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The variable resistor is mainly used for adjusting resistance values, and often applied in the voltage divider biasing circuit of the transistor. The following talks about its function and features:

Function and Features
First, the variable resistor is a kind of resistor, so it plays the role of a resistor in the electronic circuit. However, its resistance can be continuously changed within a certain range.
Due to the structure and uses of variable resistors, the failure rate is significantly higher than that of ordinary resistors.
Variable resistors are usually used in small signal circuits, and large signal variable resistors are also used in a few occasions such as valve amplifiers.

Appearance and Features of Variable Resistor
The appearance of a variable resistor is very different from that of a common resistor. It has the following features, and according to these features, we can find out the variable resistor in the circuit board :
(1) The volume of the variable resistor is larger than that of the common resistor. And there are fewer variable resistors in the circuit, which can be easily found in the circuit board.
(2) The variable resistor has three pins, and they are different from each other. One is the moving plate pin, as shown in the Figure below. The other two are fixed plate pins. The two fixed plated pins can be used interchangeably. but the moving plate pin and the fixed plate pin cannot be interchanged.
(3) There is an adjustment port on the variable resistor. Put a straight screwdriver to into this adjustment port. Rotate the screwdriver to change the position of the moving plate and adjust the resistance.
(4) The nominal resistance value is on the variable resistor. This value refers to the resistance value between two moving plate pin, or between one fixed plate pin and the moving plate pin.
(5) Vertical variable resistors are mainly used in small signal circuits. Its three pins are vertically downward and mounted on the circuit board vertically, and the resistance adjustment port is in the horizontal direction.
(6) Horizontal variable resistors are also used in small signal circuits. Its three pins are at 90° to the plane of the resistor, vertically downward. It’s installed on the circuit board horizontally, with the resistance adjusting port facing upward.
(7) The variable resistor with a small plastic case has a smaller volume. It has a circular structure, as shown in Figure 1(a), with its three pins facing downwards and the resistance adjusting port facing upwards.
(8) Figure 1(b) shows a variable resistor (wire-wound structure) used in high power occasions. It has a large volume and the moving plate can slide left and right to adjust the resistance.

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Figure 1. Variable Resistor Diagram
RachelAnne2020 11 august 2020, 9:24

Most of us kept the ordinary microprocessor in hands, but hardly anyone had an idea to cut and examine it under a scanning electronic microscope. This is precisely what made the Swedish teacher Kristian Storm to demonstrate to students the device of microchip. Photos are stunning: the quality allows us to consider the individual layers of the processor. Apparently, Soviet engineers did the same procedure that took apart and copied the western high technology. Much the same is being done now to examine the products of competitors.

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All photos are clickable and available in high definition.
xially 6 november 2011, 20:08