A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. This means that they don’t generate power but only consume it. Resistors are normally added to circuits where they complement active components like op-amps, microcontrollers and other integrated circuits. Resistors are commonly used to limit current, divide voltages and pull-up I/O lines.
Practical resistors can be composed of various compounds and films, as well as resistance wires (wire made of a high-resistivity alloy, such as nickel-chrome). Resistors are also implemented within integrated circuits, particularly analog devices, and can also be integrated into hybrid and printed circuits.
The electrical resistance of a resistor is measured in ohms (symbol: Ω). An ohm is equal to a volt per ampere. Since resistors are specified and manufactured over a very large range of values, the derived units of milliohm.
Resistors come in a variety of shapes and sizes. They might be through-hole or surface-mount. They can be a standard, static resistor, a pack of resistors or a special variable resistor.
Mounting and termination
As stated, resistors come in one of two termination-types: through-hole or surface-mount. These types of resistors are usually abbreviated as either PTH (plated through-hole) or SMD/SMT (surface-mount technology or device).
Through-hole resistors come with long, pliable leads which can be stuck into a breadboard (a solderless circuit board) or hand-soldered into a prototyping board or printed circuit board (PCB). These resistors are usually more useful in breadboarding, prototyping or in any case where you’d rather not solder small resistors.
Surface-mount resistors are usually small black rectangles, terminated on either side with even smaller, silver conductive edges. These resistors are intended to sit on top of PCBs, where they’re soldered onto mating landing pads. These resistors are usually set into place by a robot, because they are so small, and then sent through an oven where solder melts and holds them in place.
Resistors can be made out of a variety of materials. Modern resistors are most commonly made out of either a carbon, metal or metal-oxide film. In these resistors, a thin film of conductive (although still resistive) material is wrapped in a helix around and covered by an insulating material. Most of the standard, no-frills, through-hole resistors will come in a carbon-film or metal-film composition.
Some through-hole resistors can be either, wirewound or made of super-thin metallic foil. These resistors are usually more expensive, higher-end components specifically chosen for their unique characteristics like a higher power-rating, or maximum temperature range.
Surface-mount resistors are usually either thick or thin-film variety. Thick-film is usually cheaper but less precise than thin. In both resistor types, a small film of resistive metal alloy is sandwiched between a ceramic base and glass/epoxy coating, and then connected to the terminating conductive edges.