Resistors are made up of various manufacturing technologies using a wide variety of conducting and insulating materials. Normally resistors are classified based on the materials and methods used for manufacturing. The popular resistor types are Carbon Composition Resistors, Carbon Film Resistors, Metal film resistors, Foil Resistors,Wire wound Resistors, Fuse Resistors, Power Film Resistors. Among all these, Thick Film (Carbon Film) Resistors are more popular because of their low cost, small size, acceptable tolerance, and higher power rating.
Carbon Composition Resistors
- With composition resistors, the entire body of the resistor acts as the resistance element. Since there is no trimming, the tolerances on delivery are relatively high (± 10 %, ± 20 %).
- The advantages of the composition resistor are its very good high-frequency characteristics and the high capability to overload relative to the component size. These factors mean that these relatively expensive resistors are used in applications like power supplies, welding controls and as “dummy loads”.
- TCR is in the range of 1000 ppm/°C and is negative, that is when the temperature goes up the resistance goes down and when the temperature goes down, the resistance goes up.
- Composition resistors also have a positive voltage coefficient. That is the resistance will change with applied voltage, the greater the voltage, the greater the change.
- Due to their construction, they generate noise and this noise level varies with value and physical size.
- The power capability in relation to physical size is greater than Precision Wirewounds but less than Power Wirewounds.
- The main advantage of Carbon Composition resistors is their pulse handling capability. This is due to the fact that the entire rod conducts.
- Carbon Composition Resistors are suitable for any application involving high-energy pulses, where small size is important.
- Carbon Composition resistors are used as surge protection resistors.
- Carbon Composition Resistors are noisy, unstable, have large TCRs and exhibit poor long-term stability and poor resistance to soldering heat.
Carbon Film Resistors
- Carbon film resistor is the most general purpose, cheap resistor.
- Carbon film resistors have a disadvantage; they tend to be electrically noisy. That is why metal film resistors are recommended for use in analog circuits.
- Carbon Film Resistors have noise, a voltage coefficient.
- Carbon Film Resistor’s long-term stability is not as good as metal film and wire-wound resistors, but at 0.25%/year is very good.
- The frequency response of this type of resistor is among the best, far better than Wirewounds, and much better that carbon composition. The wirewound resistors are inductive at lower frequencies and lower values and somewhat capacitive at higher frequencies regardless of value. Also wirewound resistors will have a resonant frequency. And Carbon Composition Resistors will be predominately capacitive.
- Carbon film resistors are primarily divided into two categories as Thick-Film resistors and Thin-Film resistors.
- The principal difference between “thin film” and “thick film resistors” isn’t necessarily the “thickness” of the film, but rather, how the film is applied to the cylinder (axial resistors) or the surface (SMD resistors). In thick film resistors the “film” is applied using traditional screen-printing technology. Thin film resistors are made by sputtering the resistive material onto the surface of the resistor.
- Thin film resistors, like their thick film counterparts, are then usually trimmed to an accurate value by abrasive or laser trimming.
- Traditionally thick film resistors had tolerances of 1%, 2%, 5%, but temperature coefficients of thick film resistors are typically ±200 or ±250 ppm/K, depending on the resistance. Thus a 40 kelvin (70° F) temperature change can add another 1% variation to a 1% resistor.
- Thin film resistors are usually specified with tolerances of 0.1, 0.2, 0.5, and 1%, and with temperature coefficients of 5 to 25 ppm/K. They are usually far more expensive than their thick film cousins.
Metal film resistors
- Metal Film resistors are the best compromise of all resistors. When compared to Precision Wirewound resistors, metal film resistors are not accurate and have a higher TCR and are not as stable as Precision Wirewounds, but metal film resistors are much cheaper. When compared to Carbon film resistors, metal film resistors are more accurate, do not have a voltage coefficient, and have a lower temperature coefficient than Carbon Film resistors. TCRs of 25 to 100 ppm can be achieved.
- Metal film resistors are used when a higher tolerance (more accurate value) is needed. They have about ±0.05% tolerance.
- The metal film resistor is used for bridge circuits, filter circuits, and low-noise analog signal circuits.
- Metal electrode leadless face (MELF) resistor use the same metal film technology, but is a cylindrically shaped resistor designed for surface mounting.
- Note that other types of resistors (e.g., carbon composition) are also available in MELF packages.
- The metal film resistors have a very low noise level.
- Metal Film Resistors are the best, general-purpose resistors with high long-term stability, have low to very low TCRs, exhibit very low changes in resistance due to soldering, can work at low temperatures, and have low voltage coefficients.
- The frequency characteristics of metal film resistors are excellent and better than Carbon Films.
- The one area that carbon films exceed metal films is the maximum values of resistance. Carbon films can achieve higher maximum values than any other group.
- Metal Oxide Film Resistors have good long-term stability, good TCRs. They are excellent replacements for low-power wire-wound resistors.
- Metal film resistors possess very good noise characteristics and low non-linearity. Parameters such as tolerance, temperature coefficient, and stability are excellent.
- Foil resistors have the best precision and stability of any resistor available.
- One of the important parameters influencing stability is the temperature coefficient of resistance (TCR). The TCR of foil resistors is extremely low. One range of ultra-precision foil resistors offers a TCR of 0.14 ppm/°C, tolerance of ±0.005%, long-term stability (1 year) 25 ppm, (3 year) 50 ppm (further improved 5-fold 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.
- Metal foil resistors are used today in large quantities as low-ohm current measuring resistors (shunts), as well as precision resistors for measurement applications. The most important requirements for these applications are low temperature coefficient, low thermo-electrical potential difference with reference to copper, and high long-term stability. These requirements are met extremely well.
- Other technical advantages are extremely low inductivity and good pulse loading capability. Foil technology is particularly suitable for resistors in the 2 mΩ to 150 kΩ range.
- Their main advantages are better stability than metal films and lower TCRs. They have excellent frequency response, low TCR, good stability, and very accurate.
- Their main disadvantage is the maximum value is less than Metal Film Resistors.
Wire wound Resistors
- The outstanding characteristic of wirewound of resistor is the very high surface temperature it can take, up to + 450 °C, which makes it very tough indeed.
- Their areas of application are comparable to those of the composition resistor, but the high frequency characteristics of the wirewound resistor are substantially worse.
- Because wirew
ound 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.
- Wire wound resistors are a type of power resistor and are very accurate.
- Typical applications for wirewound resistors include device requiring high current handling capability, heat dissipation and resistance stability and accuracy.
- Precision Wirewound is a highly stable and accurate resistor with a very low TCR (as low as ±1 ppm/°C) and can be accurate within .005% tolerance. Tolerances as low as ±0.005% are available. These resistors available with stabilities of ±20 ppm/year.
- Precision Wirewound components are too expensive for general use. The frequency response of this type is not good. These are generally used in highly accurate DC measuring equipment, and reference resistors for voltage regulators and decoding networks.
- Precision Wirewound accuracy is mentioned at 25°C and will change with temperature.
- Precision Wirewound’s life is generally rated for 10,000 hours at rated temperature and rated load. The allowable change in resistance under these conditions is 0.10%. Extended life can be achieved if operated at lower temperatures and reduced power levels. End of life requirements are generally defined by the manufacturer or in some case by user specification, when some degradation in performance can be expected.
- In some cases, particularly if the tolerance is very low and the TC is low, the rated power is reduced to improve resistor stability through life. Precision Resistors regardless of type, are designed for maximum accuracy and not to carry power. The materials used in these resistors are highly stable heat treated materials that do change under extended heat and mechanical stress. There is little detectable noise in this type of resistor. The stability and reliability of these resistors is very good.
- “Power” generally means 1 Watt and higher, and since P = V2/R, this most often implies low resistance.
- Power wirewound resistor’s physical construction is designed to dissipate the heat.
- Power wirewound resistors have excellent high-energy pulse handling capabilities.
- Wire-wound resistors are available at values below 1 Ω, where other technologies are often not available. These resistors are bulky when compared to other resistors, and are tend to be expensive.
- Power wire-wound resistors typically have 1% tolerance, and have good long-term stability. Even power precision wire-wound resistors often have low TCRs.
- Power Wirewound Resistors are used when it is necessary to handle a lot of power. They will handle more power per unit volume than any other resistor. Some of these resistors are free wound similar to heater elements. These require some form of cooling in order to handle any appreciable amount of power. Some are cooled by fans and others are immersed in various types of liquid ranging from mineral oil to high density silicone liquids.
- To achieve the maximum power rating in the smallest package size, the core on which the windings are made must have a material with high heat conductivity. The more heat that can be radiated from the resistor, the more power that can be safely applied.
- Wire-wound resistors are bulkier than other resistors, but the main downside is cost. For example, a 1 K wire-wound resistor can cost 25 times as much as a carbon film resistor (but can dissipate10 Watts versus 0.125 Watts).
- Fuse Resistors serve a dual purpose, a resistor and a fuse. They are designed so that they will open with a large surge current. The fusing current is calculated based on the amount of energy required to melt the resistive material.
- These resistors will normally run hotter than a normal precision or power resistor so that a momentary surge will bring the resistive element up to fusing temperature. Some designs create a hot spot inside the resistor to assist in this fusing.
Power Film Resistors
- Power film resistors are similar to their respective metal film or carbon film resistors. They are manufactured and rated as power resistors, with the power rating being the most important characteristic.
- Power Film Resistors are available in higher maximum values than the Power Wirewound resistors and have a very good frequency response.
- They are generally used in applications requiring good frequency response and/or higher maximum values.
If you are looking for more on selection guidelines for resistors, you can read my earlier post titled Resistor Selection for Analog Circuit Design.
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