The Resistor

What is a Resistor?

A resistor is a component of an electrical circuit that, quite literally, "resists" the flow of electrical current. Electrical resistance is caused by collisions between electrons within the resistor, itself, and the larger the resistor, the less current can flow, for any given voltage. This is according to Ohm`s Law ? developed by 19th century German physicist Georg Ohm ? which states that V = IR, where V is the voltage of the device, I is the current flowing, and R is the resistance. In other words, R = V/I, or electrical resistance is equal to the drop in voltage across the terminals of a resistor divided by the current flowing through it. A resistor must therefore, by definition, have two terminals across which electricity must pass, and its primary use is to provide a known, and safe, amount of current in an electrical circuit.

Resistor Characteristics & Uses

Resistors are ubiquitous in the world around us today, not just in electronic equipment, but in many everyday devices. They come in many shapes and sizes, and it is their size and physical construction which determines not only their electrical resistance, but also the amount of power that they dissipate. Carbon may be packed in to a ceramic cylinder to form a carbon composition resistor, or wrapped around the outside of the same cylinder, to form a carbon film resistor and the cylinder turned and scored with a laser to achieve the correct resistance. Metal oxide, and metal film, resistors are constructed in much the same way, and for resistance at higher power levels, so-called wire wound resistors ? which feature metal wire wrapped around a central, clay, plastic or fibreglass tube ? are often used.

The ability of a resistor to limit the current flowing through an electronic device is important. Typically, an electronic device, such as an amplifier, or a head unit, has a power supply that supplies all its components, but, if parts of its circuitry were connected to the supply without limiting the current, they would likely be destroyed. Resistors are also used for heating applications ? toasters, clothes dryers, etc. ? and there are special types of resistors that are sensitive to light, temperature and strain, so resistors also find themselves in photographic light meters, bathroom scales, and other, apparently unlikely, situations.

Circuit Design

Resistors behave, physically, in the same way whether they are connected in series or in parallel, but there is an important difference in the contribution they make to the total resistance of a circuit, depending up which way they are connected.

Connecting resistors in series means connecting them, effectively, in a chain, so that the current in a circuit has only one possible path, and the same current physically flows through both resistors. In this case, the total resistance of the circuit can be found by simply adding together the value of the individual resistors (measured in Ohms, Ω). In algebraic terms, R total = R1 + R2 ? and so on, so for two 1k Ω resistors connected in series, R total = 1 + 1 = 2k Ω.

Conversely, connecting resistors in parallel means connecting them such that there heads are connected together, as are their tails, but the current in the circuit is divided, with some flowing through each parallel branch. The voltage across each resistor is, however, the same. In this case, the total resistance of the circuit can be calculated from 1/R total = 1/R1 + 1/R2 ?, so for two 1k Ω resistors in parallel, 1/R total = 1/1 + 1/1, so R total = 1/2 = 0.5k Ω. So, we can see that, simply by changing the way in which the resistors are connected, we have reduced the overall resistance of the circuit by a factor of 4.

Resistors are used in integrated circuits ? semiconductors wafers bearing thousands, or millions, of resistors, capacitors and transistors ? which can function as computer memory, or microprocessors. Similarly, resistors can be built into printed circuit boards ? thin plates containing chips and other electronic components ? which are used as motherboards, expansion boards, etc. in the world of computing. Two, or four, layer printed circuit boards in a variety of sizes ? typically up to 144 square inches ? are available online and, in some cases, online discount codes, or other discounts, are available.