The humble resistor spans more than nine orders of magnitude in value
Few components are plainer than a resistor, a two-legged part whose whole job is to get in the way of current. Yet commercial resistors are made in values ranging across more than nine orders of magnitude, and choosing the wrong one can do more than spoil a circuit. An overloaded resistor can scorch a board or even start a fire.
Their uses are everywhere. Resistors limit current, set signal levels, split voltages, bias transistors and terminate transmission lines, and chunky high-power versions shed many watts as heat in motor controls or when testing generators. Variable types turn up as volume knobs and lamp dimmers, or as sensors that respond to heat, light, humidity or pressure. The rule behind them is Ohm's law: put a 300 ohm resistor across a 12 volt battery and 0.04 amperes flows.
Combining them is simple arithmetic. In a row, resistances add. Side by side, the reciprocals add, so a 10, a 5 and a 15 ohm resistor in parallel give only about 2.7 ohms together. Tangled networks that are neither can need matrix methods to untangle. On diagrams, engineers often write values with a letter instead of a decimal point, so 8K2 means 8.2 kilohms.
Heat is the practical limit. The power a resistor absorbs turns into warmth its body must shed, which is why parts are rated by maximum dissipation. Small ones in ordinary electronics are rated at a tenth to a quarter of a watt and seldom come close. Power resistors, loosely anything from 1 watt up, are bulkier and may need heat sinks. Designers derate them for thin air at altitude or poor ventilation, and flameproof versions exist that will not ignite however long they are overloaded. Voltage ratings matter too: one quarter-watt part rated at 750 volts and 100 megohms could never actually reach its power limit.
Construction has shifted over the decades. Carbon composition resistors, a baked mix of powdered carbon and ceramic, dominated until the 1960s; now they are pricey and kept mainly for power supplies, welding gear and authentic repairs of vintage equipment. Carbon film types, with a helix cut into a thin coating, run quieter. Stacks of compressed carbon discs can even act as adjustable loads for testing car batteries.
Source: Resistor
