Radio & Electronic Projects: What is a resistor?

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Introduction


Figure 1 Circuit symbols for resistors

Materials that carry electricity easily are called conductors. They include all metals and salt water, for example. We use wire as a conductor, and the ease with which it passes an electric current depends upon the material, its thickness and its length. Silver (symbol Ag), gold (Au), copper (Cu) and aluminum (Al) are the best metallic conductors. Most wires are made of copper, although the best conductor, weight for weight, is aluminum.

Materials that don't carry current (or, at least, do so very badly) are called insulators, and they include dry wood, rubber, plastic and glass among their number. Wires are often coated with a layer of insulator to prevent adjacent wires touching and causing an accident.

Resistors If there wasn't such a thing as resistance, the subject of electronics wouldn't exist; only infinite currents would flow and voltages wouldn't exist either! We need to reduce the flow of current if we are to make current do something useful for us. Components that resist the flow of current are called resistors, and they are said to have a resistance which is measured in ohms (Ω), named after Georg Ohm, who formulated the law (also named after him) by which the voltage and current through a conductor are related.

His law gave rise to the formula everyone remembers:

I = V/R , where

I is the current flowing, measured in amps, V is the voltage across the conductor, and R is the resistance of the conductor, measured in ohms.

From this equation, you can see that, for a constant value of voltage, V, if the resistance goes up, the current will go down, and vice versa. The circuit symbols for resistors are shown in Figure 1. You will find the upper symbol in older magazines; it is still preferred by many engineers. The lower symbol is the prevalent standard symbol.

Resistors are made in several ways, the cheapest using carbon; another type is usually made from a ceramic cylinder (used only as a support) on which is placed a very thin film of metal -- the thinner the film, the greater the resistance. All resistors are coated with a thin film of insulation, for the same reason we discussed earlier.

Figure 2 Some examples of resistor color codes; top 4700Ω (4.7 kΩ) and bottom 100Ω.

The color code

Each resistor has colored bands on it which enable us to see what value of resistance it has. There are normally three (but sometimes four) at one end, and a single one at the other (see Figure 2). The colors indicate figures, according to the list below.

Using the color codes is easy, once you see the logic behind it. Hold the resistor so that the single band is towards the right. The three colors on the left are read in the normal order from left to right. The first two bands always indicate numbers; the third band gives the number of zeros to add to the right of these two numbers. So, looking at the top resistor in Figure 2, yellow, violet, red means 4, 7, and two zeros, giving 4700 ohms! Looking at the lower resistor, brown, black, brown means 1, 0, and one zero, giving 100 ohms.

Remembering the order of the colors may be difficult at first. The colors from red to violet are the colors of the rainbow, in order, so if you know those, you're almost there! Around those colors are black and brown below the red, and grey and white above the violet, which you can imagine as getting brighter from black to white (well, almost!). It won't be long before you don't need to remember them at all.

The isolated band on the right-hand side is not part of the resistor's value; it indicates its tolerance, i.e. how close it might be to the indicated value. A brown band indicates ±1%, a red band ±2%, a gold band ±5% and a silver band ±10%. For example, a resistor marked as being 100 ohms with a ±5% tolerance will have an actual value somewhere between 95 ohms and 105 ohms.

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