Resistors

Resistors are electrical components that oppose the flow of either direct or alternating current. They are used to protect, operate, or control circuits. Voltages can be divided with the use of resistors. In combination with other components, resistors can also be used to make electrical waves into shapes most suited to the designer’s requirements. Resistors can have a fixed value of resistance, or they may be variable or adjustable within a certain range. Typically, resistor values are expressed in ohms, a unit of resistance. Materials of construction for resistors include carbon composition, carbon film, ceramic composition, metal alloy, metal film, metal oxide, thick film, thin film, and wire. Common lead types include axial leads, gull-wing leads, J-leads, radial leads, screw terminals, and tab terminals. Resistors without leads are also available.

Performance specifications to consider when searching for resistors include resistance range, tolerance, power rating, continuous operating voltage, temperature coefficient, configuration, and operating temperature. Power rating is the maximum power supported by the resistor. Resistors can be operated at any combination of voltage and current as long as the power rating is not exceeded, and provided that neither the current rating nor the voltage rating is exceeded. Typically, power rating is measured in watts (W) and released as heat energy. Continuous operating voltage is measured in volts (V). The temperature coefficient of resistance (TCR) parameter measures the rate at which the nominal resistance value changes as a function of temperature. TCR for resistors is expressed in parts-per million (ppm) per degree Celsius, or ppm/C. .

Physical specifications for resistors include mounting style and package type. Mounting styles include surface mount technology (SMT) and through-hole technology (THT), as well as bold-on (chassis) and panel-mounted methods. Surface mount components are a direct response to cost reduction efforts that center around improved printed circuit board (PCB) production. Automatic or robotic pick and place equipment can pick and place surface mount devices (SMD) on a PCB with both speed and accuracy. Typically, the pad on the circuit board is coated with a paste like formulation of solder and flux. Unlike SMDs, through-hole resistors connect to a printed circuit board by inserting a terminal or lead through a hole in the board and soldering it to the opposite side. Bolt-on or chassis-style resistors use a metal-to-metal connection for maximum heat dissipation. Panel-mounted resistors use a threaded shaft with a bushing.

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Part Numbers for Resistors