In general terms, a resistor is a passive electrical component (with two terminals) that is designed to limit the flow of electric current through a circuit. Resistors work by dissipating electrical energy as heat and reducing the flow of electricity through the resistor. Resistors are found in almost all electrical networks and electronic circuits and are essential to precisely controlling the amount of current and voltage in circuits.
Resistors come in several formats. The most common formats are semiconductor chip resistors (e.g., surface mount devices, SMD), radial lead resistors, and axial lead resistors (Figure 2.)
There are thousands of different resistor types. Resistors are constructed using various techniques that produce diverse performance characteristics. However, modern fixed-value resistors can be classified into four broad groups by material and construction type:
- Carbon Composition Resistors
- Film (and Cermet) Resistors
- Wire-wound Resistors
- Semiconductor Resistors
Carbon composition resistors
Carbon composition resistors are quite standard, inexpensive resistors made of carbon dust or graphite paste mixed with a binder and baked onto a ceramic carrier under high pressure and temperature. They are qualified for only relatively low wattages and have high tolerances; that is, due to the manufacturing process of carbon composition resistors, they can have values that vary (more than others) from what is expected, as read on the resistor’s band. A significant advantage of carbon composition resistors is their ability to withstand high energy pulses. However, they are not recommended if electrical noise is going to be a problem as they warm up. All resistors produce some level of noise that’s related to heating. Thermal noise comes from electron holes moving around in the conductor. Larger value resistors create more noise than lower value resistors, all other things being equal. When current flows through a carbon resistor, the entire carbon composition body conducts the energy which results in a higher energy capacity. Carbon composition resistors may be less expensive than other resistors, but they are not the best regarding temperature coefficient, noise, voltage dependence (different value based on the voltage that’s applied), and load. Modern applications include circuit protection (surge or discharge protection), high voltage power supplies, high power lighting, and welding.
Film or Cermet resistors
Film resistors are made of either metal film, carbon film, or metal oxide film deposited on an insulating ceramic rod or another substrate. Film resistors are preferable to carbon composite resistors when greater tolerance is required (<1%), and they are available in substantial values (mega-ohms). Film resistors are also less noisy and more stable at higher temperatures than carbon composite resistors. The resistive value is controlled by varying the thickness of the film. Therefore they can be grouped into either “thin-film resistors” or “thick-film resistors.” Lasers are used to cut high-precision patterns into the film, which changes the conductive or resistive path. Thin-film resistors have a low TCR, long-term stability, and are good for applications that require high precision.
The laser cut patterns are part of what makes film resistors capable of closer tolerances than the simpler carbon composition resistors. Film resistors provide a very high resistor value (ohms) versus other types of resistors. Film resistors are typically used for very low wattage applications and have good characteristics for tolerance, stability, and TCR. They have little noise and high linearity because of a small voltage coefficient. There are many applications, depending on the film type. For example, metal film resistors are used for active filters or bridge circuits. Carbon film resistors are commonly used in radar, x-ray equipment, and power supplies. Metal oxide resistors are typically used in applications with high endurance demands.
Cermet resistors are a type of thick film resistor for which a thicker conducting paste is used. The paste is a mix of both ceramic and metal, thus the term “cermet.” Cermet resistors possess qualities of low noise, good temperature stability, and decent voltage ratings.
Wire-wound resistors have very high wattage ratings and are made by winding a thin wire on an insulated core (usually a ceramic tube). The resistance value depends on the resistivity of the wire, the cross-section, and the length.
Wirewound resistors are mostly produced with alloys because pure metals have a high-temperature coefficient of resistance (TCR). Common alloys that are used as resistor wires are copper-, silver-, nickel chromium-, iron chromium-, and iron chromium aluminum-alloys. However, pure metals such as tungsten may be used for high-temperature applications. All resistors have an associated parasitic capacitance and inductance, but wire wound resistors have a notably higher inductive value due to its wire winding. Parasitic capacitance and inductance influence the current flow in an alternating current circuit (typically an undesirable effect). Wire wound resistors are mainly produced for lower value resistances, have comparatively higher power ratings, and are commonly used in motor control and power supplies where energy associated with in-rush current must be dissipated. Wire wound resistors are not used in audio and RF circuits due to higher-than-average inductance values that will vary with changes in frequency, causing an unpredictable response. Wire wound resistors are common for use as heating elements in appliances like toasters and electric heaters.
Foil resistors have a thin bilk metal foil, only a few micrometers thick, that is cemented to a ceramic material substrate. They are highly stable and reliable, with a low TCR, and are the most cost-effective regarding constructing high-performing networks of resistors for precision applications.
Semiconductor resistors are merely resistors formed in a semiconductor substrate such as silicon. Semiconductor resistors range in conductivity between a conductor (such as copper or gold) and an insulator (such as glass). Conductivity increases with temperature, which is behavior that is the opposite of metal. The most common types of resistors built on semiconductor wafers are 1) diffused resistors; 2) ion-implanted resistors; 3) thin-film resistors, and 4) polysilicon resistors.