Resistor

The electronic component designed to introduce a certain electrical resistance between two points of an electrical circuit is called resistance or resistor. In other cases, such as irons, heaters, etc., resistances are used to produce heat taking advantage of the Joule effect. It is a material made up of carbon and other resistive elements to reduce the current that passes through. It opposes the flow of current.

The maximum current and maximum potential difference in a resistor is conditioned by the maximum power that its body can dissipate. This power can be identified visually from the diameter without any other indication being necessary. The most common values are 0.25 W, 0.5 W and 1 W.

There are resistors whose value can be adjusted manually called potentiometers, rheostats or simply variable resistors. Devices are also produced whose resistance varies depending on external parameters, such as thermistors, which are resistors that vary with temperature; the varistors that depend on the voltage to which they are subjected, or the photoresistors that do so according to the light received.

The electrical function of a resistor is specified by its resistance: common commercial resistors are manufactured over a range of more than nine orders of magnitude. The nominal value of the resistance is within the manufacturing tolerance indicated on the component.

Behavior in a circuit

Resistors are used in circuits to limit the value of the current or to set the value of the voltage, according to Ohm's Law. Unlike other electronic components, resistors have no defined polarity.

Coding Systems

Color code

To characterize a resistor, three values are needed: electrical resistance, maximum dissipation and precision or tolerance. These values are normally indicated on the package depending on its type; For the type of axial encapsulation, which can be seen in the photographs, these values are labeled with a color stripe code.

These values are indicated with a set of colored stripes over the body of the element. They are three, four or five stripes; Leaving the tolerance stripe (usually silver or gold) on the right, they are read from left to right. The last line indicates the tolerance (precision). Of the remaining ones, the last one is the multiplier and the others indicate the significant figures of the resistance value.

The value of the electrical resistance is obtained by reading the figures as a one, two or three-digit number; It is multiplied by the multiplier and the result is obtained in Ohms (Ω). The temperature coefficient is only applied to high precision resistors or tolerances less than 1%.

Band color		Value of 1.a significant figure	Value of 2.a significant figure	Multiplier	Tolerance	Temperature coefficient
Black		0	0	1	-	-
Brown		1	1	10	±1 %	100 ppm/°C
Red		2	2	100	±2 %	50 ppm/°C
Orange		3	3	1000	-	15 ppm/°C
Yellow		4	4	10 000	±4 %	25 ppm/°C
Green		5	5	100 000	±0.5 %	20 ppm/°C
Blue		6	6	1 000	±0.25 %	10 ppm/°C
Morado		7	7	10 000	±0.1 %	5 ppm/°C
Grey		8	8	100 000	±0.05 %	1 ppm/°C
White		9	9	1 000 000 000	-	-
Dorado		-	-	0.1	±5 %	-
Plated		-	-	0.01	±10 %	-
Pink		-	-	0.001	-	-
None		-	-	-	±20 %	-

How to read the value of a resistor

In a resistor we generally have four colored lines, although we can find some that contain five lines (4 colored and 1 indicating tolerance). Let's take the most general one as an example, those with four lines. With the tolerance band on the right, we read the remaining bands from left to right, as follows:

The first two bands make up a two-digit integer:^{[citation-required]}

• The first line represents the digit of the dozens.
• The second line represents the digit of the units.

Then:

• The third line represents the power of 10 by which the number is multiplied.

The numerical result is expressed in Ohms.

For example:

• We observe the first line: green = 5
• We observe the second line: yellow = 4
• We observe the third line: red =100
• We unite the values of the first two lines and multiply by the value of the third

54 X 10² = 5400Ω or 5.4 kΩ and this is the value of the resistance expressed in Ohms

Examples

• The characterization of a resistance of 2.700.000 Ω (2.7 MΩ), with a tolerance of ±10%, would be represented in the Figure 3:

1.a figure: red (2)

2.a figure: violet (7)

Multiplier: green (100000)

Tolerance: silver (±10%)

• The Value of Resistance Figure 4 is 75 Ω and ±2% tolerance since:

1.a figure: violet (7)

2.a figure: green (5)

3.a figure: black (0)

Multiplier: gold (10)^-1)

Tolerance: red (±2%)

Coding of surface mount resistors

Resistors when found in circuits with surface mount technology have numerical values printed in a code similar to that used on axial resistors.

Standard-tolerance resistors in these types of mounts (Standard-tolerance Surface Mount Technology) are marked with a three-digit code, in which the first two digits represent the first two significant digits and the third digit represents a power of ten (the number of zeros).

Coding in SMD Resistors

In SMD or surface mount resistors, their coding is more usual is:

	1.a Figure = 1.♪ Number 2.a Figure = 2nd number 3.a Figure = Multiplier	In this example resistance has a value of: 1200 ohmos = 1.2 kΩ
	1.a Figure = 1.♪ Number "R" indicates decimal coma 3rd Figure = 2nd Number	In this example resistance has a value of: 1.6 ohmos
	"R" indicates decimal comma ("0") 2nd Figure = 2nd Number 3rd Figure = 3rd Number	In this example resistance has a value of: 0.22 ohms

• For example:

Resistors less than 100 Ω are written: 100, 220, 470, etc. The final number zero represents ten to the power of zero, which is 1.

• For example:

Sometimes these values are marked "10" or "22" to prevent errors.

Resistors smaller than 10 Ω have an n#39;R' to indicate the position of the decimal point.

• For example:

Precision resistors are marked with four-digit codes, in which the first three digits are the significant numbers and the fourth is the power of ten.

• For example:

The values y#34;000" and n#34;0000" They sometimes appear in surface mount links, because they have approximately zero resistance.

Coding for Industrial Use

Format: XX 99999 or XX 9999X [two letters]<space>[resistor value (three/four digits)]<nospace>[tolerance code(numeric/alphanumeric - one digit/one letter)]

The operational temperature range distinguishes commercial, industrial and military types of components.

• Commercial type: 0 °C to 70 °C
• Industrial Type: −40 °C to 85 °C (sometimes −25 °C to 85 °C)
• Military type: −55 °C to 125 °C (sometimes -65 °C to 275 °C)
• Standard Type: -5 °C to 60 °C

Fixed resistors

Cable layout

Through-hole components usually have "wires" that leave the body "axially", that is, in a line parallel to the longest axis of the piece. Others have wires that exit their body "radially" instead. Other components may be SMT (surface mount technology), while high-power resistors may have one of their leads designed into the heat sink.

Carbon composition

Carbon composition resistors (RCC) consist of a solid cylindrical resistive element with embedded leads or metal caps into which the leads are fixed. The body of the resistor is protected with paint or plastic. Early 20th century carbon composition resistors had uninsulated bodies; Lead wires were wound around the ends of the resistance element rod and soldered. The finished resistor was painted to color code its value.

Carbon composition resistors are still available, but are relatively expensive. Values vary from fractions of an ohm to 22 megohms. Due to their high price, these resistors are no longer used in most applications. However, they are used in power supplies and soldering controls. They are also in demand for the repair of older electronic equipment where authenticity is an important factor.

Carbon cell

A carbon stack resistor is made up of a stack of carbon discs compressed between two metal contact plates. Adjusting the clamping pressure modifies the resistance between the plates. These resistors are used where an adjustable load is required, for example in testing car batteries or radio transmitters. A carbon cell resistor can also be used as a speed control for small household appliance motors (sewing machines, hand mixers) with powers up to a few hundred watts. A carbon cell resistor can be incorporated into automatic voltage regulators. for generators, where the carbon stack controls the field current to maintain a relatively constant voltage. The principle also applies in the carbon microphone.

Carbon film

A carbon film is deposited on an insulating substrate and a helix is cut into it to create a long, narrow resistive path. The variation in shapes, coupled with the resistivity of the amorphous carbon (ranging from 500 to 800 μΩ m), can provide a wide range of resistance values. Compared to carbon composition, they are characterized by low noise, due to the precise distribution of pure graphite without binder. Carbon film resistors feature a power range of 0.125 W to 5 W at 70 °C. Available resistors range from 1 ohm to 10 megohms. The carbon film resistor has an operating temperature range of -55°C to 155°C. It has a maximum working voltage range of 200 to 600 volts. Specialty carbon film resistors are used in applications requiring high pulse stability.

Printed carbon resistors

Carbon composition resistors can be printed directly on printed circuit board (PCB) as part of the PCB manufacturing process. Although this technique is most common on hybrid PCB modules, it can also be used on standard fiberglass PCBs. Tolerances are usually quite large, and can be on the order of 30%. A typical application would be non-critical pull-up resistors.

Thick and thin film

.

Thick film resistors became popular during the 1970s, and most SMDs (surface mount device) today are of this type. The resistive element of thick films is 1000 times thicker than that of thin films, but the main difference is how the film is applied to the cylinder (axial resistors) or to the surface (SMD resistors).

Thin film resistors are manufactured by sputtering the resistive material onto an insulating substrate. The film is then recorded in a similar way to the old (subtractive) process of manufacturing printed circuit boards; That is, the surface is coated with a photoresist material, then covered with a pattern film, irradiated with ultraviolet light, and then the exposed photoresist layer is developed and the underlying thin film is etched.

Thick film resistors are manufactured using screen printing and stenciling processes.

Metallic film

A common type of axial leaded resistor today is the metal film resistor. Lead-free metal electrode resistors typically use the same technology.

Metal film resistors are usually coated with nickel-chromium (NiCr), but can be coated with any of the cermet materials mentioned above for thin film resistors. Unlike thin film resistors, the material can be applied using techniques other than sputtering (although this is one of the techniques). Additionally, unlike thin film resistors, the resistance value is determined by cutting a helix through the coating rather than etching it. 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 usually between 50 and 100 ppm/K. Metal film resistors have good noise characteristics and low nonlinearity due to at a low stress coefficient. Also beneficial are its close tolerance, low temperature coefficient, and long-term stability.

Metal oxide film

Metal oxide film resistors are made from metal oxides, resulting in higher operating temperatures and greater stability and reliability than metal film resistors. They are used in applications with high resistance requirements.

Cable winding

.

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 to two caps or rings, attached to the ends of the core. The assembly is protected with a layer of paint, molded plastic or a high-temperature baked enamel. These resistors are designed to withstand unusually high temperatures of up to 450 °C. The leads of low-power wirewound resistors are usually between 0.6 and 0.8 mm in diameter and are tinned for easy soldering. For higher power wirewound resistors, a ceramic outer shell or an aluminum outer shell is used over an insulating layer; If the outer casing is ceramic, these resistors are sometimes described as 'cement' resistors, although they do not actually contain any traditional Portland cement. Aluminum cased resistors are designed to be connected to a heat sink to dissipate heat; The power rating depends on its use with a suitable heat sink, for example a 50W rated power resistor will overheat at a fraction of the power dissipated if not used with a heat sink. Large wirewound resistors can have a power rating of 1000 watts or more.

Because wirewound resistors are electromagnetic, they have more undesirable inductance than other types of resistors, although winding the wire in sections with alternating directions can minimize inductance. Other techniques employ bifilar winding, or a thin flat former (to reduce the cross-sectional area of the coil). For the most demanding circuits, Ayrton-Perry wirewound resistors are used.

The applications of wirewound resistors are similar to those of composition resistors, with the exception of high frequency. The high frequency response of wirewound resistors is substantially worse than that of a composition resistor.

Sheet metal resistance

In 1960 Felix Zandman and Sidney J. Stein presented a development of very high stability sheet resistance.

The primary resistance element of a foil resistor is a sheet of chromium-nickel alloy several microns thick. Chromium-nickel alloys are characterized by having a high electrical resistance (about 58 times that of copper), a small temperature coefficient and a high resistance to oxidation. Some examples are Chromel A and Nichrome V, whose typical composition is 80 Ni and 20 Cr, with a melting point of 1420 °C. When iron is added, the chromium-nickel alloy becomes more ductile. Nichrome and Chromel C are examples of an alloy containing iron. The typical composition of Nichrome is 60 Ni, 12 Cr, 26 Fe, 2 Mn and that of Chromel C, 64 Ni, 11 Cr, Fe 25. The melting temperature of these alloys is 1350° and 1390 °C, respectively..

Since their introduction in the 1960s, foil resistors have had the best precision and stability of any resistor available. One of the important stability parameters is the temperature coefficient of resistance (CTR). The CTR of foil resistors is extremely low, and has been improved over the years. A range of ultra-precision foil resistors offers a TCR of 0.14 ppm/°C, tolerance ±0.005%, long-term stability (1 year) 25 ppm, (3 years) 50 ppm (improved 5 times by hermetic sealing), stability under load (2000 hours) 0. 03%, thermal EMF 0.1 μV/°C, noise -42 dB, voltage coefficient 0.1 ppm/V, inductance 0.08 μH, capacitance 0.5 pF.

The thermal stability of this type of resistors also has to do with the opposite effects of the electrical resistance of the metal that increases with temperature, and which is reduced by thermal expansion that leads to an increase in the thickness of the sheet, whose other dimensions are limited by a ceramic substrate.

Measuring elements

A shunt or current shunt is a special type of current sensing resistor, which has four terminals and a value in milliohms or even micro-ohms. Current measuring instruments, by themselves, usually accept only limited currents. To measure high currents, the current passes through the shunt through which the voltage drop is measured and interpreted as current. A typical shunt consists of two solid metal blocks, sometimes brass, mounted on an insulating base. Between the blocks, and welded to them, are one or more strips of manganin alloy with low temperature coefficient of resistance. Large screws threaded into the blocks make the current connections, while much smaller screws provide the voltmeter connections. Shunts are rated for full scale current, and often have a voltage drop of 50 mV at rated current. These meters are matched to the rated current of the shunt using a suitably marked dial; No changes need to be made to the other parts of the meter.

Network resistance

In high-current industrial applications, a grid resistor is a large convection-cooled lattice of stamped metal alloy strips connected in rows between two electrodes. These industrial grade resistors can be as large as a refrigerator; Some designs can handle more than 500 amps of current, with a resistance range extending below 0.04 ohms. They are used in applications such as dynamic braking and load banking for locomotives and trams, neutral grounding for industrial AC distribution, control loads for cranes and heavy equipment, generator load testing, and filtering. of harmonics for electrical substations.

The term grid resistor is sometimes used to describe a resistor of any type connected to the control grid of a vacuum tube. This is not a resistor technology; It is an electronic circuit topology.

Variable resistors

Adjustable resistances

A resistor can have one or more fixed tap points so that the resistance can be changed by moving the connecting wires to different terminals. Some wirewound power resistors have a connection point that can slide along the resistance element, allowing a larger or smaller portion of the resistor to be used.

Where continuous adjustment of the resistance value is required during equipment operation, the sliding resistance tap can be connected to a knob accessible to an operator. Such a device is called a rheostat and has two terminals.

Potentiometers

A potentiometer (colloquially, pot) is a three-terminal resistor with a continuously adjustable tapping point controlled by the rotation of a shaft or knob or by a linear slider. The name potentiometer comes from its function as a voltage divider. adjustable to provide a variable potential at the terminal connected to the tap point. Controlling volume on an audio device is a common application of a potentiometer. A typical low-power potentiometer (see drawing) is constructed with a flat resistance element (B) of carbon composition, metal film or conductive plastic, with an elastic phosphor bronze (C) that moves along the surface. An alternative construction is resistance wire wound in a shape, with the wiper sliding axially along the coil. These have lower resolution, since as the wiper moves, the resistance changes in steps equal to the resistance. in a single turn.

High resolution multiturn potentiometers are used in precision applications. These have wire wound resistance elements typically wound on a helical mandrel, with the wiper moving on a helical track when the control is turned, making continuous contact with the wire. Some include a conductive plastic resist coating over the cable to improve resolution. These usually offer ten turns of their axes to cover their full range. They are typically configured with dials that include a simple turn counter and a graduated dial, and can typically achieve three-digit resolution. Electronic analog computers used them in quantity to set coefficients, and the delayed scan oscilloscopes of recent decades included one on their panels.

Decade box

A resistor decades box or resistor substitution box is a unit containing resistors of many values, with one or more mechanical switches allowing any of the various discrete resistors offered by the box to be dialed. The resistor typically has a high accuracy, ranging from a laboratory/calibration accuracy of 20 parts per million, to a field accuracy of 1%. There are also economical boxes with less precision. All types offer a convenient way to quickly select and change a resistor in laboratory, experimentation and development work, without the need to place resistors one by one, or store each value. The range of resistances provided, maximum resolution and precision characterize the box. For example, one box offers resistances from 0 to 100 megohms, maximum resolution of 0.1 ohms, accuracy of 0.1%.

Special devices

There are several devices whose resistance changes with various magnitudes. The resistance of NTC thermistors has a strong negative temperature coefficient, making them useful for measuring temperatures. Since their resistance can be large until they are allowed to heat up due to the passage of current, they are also commonly used to prevent excessive current surges when equipment is turned on. Likewise, the resistance of a humidor varies with humidity. One type of photodetector, photoresistor, has a resistance that varies with illumination.

The strain gauge, invented by Edward E. Simmons and Arthur C. Ruge in 1938, is a type of resistance that changes value with applied voltage. You can use a single resistor, or a pair (half-bridge), or four resistors connected in a Wheatstone bridge configuration. The strain gauge is attached with adhesive to an object that is subjected to mechanical deformation. With the strain gauge and a filter, an amplifier and an analog/digital converter, the deformation of an object can be measured.

A related, but more recent, invention uses a quantum tunneling compound to detect mechanical stress. A current passes whose magnitude can vary by a factor of 10¹² in response to changes in applied pressure.

Special varieties

• Cermet
• Phenolic
• Tantalio
• Water resistance

Precision resistors

Precision or metal foil resistors, also known by their English name foil resistors, are those whose value is adjusted with errors of 100 parts per million or less and also have a very wide variation. small with temperature, of the order of 10 parts per million between 25 and 125 degrees Celsius. This component has a very special use in analog circuits, with very narrow specifications adjustments. The resistance achieves such high precision in its value and in its temperature specification, because it must be considered as a system, where the materials that make it up interact to achieve its stability. A very thin sheet of metal is glued to an insulator such as glass or ceramic. As the temperature increases, the thermal expansion of the metal is greater than that of glass or ceramic and when it is glued to the insulator, it produces a force in the metal that makes it It compresses, reducing its electrical resistance, since the coefficient of variation of resistance of the metal with temperature is almost always positive, the almost linear sum of these factors means that the resistance does not vary or does so minimally.

This component had its origin in several countries and at different times. By the 1950s, some companies and academic technology centers, especially in the United States, began to investigate new component techniques that would adapt to the nascent semiconductor industry. The new electronic systems had to be more stable and more compact and the industry at that time placed more emphasis on precision and stability of behavior with changes in temperature. In resistor technology there were two emerging types: those made with very thin metal films, deposited on insulating substrates, such as glass or ceramics, and whose deposit was carried out with metallic evaporation techniques. Then there were resistors made with metal sheets, whose thicknesses were greater than those made with metal films. The metal sheets were glued to insulating substrates, such as glass or ceramics.

Investigating the origin of this latest technology we arrived at Duncan and John Cox, who patented a resistor for heating use in 1951. Although the purpose of this component was to be used as a heating element, the novelty of the The same resided in its geometric construction, the shape of the resistive lines were adopted by companies dedicated to the manufacture of metal sheet resistors made in 1979 by Benjamín Solow, or in its improved version of 1983 made by Josph Szware, The realization of a thin metal film resist requires advanced experience in multiple disciplines such as vector drawing, metallurgy, high temperature adhesives, very fine line photolithography, fine etching and chemical passivation, which masks it against the action of external agents, such as a chemical or electrochemical reaction, which reduces or is completely prevented, design of the shape of the conductors to minimize inductance, capacitance, excess noise and hot spots (points of high current density), voltage analysis mechanical components and their thermal components, encapsulation that reduce mechanical stress and in-process and post-process manufacturing operations to improve reliability.

Thin metal film resistors are used in harsh environmental areas such as those found in space. The demands of the aerospace industry differ from business requirements in one important area: reliability must continue. In some cases, there is only one chance to complete the mission and the system cannot be returned to the workshop for repairs. Some systems must transit deep space for 10 years or more before activating. with great long-term reliability.

A simple example is its use in operational amplifiers, the gain is established by the ratio of the feedback resistance to the input resistance. With differential amplifiers, the ratio is based on a set of four resistors. In both cases, any change in the ratios of these resistances directly affects the function of the circuit. These can change due to different temperature coefficients experiencing heating, whether internal or external, therefore it is common for many circuits to depend on many application-related stability characteristics, all at the same time on the same devices.

Piezoresistive effect

As initially indicated, there is a force interaction effect between the metal sheet and the substrate; The metal sheet behaves like a strain gauge, which is a sensor based on the piezoresistive effect; a stress that deforms the gauge will produce a variation in its electrical resistance.

This sensor, in its basic form, was used for the first time in 1936. The discovery of the principle was made in 1856 by Lord Kelvin, who loaded copper and iron wires, producing mechanical tension in them and recording a increase in electrical resistance with the tensile strain of the wire, observed that iron wire has a greater increase in resistance than copper wire, when subjected to the same strain.

From experiments carried out by Lord Kelvin in 1856 it turns out that when a metal is subjected to a mechanical force, a change in its electrical resistance occurs. Thus, subjecting the metal to a force that stretches it produces an increase in its resistance, and if we apply compression, its electrical resistance decreases. This effect eventually opened a new field of measurements. An increase in temperature in a metal produces two effects, an expansion and an increase in its electrical resistance.

In 1959, William T. Bean introduced a metal sheet type strain gauge, with a Cox geometry used to measure the unit strain of materials. subjected to mechanical forces, several points must be highlighted in this development: 1) it uses a metal sheet with Cox geometry, 2) it uses metals such as constantan or nichrome and 3) the use of a photographic method and then the use of chemical erosion to perform the resistive model. Studying this development, it can be speculated that technicians who used strain gauges, measuring the mechanical properties of glasses and ceramics, found a very small variation in resistance with temperature, due precisely to the effect initially mentioned.

The first description of this system, using geometric, physical and chemical properties, such as Cox geometry, the Kelvin effect and the use of the nickel-chromium alloy, all of which were integrated into a component, was carried out by Zandman in 1970.