## 180 k ohm-1/4 watt Resistance

A **resistor** is a passive **two-terminal electrical component** that implements electrical resistance as a circuit element. Resistors act to reduce **current flow**, and, at the same time, act to **lower voltage** levels within circuits. In electronic circuits, resistors are used to **limit current flow**, to adjust signal levels, bias active elements, and terminate transmission lines among other uses.

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## Definitions and Calculations

### Resistor and Resistance

A **resistor** is a passive electrical component that creates **electrical resistance** in electronic circuits. Resistors can be found in almost all electrical circuits. They are used for various purposes, for example, to limit electric current, as voltage dividers, to provide bias to active circuit elements, to terminate transmission lines, in resistor-capacitor circuits as a timing component… The list is endless.

The electrical resistance of a resistor or an electrical conductor is a measure of the opposition to the flow of electric current. The SI unit for resistance is the ohm. Any material shows some resistance except superconductors, which have zero resistance. More information about resistance, resistivity and conductance.

## Resistor Tolerance

Of course, it is possible to make a resistor with very precise resistance, however, it will be insanely expensive. Besides, high precision resistors are relatively rarely used. There are very expensive resistors used for measurements. Here we will talk about inexpensive resistors used in electric circuits, which do not require high precision. In many cases, ±20% of precision is enough. For a 1 kilohm resistor, this means that any resistor with a value in the range of 800 ohms to 1200 ohms is acceptable. For some critical components, the tolerance can be specified as ±1% or even ±0.05%. At the same time, it is hard to find 20% resistors today — they were common at the beginning of the transistor radio era. 5% and 1% resistors are very common today. They were relatively expensive in the past, but not anymore.

## Power Dissipation

When an electric current passes through a resistor, it is heated and the electrical energy is converted into the thermal energy, which it dissipates. This energy must be dissipated by the resistor without excessive raising its temperature. And not only its temperature but also the temperature of components surrounding this resistor. The power consumed by a resistor is calculated as

where *V* in volts is the voltage across the resistor of resistance *R* in ohms and *I* is the current in amps flowing through it. The power that a resistor can safely dissipate for an indefinite period of time without degrading its performance is called the **resistor power rating** or **resistor wattage rating**. Generally, the larger the resistor package, the more power it can dissipate. Resistors of different power ratings are produced, most commonly from 0.01 W to hundreds of watts. Carbon resistors are commonly produced in power ratings of 0.125 to 2 watts.

## Preferred Values

Although it is possible to produce resistors of any value, it is more useful to make a limited number of components, especially considering that any manufactured resistor is subject to a certain tolerance. More precision resistor’s costs are much higher than their less precise counterparts. Common logic dictates to choose a logarithmic scale of values so that all values are equally spaced on a logarithmic scale and match the tolerance of the range. For example, for a tolerance of ±10%, it makes sense to cover a decade (the interval from 1 to 10, 10 to 100, etc.) in 12 steps: 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2, then 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82. These values are called preferred values and are standardized as **E series** of preferred numbers, which are used not only for resistors, but also for capacitors, inductors, and Zener diodes. Each E-series (E3, E6, E12, E24, E48, E96, and E192) subdivides a decade into 3, 6, 12, 24, 48, 96 and 192 steps. Note that the E3 series is obsolete and is almost not used anymore.

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