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Circuit Components: The Resistor

The resistor is the simplest, most basic electronic component. In an electronic circuit, the resistor limits the flow of electrical current through itself. It accomplishes this by absorbing some of the electrical energy applied to it, and then dissipating that energy as heat. By doing this, the resistor provides a means of limiting or controlling the amount of electrical current that can pass through a given circuit.

Resistors, such as the two pictured to the right, have two ratings, or values, associated with them. First, of course is the resistance value itself. This is measured in units called ohms and symbolized by the Greek letter Omega (Ω). The second rating is the amount of power the resistor can dissipate as heat without itself overheating and burning up. Typical power ratings for modern resistors in most applications are ½ watt and ¼ watt, which are the two sizes shown in the figure. High-power applications can require high-power resistors of 1, 2, 5, or 10 watts, or even higher.

A general rule of thumb is to always select a resistor whose power rating is at least double the amount of power it will be expected to handle. That way, it will be able to dissipate any heat it generates very quickly, and will operate at normal temperatures.

For purposes of physical comparison, the larger resistor to the right is rated at ½ watt; its body is a cylinder 3/8" long and 1/8" in diameter. The smaller resistor, rated at ¼ watt, is of the same shape but is only 1/4" long and 1/16" in diameter.

The traditional construction of ordinary, low-power resistors is as a solid cylinder of a carbon composition material. This material is of an easily-controlled content, and has a well-known resistance to the flow of electrical current. The carbon cylinder is molded around a pair of wire leads at either end to provide electrical connections. The length and diameter of the cylinder are controlled in order to define the resistance value of the resistor — the longer the cylinder, the greater the resistance; the greater the diameter, the less the resistance. At the same time, the larger the cylinder, the more power it can dissipate as heat. Thus, the combination of the two determines both the final resistance and the power rating.

A newer, more precise method is shown to the left. The manufacturer coats a cylindrical ceramic core with a uniform layer of resistance material, with a ring or cap of conducting material over each end. Instead of varying the thickness or length of the resistance material along the middle of the ceramic core, the manufacturer cuts a spiral groove around the resistor body. By changing the angle of the spiral cut, the manufacturer can very accurately adjust the length and width of the spiral stripe, and therefore the resistance of the unit. The wire leads are formed with small end cups that just fit over the end caps of the resistor, and can be bonded to the end caps.

With either construction method, the new resistor is coated with an insulating material such as phenolic or ceramic, and is marked to indicate the value of the newly finished resistor.

High-power resistors are typically constructed of a resistance wire (made of nichrome or some similar material) that offers resistance to the flow of electricity, but can still handle large currents and can withstand high temperatures. The resistance wire is wrapped around a ceramic core and is simply bonded to the external connection points. These resistors are physically large so they can dissipate significant amounts of heat, and they are designed to be able to continue operating at high temperatures.

These resistors do not fall under the rule of selecting a power rating of double the expected power dissipation. That isn't practical with power dissipations of 20 or 50 watts or more. So these resistors are built to withstand the high temperatures that they will produce in normal operation, and are always given plenty of physical distance from other components so they can still dissipate all that heat harmlessly.

Regardless of power rating, all resistors are represented by the schematic symbol shown to the right. It can be drawn either horizontally or vertically, according to how it best fits in the overall diagram.

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