In this post we’ll give you a complete explanation regarding resistor types, specifications and purposes.
A resistor is an electrical component or device made explicitly to have a certain magnitude of resistance, expressed in ohms and is designed to reduce or limit the current. The unit of measure for resistance (R) is the ohm, which was named for a German scientist named Georg S. Ohm. The symbol used to represent resistance is the Greek letter omega (Ω).
Resistors must operate reliably in their environment, including electric field intensity, temperature, humidity, radiation, and other effects. Some resistors are designed explicitly to convert electric energy to heat energy. Others are used in control circuits, where they modify electric signals and energy to achieve desired effects.
Types of resistors
- Fixed Resistors
- Variable Resistors
Fixed resistors are units whose resistance never changes and are produced in several different geometries and modes of construction.
The cheapest method of making a resistor involves mixing powdered carbon (a fair electrical conductor) with a non-conductive solid or paste, pressing the resulting clay into a cylindrical shape, inserting wire leads in the
ends, and then letting the whole mass harden. The resistance of the final product depends on the ratio of carbon to the non-conducting material, and also on the physical distance between the wire leads.
The carbon-composition resistors are in a wide range of ohmic values. This kind of resistor is non-reactive, meaning that it introduces almost pure resistance into the circuit, and essentially no inductive reactance or capacitive reactance. This property makes carbon-composition resistors useful in the construction of radio receivers and transmitters, where the slightest extraneous reactance can cause trouble.
Carbon resistors are very popular for most applications because they are inexpensive and readily available. They are made in standard values that range from about 1 ohm to about 22 megohms (MV), and they can be obtained in power ratings of 1⁄8, 1⁄4, 1⁄2, 1, and 2 watts. The power rating of the resistor is indicated by its size. A 1⁄2-watt resistor is approximately 3⁄8 inch in length and 1⁄8 inch in diameter. A 2-watt resistor has a length of approximately 11⁄16 inches and a diameter of approximately 5⁄16 inch (Figure 5–4). The 2-watt resistor is larger than the 1⁄2 -watt or 1-watt because it must have a larger surface area to be able to dissipate more heat. Although carbon resistors have a lot of desirable characteristics, they have one characteristic that is not desirable. Carbon resistors will change their value with age or if they are overheated. Carbon resistors generally increase instead of a decrease in value.
The resistance can also be obtained with a length of wire made from poorly conducting material. The wire can take the form of a coil wound around a cylindrical form. The resistance depends on how well the wire conducts, on its diameter or gauge, and on its total stretched-out length. This is called a wire-wound resistor.
Wire-wound resistors can be found in various case styles and sizes. These resistors are generally used when a high power rating is needed. Wire-wound resistors can operate at higher temperatures than any other type of resistor. A wire-wound resistor usually has a hollow center. This type of resistor should be mounted vertically and not horizontally. The center of the resistor is hollow for a very good reason. When the resistor is mounted vertically, the heat from the resistor produces a chimney effect and causes air to circulate through the center. This increase of airflow dissipates heat at a faster rate to help keep the resistor from overheating. The disadvantage of wire-wound resistors is that they are expensive and generally require a large amount of space for mounting. They can also exhibit an amount of inductance in circuits that operate at high frequencies. This added inductance can cause problems for the rest of the circuit.
Another type of fixed resistor is the metal film resistor. Metal film resistors are constructed by applying a film of metal to a ceramic rod in a vacuum. The resistance is determined by the type of metal used to form the film and the thickness of the film. Typical thicknesses for the film are from 0.00001 to 0.00000001 inches. Leads are then attached to the film coating, and the entire assembly is covered with a coating. These resistors are superior to carbon resistors in several respects. Metal film resistors do not change their value with age, and their tolerance is generally better than carbon resistors. Tolerance indicates the plus and minus limits of a resistor’s ohmic value. Carbon resistors commonly have a tolerance range of 20%, 10%, or 5%. Metal film resistors generally range intolerance from 2% to 0.1%. The disadvantage of the metal film resistor is that it is expensive.
Another type of fixed resistor that is constructed in a similar manner is the carbon film resistor. This resistor is made by coating a ceramic rod with a film of carbon instead of metal. Carbon film resistors are less expensive to manufacture than metal film resistors and can have a higher tolerance rating than composition carbon resistors.
The cylindrical form consists of an insulating substance, such as porcelain, glass, or thermoplastic. The film can be deposited on this form by various methods, and the value tailored as desired. Metal-film resistors can be manufactured to extremely close tolerances. Film-type resistors usually have low to medium-high resistance.
Film-type resistors, like carbon-composition resistors, have little or no inductive reactance—a big asset in high-frequency AC applications. However, film-type resistors generally can’t handle as much power as carbon-composition or wire-wound types of comparable physical size.
Integrated-Circuit (IC) Resistors
A semiconductor wafer is known as an integrated circuit (IC), also called a chip, allows for the fabrication of resistors on its surface. The thickness of the resistive layer and the types and concentrations of impurities added, determine the resistance of the component. Because of its microscopic size, a typical IC resistor can handle only a tiny amount of power. However, this
limitation rarely poses a problem because of the entire circuit on the chip functions at nano-power levels (on the order of nanowatts, units of 10-9 W) or micro-power levels (on the order of microwatts, units of 10-6 W).
A variable resistor is a resistor whose values can be changed or varied over a range. Variable resistors can be obtained in different case styles and power ratings. Resistive wire is wound in a circular pattern, and a sliding tap makes contact with the wire. The value of resistance can be adjusted between one end of the resistive wire and the sliding tap. If the resistive wire has a total value of 500 ohms, the resistor can be set between the values of 0 and 500 ohms.
Variable resistors are known by several common names. The most popular name is pot, which is shortened from the word potentiometer. Most potentiometers can handle only low levels of current, at low to moderate voltages. You’ll encounter two major designs in your electronics work: the linear-taper potentiometer and the audio-taper potentiometer.
A linear-taper potentiometer uses a strip of resistive material with constant density all the way around. As a result, the resistance between the center terminal and either end terminal changes at a steady rate as the control shaft rotates. Engineers usually prefer linear-taper potentiometers in electronic test instruments. Linear-taper potentiometers also exist in some consumer electronic devices.
Suppose that a linear-taper potentiometer has a value of 0 to 270 ohms. In most units, the shaft rotates through a total angular displacement of about 270°. The resistance between the center and one end terminal increases along with the number of angular degrees that the shaft turns away from that end terminal. The resistance between the center and the other end terminal equals 270 minus the number of degrees that the control shaft subtends with respect to that terminal. The resistance between the middle terminal and either end terminal, therefore, constitutes a linear function of the angular
In some applications, linear taper potentiometers don’t work well. The volume control of a radio receiver or hi-fi audio amplifier provides a good example. Humans perceive sound intensity according to the logarithm of the
actual sound power, not in direct proportion to the actual power. If you use a linear-taper potentiometer to control the volume (or gain ) of a radio receiver or audio system, the sound volume (as you hear it) will change slowly in some parts of the control range, and rapidly in other parts of the control range. The device will work, but not in a user-friendly fashion.
An audio-taper potentiometer, if properly selected and correctly installed, can compensate for the way in which people perceive sound levels. The resistance between the center and either end terminal varies according to a nonlinear function of the angular shaft position. Some engineers call this type of device a logarithmic-taper potentiometer or log-taper potentiometer because the function of resistance versus angular displacement follows a logarithmic curve. As you turn the shaft, the sound intensity seems to increase in a linear manner, even though the actual power variation is logarithmic.
A potentiometer can employ a straight strip of resistive material rather than a circular strip so that the control moves up and down, or from side to side, in a straight line. This type of variable resistor called a slide potentiometer, finds applications in hi-fi audio graphic equalizers, as gain controls in some amplifiers, and in other applications where operators prefer a straight-line control movement to a rotating control movement. Slide potentiometers exist in both linear-taper and audio-taper configurations.
A variable resistor can employ a wire-wound element, rather than a solid strip of resistive material. We call this type of device a rheostat. It can have either a rotary control or a sliding control, depending on whether the resistive wire is wound around a donut-shaped form (toroid ) or a cylindrical form (solenoid ). Rheostats exhibit inductive reactance as well as resistance.
They share the advantages and disadvantages of fixed wire-wound resistors. We can’t adjust a rheostat in a perfectly smooth “continuum” as we can do with a potentiometer because the movable contact slides along the wire coil from a certain point on one turn to the adjacent point on the next turn.
The smallest possible increment of resistance therefore equals the amount of resistance in one turn of the coil.
Rheostats find applications in heavy-duty systems, such as variable voltage power supplies, designed for use with electron-tube amplifiers.
When we chose a resistor for a particular application, we must obtain a unit that has the correct properties, or specifications. Here are some of the most important specifications to watch for.
In theory, a resistor can have any ohmic value from the lowest possible (such as a shaft of solid silver) to the highest (dry air). In practice, we’ll rarely find resistors with values less than about 0.1 ohm or more than about 100 M.
Resistors are manufactured with ohmic values in power-of-10 multiples of numbers from the set:
1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2,
We’ll routinely see resistances such as 47 ohms, 180 ohms, 6.8 k, or 18 M, but we’ll hardly ever find resistors with values such as 314 ohms, 2.54 k, or 6.132 M.
Additional basic resistances exist, intended especially for tight-tolerance (or precision ) resistors: power-of-10 multiples of numbers from the set:
1.1, 1.3, 1.6, 2.0, 2.4, 3.0, 3.6, 4.3, 5.1, 6.2, 7.5, 9.1,
The first set of numbers above represents standard resistance values available in tolerances of plus or minus 10 percent (±10%). This means that the resistance might be as much as 10% more or less than the indicated amount. In the case of a 330-ohm resistor, for example, the value can be larger or smaller than the rated value by as much as 33 ohms, and still adhere to the rated tolerance.
Engineers calculate resistor tolerance figures on the basis of the rated resistance, not the measured resistance. For example, we might test a “330-ohm” resistor and find it to have an actual resistance of 317 ohms; this discrepancy would still put the component within ±10% of the specified value. But if we test it and find it to have a resistance of 210 ohms, its actual value falls outside the rated range, so it constitutes a “reject.”
The second set of numbers listed above, along with the first set, represents all standard resistance values available in tolerances of plus or minus 5 percent (±5%). A 330-ohm, 5% resistor will have an actual value of 330 ohms plus or minus 16 ohms.
For applications requiring exceptional precision, resistors exist that boast tolerances tighter than ±5%. We might need a resistor of such quality in a circuit or system where a small error can make a big difference. In most audio and RF oscillators and amplifiers, we’ll usually use resistors having ±10% or ±5% tolerances. In some applications, we can even get away with a ±20% tolerance.
A manufactured resistor always bears a specification that tells us how much power it can safely dissipate. The dissipation rating indicates continuous duty, which means that the component can dissipate a certain amount of power constantly and indefinitely.
We can calculate how much current a given resistor can handle using the formula for power P (in watts) in terms of current I (in amperes) and resistance R (in ohms), as follows: P = U*I and U=R*I
We can effectively multiply the power rating for a given resistor by connecting identical units in series-parallel matrices of 2 × 2, 3 × 3, 4 × 4, or larger. If we need a 47-ohm, 45-W resistor but we have only a lot of 47-ohm, 1-W resistors available, we can connect seven sets of seven resistors in parallel (a 7 × 7 series-parallel matrix) and get a 47-ohm resistive
component that can handle up to 7 × 7 W, or 49 W.
Resistor power dissipation ratings, like the ohmic values, are specified with a margin for error. A good engineer never tries to “push the rating” and use, say, a ¼-W resistor in a situation where it will need to draw 0.27 W. In
fact, good engineers usually include their own safety margin, in addition to that offered by the vendor. Allowing a 10% safety margin, for example, we should never demand that a ¼-W resistor handle more than about 0.225 W, or expect a 1-W resistor to dissipate more than roughly 0.9 W.
All resistors change value when the temperature rises or falls dramatically. Because resistors dissipate power by design, they get hot in operation.
Sometimes the current that flows through a resistor does not rise high enough to appreciably heat the component. But in some cases it does, and the heat can cause the resistance to change. If this effect becomes great enough, a sensitive circuit will behave differently than it did when the resistor was still cool. In the worst-case scenario, an entire device or system
can shut down because of a single hot resistor.
Resistor manufacturers do various things to prevent problems caused by resistors changing value when they get hot. In one scheme, resistors are specially manufactured so that they don’t appreciably change value when they heat up. We call these components temperature compensated. As you might expect, a temperature-compensated resistor can cost several times as much as an ordinary resistor.
Rather than buy a single temperature-compensated resistor, we can employ a single resistor or a series-parallel matrix of resistors with a power rating several times higher than we ever expect the component to dissipate.
This technique, called over-engineering, keeps the resistor or matrix from reaching temperatures high enough to significantly change the resistance.
Alternatively, we might take several resistors, say five of them, each with five times the intended resistance, and connect them all in parallel. Or we can take several resistors, say four of them, each with about ¼ of the intended resistance, and connect them in series.
Whatever trick we employ to increase the power-handling capability of a component, we should never combine resistors with different ohmic values or power ratings into a single matrix. If we try that, then one of them might end up taking most of the load while the others “loaf,” and the combination will perform no better than the single hot resistor we started with. Whenever we want to build a resistor matrix to handle high current or keep cool under load, we should always procure a set of identical components.
Purpose of the Resistor
Resistors play diverse roles in electrical and electronic equipment, despite the fact that their only direct action constitutes an interference with the flow of current. Common applications include the following:
- Voltage division
- Current limiting
- Power dissipation
- Bleeding off charge
- Impedance matching
A voltage divider is a simple circuit that turns a large voltage into a smaller one. Using just two series resistors and an input voltage, we can create an output voltage that is a fraction of the input. The resistors dissipate some power in doing this job, but the resulting potential differences ensure that an external circuit or system operates properly. For example, a well-engineered voltage divider allows an amplifier to function efficiently, reliably, and with a minimum of distortion.
In a bipolar transistor, a field-effect transistor, or an electron tube, the term bias means that we deliberately apply a certain DC voltage to one electrode relative to another, or relative to electrical ground. Networks of resistors can accomplish this function.
A radio transmitting amplifier works with a different bias than an oscillator or a low-level receiving amplifier. Sometimes we must build low resistance voltage dividers to bias a tube or transistor; in some cases, a single resistor will do.
The figure below shows a bipolar transistor that obtains its bias from a pair of resistors in a voltage-divider configuration.
A sensitive amplifier designed for radio reception offers a good example of an application in which a current-limiting resistor in series with the power supply or battery output keeps the transistor from dissipating too much power as heat. Without such a resistor to limit or control the current, the transistor would carry a lot of DC that wouldn’t contribute to the signal amplification process, and might actually degrade it.
The figure below shows a current-limiting resistor connected between the emitter of a bipolar transistor and electrical ground, which also constitutes the negative power-supply connection (not shown). We can supply the signal input across that resistor (between E and ground) or at the base of the transistor (B). We would normally take the signal output from the collector (C).
In some applications, we want a resistor to dissipate power as heat. Such a resistor might constitute a “dummy” component so that a circuit “sees” the resistor mimic the behavior of something more complicated.
When testing a radio transmitter, for example, we can install a massive resistor in place of the antenna. This engineering trick allows us to test the transmitter for long periods at high power levels without interfering with on-the-air communications. The transmitter output heats the resistor without radiating any signal. However, the transmitter “sees” the resistor as if it were a real antenna—and a perfect one, too, if the resistor has the correct ohmic value.
We might take advantage of a resistor’s power-dissipating ability at the input of a power amplifier, such as the sort used in hi-fi audio equipment.
Sometimes the circuit driving the amplifier (supplying its input signal) produces too much power. A resistor, or network of resistors, can dissipate the excess power so that the amplifier doesn’t receive too much input signal.
In any type of amplifier, overdrive (an excessively strong input signal) can cause distortion, inefficiency, and other problems.
Bleeding Off Charge
A high-voltage, DC power supply employs capacitors (sometimes along with other components) to smooth out the current pulsations, known as ripple. These filter capacitors acquire an electric charge and store it for a while. In some power supplies, filter capacitors can hold the full output voltage of the supply, say something like 750 V, for a long time even after we power the whole system down. Anyone who attempts to repair or test such a power supply can receive a deadly shock from this voltage.
If we connect a bleeder resistor in parallel with each individual filter capacitor in a power supply, the resistors will drain the capacitors’ stored charge, sparing personnel who service or test the power supply the risk of electrocution. In the figure below the bleeder resistor R should have a value high enough so that it doesn’t interfere with the operation of the power supply, but low enough so that it will discharge the capacitor C in a short time after power-down.
Even if a power supply has bleeder resistors installed, the wise engineer or technician, wearing heavy, insulated gloves, will short out all filter capacitors, using a screwdriver or other metal tool with an insulated handle, after power-down and before working on the circuit. Even if the supply has bleeder resistors, they can take a while to get rid of the residual charge.
Besides that, bleeder resistors sometimes fail!
We encounter a more sophisticated application for resistors in the coupling between two amplifiers, or in the input and output circuits of amplifiers. In order to produce the greatest possible amplification, the impedances between the output of a given amplifier and the input of the next must precisely agree. The same holds true between a source of the signal and the input of an amplifier. The principle also applies between the output of an amplifier and a load, whether that load is a speaker, a headset, or whatever. We might think of impedance as the AC “big brother” of DC resistance.
The color code for Resistors
Each resistor has colored bands on it which enable us to see what value of resistance it has. There are normally three (but sometimes four) at one end, and a single one at the other. The colors indicate figures, according to the list below.
Using the color codes is easy, once you see the logic behind it. Hold the resistor so that the single band is towards the right. The three colors on the left are read in the normal order from left to right. The first two bands always indicate numbers; the third band gives the number of zeros to add to the right of these two numbers.
So, looking at the top resistor in the figure below, yellow, violet, red means 4, 7, and two zeros, giving 4700 ohms! Remembering the order of the colors may be difficult at first. The colors from red to violet are the colors of the rainbow, in order, so if you know those, you’re almost there! Around those colors are black and brown below the red, and grey and white above the violet, which you can imagine as getting brighter from black to white. It won’t be long before you don’t need to remember them at all.
The isolated band on the right-hand side is not part of the resistor’s value; it indicates its tolerance, i.e. how close it might be to the indicated value. A brown band indicates ±1%, a red band ±2%, a gold band ±5% and a silver band ±10%. For example, a resistor marked as being 100 ohms with a ±5% tolerance will have an actual value somewhere between 95 ohms and 105 ohms.
If there wasn’t such a thing as resistance, the subject of electrics wouldn’t exist; only infinite currents would flow and voltages wouldn’t exist either!
We hope you liked our post about resistor types, specifications and purposes and understood the resistors color code.