produces a voltage across its terminals that is proportional to the electric current passing through it in accordance with Ohm's law: V = IR Resistors are elements of electrical networks and electronic circuits and are ubiquitous in most electronic equipment. Practical resistors can be made of various compounds and films, as well as resistance wire (wire made of a high-resistivity alloy, such as nickel/chrome). The primary characteristics of a resistor are the resistance, the tolerance, maximum working voltage and the power rating. Other characteristics include temperature coefficient, noise, and inductance. Less well-known is critical resistance, the value below which power dissipation limits the maximum permitted current flow, and above which the limit is applied voltage. Critical resistance is determined by the design, materials and dimensions of the resistor. Resistors can be integrated into hybrid and printed circuits, as well as integrated circuits. Size, and position of leads (or terminals) are relevant to equipment designers; resistors must be physically large enough not to overheat when dissipating their power. Units The ohm (symbol: Ω) is the SI unit of electrical resistance, named after Georg Simon Ohm. Commonly used multiples and submultiples in electrical and electronic usage are the milliohm (1x10−3), kilohm (1x103), and megohm (1x106). [edit] Theory of operation [edit] Ohm's law The behavior of an ideal resistor is dictated by the relationship specified in Ohm's law: Ohm's law states that the voltage (V) across a resistor is proportional to the current (I) through it where the constant of proportionality is the resistance (R). Equivalently, Ohm's law can be stated: This formulation of Ohm's law states that, when a voltage (V) is maintained across a resistance (R), a current (I) will flow through the resistance. This formulation is often used in practice. For example, if V is 12 volts and R is 400 ohms, a current of 12 / 400 = 0.03 amperes will flow through the resistance R. Thick and thin film Thick film resistors became popular during the 1970s, and most SMD (surface mount device) resistors today are of this type. The principal difference between thin film and thick film resistors is not the actual thickness of the film, but rather how the film is applied to the cylinder (axial resistors) or the surface (SMD resistors). Thin film resistors are made by sputtering (a method of vacuum deposition) the resistive material onto an insulating substrate. The film is then etched in a similar manner to the old (subtractive) process for making printed circuit boards; that is, the surface is coated with a photo-sensitive material, then covered by a pattern film, irradiated with ultraviolet light, and then the exposed photo-sensitive coating is developed, and underlying thin film is etched away. Thick film resistors are manufactured using screen and stencil printing processes[1]. Because the time during which the sputtering is performed can be controlled, the thickness of the thin film can be accurately controlled. The type of material is also usually different consisting of one or more ceramic (cermet) conductors such as tantalum nitride (TaN), ruthenium dioxide (RuO2), lead oxide (PbO), bismuth ruthenate (Bi2Ru2O7), nickel chromium (NiCr), and/or bismuth iridate (Bi2Ir2O7). The resistance of both thin and thick film resistors after manufacture is not highly accurate; they are usually trimmed to an accurate value by abrasive or laser trimming. Thin film resistors are usually specified with tolerances of 0.1, 0.2, 0.5, or 1%, and with temperature coefficients of 5 to 25 ppm/K. Thick film resistors may use the same conductive ceramics, but they are mixed with sintered (powdered) glass and some kind of liquid so that the composite can be screen-printed. This composite of glass and conductive ceramic (cermet) material is then fused (baked) in an oven at about 850 °C. Thick film resistors, when first manufactured, had tolerances of 5%, but standard tolerances have improved to 2% or 1% in the last few decades. Temperature coefficients of thick film resistors are high, typically ±200 or ±250 ppm/K; a 40 kelvin (70 °F) temperature change can change the resistance by 1%. Thin film resistors are usually far more expensive than thick film resistors. For example, SMD thin film resistors, with 0.5% tolerances, and with 25 ppm/K temperature coefficients, when bought in full size reel quantities, are about twice the cost of 1%, 250 ppm/K thick film resistors. [edit] Metal film A common type of axial resistor today is referred to as a metal-film resistor. Metal electrode leadless face (MELF) resistors often use the same technology, but are a cylindrically shaped resistor designed for surface mounting. Note that other types of resistors (e.g., carbon composition) are also available in MELF packages. Metal film resistors are usually coated with nickel chromium (NiCr), but might be coated with any of the cermet materials listed above for thin film resistors. Unlike thin film resistors, the material may be applied using different techniques than sputtering (though that is one such technique). Also, unlike thin-film resistors, the resistance value is determined by cutting a helix through the coating rather than by etching. (This is similar to the way carbon resistors are made.) The result is a reasonable tolerance (0.5, 1, or 2%) and a temperature coefficient that is generally between 50 and 100 ppm/K.[4]. Metal film resistors process good noise characteristics and low non-linearity. Also beneficial are the components efficient tolerance, temperature coefficient and stability[1]. [edit] Wirewound Types of windings in wire resistors: 1 - common 2 - bifilar 3 - common on a thin former 4 - Ayrton-Perry Wirewound resistors are commonly made by winding a metal wire, usually nichrome, around a ceramic, plastic, or fiberglass core. The ends of the wire are soldered or welded to two caps or rings, attached to the ends of the core. The assembly is protected with a layer of paint, molded plastic, or an enamel coating baked at high temperature. Because of the very high surface temperature these resistors can withstand temperatures of up to +450°C[1]. Wire leads in low power wirewound resistors are usually between 0.6 and 0.8 mm in diameter and tinned for ease of soldering. For higher power wirewound resistors, either a ceramic outer case or an aluminum outer case on top of an insulating layer is used. The aluminum-cased types are designed to be attached to a heat sink to dissipate the heat; the rated power is dependent on being used with a suitable heat sink, e.g., a 50 W power rated resistor will overheat at a fraction of the power dissipation if not used with a heat sink. Large wirewound resistors may be rated for 1,000 watts or more. Because wirewound resistors are coils they have more undesirable inductance than other types of resistor, although winding the wire in sections with alternately reversed direction can minimize inductance. Other techniques employ bifilar winding, or a flat thin former (to reduce crosssection area of the coil). For most demanding circuits resistors with Ayrton-Perry winding are used. Applications of wirewound resistors are similar to those of composition resistors with the exception of the high frequency. The high frequency of wirewound resistors is substantially worse than that of a composition resistor[1] Resistance standards The primary standard for resistance, the "mercury ohm" was initially defined in 1884 in as a column of mercury 106mm long and 1 square millimeter in cross-section, at 0 degrees Celsius. Difficulties in precisely measuring the physical constants to replicate this standard result in variations of as much as 30 ppm. From 1900 the mercury ohm was replaced with a precision machined plate of manganin[12]. Since 1990 the international resistance standard has been based on the quantized Hall effect discovered by Klaus von Klitzing, for which he won the Nobel Prize in Physics in 1985.[13] Resistors of extremely high precision are manufactured for calibration and laboratory use. They may have four terminals, using one pair to carry an operating current and the other pair to measure the voltage drop; this eliminates errors caused by voltage drops across the lead resistances, because no current flows through voltage sensing leads. It is important in small value resistors (100–0.0001 Ohm) where lead resistance is significant or even comparable with respect to resistance standard value.[14] Four-band resistors Four-band identification is the most commonly used color-coding scheme on resistors. It consists of four colored bands that are painted around the body of the resistor. The first two bands encode the first two significant digits of the resistance value, the third is a power-of-ten multiplier or number-of-zeroes, and the fourth is the tolerance accuracy, or acceptable error, of the value. The first three bands are equally spaced along the resistor; the spacing to the fourth band is wider. Sometimes a fifth band identifies the thermal coefficient, but this must be distinguished from the true 5-color system, with 3 significant digits. For example, green-blue-yellow-red is 56×104 Ω = 560 kΩ ± 2%. An easier description can be as followed: the first band, green, has a value of 5 and the second band, blue, has a value of 6, and is counted as 56. The third band, yellow, has a value of 104, which adds four 0's to the end, creating 560,000 Ω at ±2% tolerance accuracy. 560,000 Ω changes to 560 kΩ ±2% (as a kilo- is 103). Each color corresponds to a certain digit, progressing from darker to lighter colors, as shown in the chart below. Color 1st band 2nd band 3rd band (multiplier) 4th band (tolerance) Temp. Coefficient Black 0 0 ×100 Brown 1 1 ×101 ±1% (F) 100 ppm 2 Red 2 2 ×10 ±2% (G) 50 ppm 3 Orange 3 3 ×10 15 ppm 4 Yellow 4 4 ×10 25 ppm 5 Green 5 5 ×10 ±0.5% (D) 6 Blue 6 6 ×10 ±0.25% (C) 7 Violet 7 7 ×10 ±0.1% (B) 8 Gray 8 8 ×10 ±0.05% (A) 9 White 9 9 ×10 Gold ×10−1 ±5% (J) −2 Silver ×10 ±10% (K) None ±20% (M) There are many mnemonics for remembering these colors. RTD's (Resistance Temperature Detectors) are designed to meet IEC Publication 751, DIN43760, JIS16041989 and BS1904-1984. They are normally supplied to Class B, but can be manufactured to Class A as an option. RTD's offer greater repeatability and interchangeability than thermocouples or thermistors over the standard temperature scale from -260ºC to 630ºC (-436 to 1166ºF). RTD Circuitry 2-wire circuit Shown is a 2-wire RTD connected to a typical Wheatstone bridge circuit. Es is the supply voltage; Eo is the output voltage; R1, R2, and R3 are fixed resistors; and RT is the RTD. In this uncompensated circuit, lead resistance L1 and L2 add directly to RT. 4-wire circuit 4-wire RTD circuits not only cancel lead wires but remove the effects of mismatched resistances such as contact points. A common version is the constant current circuit shown here. Is drives a precise measuring current through L1 and L4; L2 and L3 measure the voltage drop across the RTD element. Eo must have high impedance to prevent current flow in the potential leads. 4-wire circuits may be usable over a longer distance than 3-wire, but you should consider using a transmitter in electrically noisy environments. 3-wire circuit In this circuit there are three leads coming from the RTD instead of two. L1 and L3 carry the measuring current while L2 acts only as a potential lead. No current flows through it while the bridge is in balance. Since L1 and L3 are in separate arms of the bridge, resistance is canceled. This circuit assumes high impedance at Eo and close matching of resistance between wires L2 and L3. TEMPCO matches RTD leads within 5%. As a rule of thumb, 3 wire circuits can handle wire runs up to 100 feet. If necessary you can connect a 2-wire RTD to a 3-wire circuit or 4-wire circuit, as shown. As long as the junctions are near the RTD, as in a connection head, errors are negligible. Platinum resistance thermometers (PRTs) offer excellent accuracy over a wide temperature range (from -200 to +850 °C). Standard Sensors are are available from many manufacturers with various accuracy specifications and numerous packaging options to suit most applications. Unlike thermocouples, it is not necessary to use special cables to connect to the sensor. The principle of operation is to measure the resistance of a platinum element. The most common type (PT100) has a resistance of 100 ohms at 0 °C and 138.4 ohms at 100 °C. There are also PT1000 sensors that have a resistance of 1000 ohms at 0 °C. The relationship between temperature and resistance is approximately linear over a small temperature range: for example, if you assume that it is linear over the 0 to 100 °C range, the error at 50 °C is 0.4 °C. For precision measurement, it is necessary to linearise the resistance to give an accurate temperature. The most recent definition of the relationship between resistance and temperature is International Temperature Standard 90 (ITS-90). This linearisation is done automatically, in software, when using Pico signal conditioners. The linearisation equation is: Rt = R0 * (1 + A* t + B*t2 + C*(t-100)* t3) Where: Rt is the resistance at temperature t, R0 is the resistance at 0 °C, and A= 3.9083 E-3 B = -5.775 E-7 C = -4.183 E -12 (below 0 °C), or C = 0 (above 0 °C) For a PT100 sensor, a 1 °C temperature change will cause a 0.384 ohm change in resistance, so even a small error in measurement of the resistance (for example, the resistance of the wires leading to the sensor) can cause a large error in the measurement of the temperature. For precision work, sensors have four wires- two to carry the sense current, and two to measure the voltage across the sensor element. It is also possible to obtain three-wire sensors, although these operate on the (not necessarily valid) assumption that the resistance of each of the three wires is the same. The current through the sensor will cause some heating: for example, a sense current of 1 mA through a 100 ohm resistor will generate 100 µW of heat. If the sensor element is unable to dissipate this heat, it will report an artificially high temperature. This effect can be reduced by either using a large sensor element, or by making sure that it is in good thermal contact with its environment. Using a 1 mA sense current will give a signal of only 100 mV. Because the change in resistance for a degree celsius is very small, even a small error in the measurement of the voltage across the sensor will produce a large error in the temperature measurement. For example, a 100 µV voltage measurement error will give a 0.4 °C error in the temperature reading. Similarly, a 1 µA error in the sense current will give 0.4 °C temperature error. Because of the low signal levels, it is important to keep any cables away from electric cables, motors, switchgear and other devices that may emit electrical noise. Using screened cable, with the screen grounded at one end, may help to reduce interference. When using long cables, it is necessary to check that the measuring equipment is capable of handling the resistance of the cables. Most equipment can cope with up to 100 ohms per core. The type of probe and cable should be chosen carefully to suit the application. The main issues are the temperature range and exposure to fluids (corrosive or conductive) or metals. Clearly, normal solder junctions on cables should not be used at temperatures above about 170 °C. Sensor manufacturers offer a wide range of sensors that comply with BS1904 class B (DIN 43760): these sensors offer an accuracy of ±0.3 °C at 0 °C. For increased accuracy, BS1904 class A (±0.15 °C) or tenth-DIN sensors (±0.03 °C). Companies like Isotech can provide standards with 0.001 °C accuracy. Please note that these accuracy specifications relate to the SENSOR ONLY: it is necessary to add on any error in the measuring system as well. Related standards are IEC751 and JISC1604-1989. IEC751 also defines the colour coding for PRT sensor cables: the one or two wires attached to one end of the sensor are red, and the one or two wires at the other end are white. STANDARDS FOR RESISTANCE TEMPERATURE SENSORS (RTDs) There are numerous standards around the world and the goal of this part of the site is to eventually catalog most, if not all, of them. This is a start with links to many of the organizations involved, if they are on the Web. The ones we know the best are those published by ASTM and those standards that deal with radiation thermometers. We shall be searching the Web and the Net for more details. If you can add to this page, just email your information and we will include it as we are able to verify and edit the page. ASTM Standards Related to Temperature and Calibration: E 1594-99 Standard Guide for the Expression of Temperature E 344-01a...Terminology Relating to Thermometry and Hygrometry E 563-97 Standard Practice for Preparation and Use of Freezing Point Reference Baths E 1502-98 Standard Guide for the Use of Freezing Point Cells for Reference Temperatures E1750-02 Standard Guide for Use of Water Triple Point Cells ASTM Standards Related to Resistance Temperature Detectors E 644-98 Standard Test Methods for Testing Industrial Resistance Thermometers E 1137-97 Standard Specification for Industrial Platinum Resistance Thermometers E 1652-00 Standard Specification for Magnesium Oxide and Aluminum Oxide Powder and Crushable Insulators Used in the Manufacture of Metal-Sheathed Platinum Resistance Thermometers, Base Metal Thermocouples, and Noble Metal Thermocouples Other Standards Related to Resistance Temperature Detectors German Industrial Standards Organization (DIN) DIN 43760 references nickel precision DIN IEC 751 reference platinum precision resistance thermometers. ARTICLE INDEX Thermocouples, Thermistors & RTDs Thermistors RTDs Indications All Pages RTDs An RTD is a resistance temperature detector. The principle of RTD function is similar to a thermistor. Unlike thermocouples, RTDs are not self-powered. A current must be passed through the RTD, the same as with thermistors, and the change of voltage with temperature is measured. Materials for RTDs can be gold, silver, copper or platinum. Platinum, however, has become the most-used metal for RTDs. A thin film of platinum or a thin platinum wire is deposited on a flat ceramic material and sealed. Platinum has a nearly linear temperature versus resistance relationship. The Callendar-Van Dusen equation approximates the RTD curve: RT=R0+R0α [T–δ(T/100–1)(T/100)–β(T/100–1)(T3/100)] where: RT = Resistance at temperature T. R0 = Resistance at 0ºC α = Temperature coefficient at T=0ºC δ = Constant (platinum=1.49). β = Constant (when termperature is above 0ºC , β=0). The lowest practically used resistance of RTDs is 100W at 0°C. The operating temperature range is from –220°C to 850°C. RTDs have a self-heating error that depends on the electrical energy input. The formula for the self-heating error is: dt = P/EK where: dt = Self-heating in K. P = Electrical energy in mW. 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