Guide for Variable Resistor 2023

Variable Resistor are mainly two types of trimmer and potentiometer, whose most important feature is that the resistance value can be continuously adjustable within a certain range.

(1) Trimmer Resistors

Trimmer resistors are also known as trimmer potentiometers and semi-adjustable resistors, whose shape can be seen in the figure below. Its resistance value can be changed within a certain range, and is often used in circuits that occasionally need to adjust the resistance value, for example, as a bias resistor for transistors, bridge balancing resistors, etc.

The structure principle of trimming resistors can be illustrated by the WH7-A type vertical trimming resistor shown below. Compared with the fixed resistor, the trimmer resistor has an additional contact pin that slides between the two fixed resistor tab pins, where the resistance value between the two fixed resistor tab pins is fixed, and the resistance value is called the nominal resistance value of this trimmer resistor. And the resistance value between the sliding contact pin and any of the fixed resistor tab pins can be changed with the rotation of the sliding contact. In this way, the purpose of adjusting the voltage or current in the circuit can be achieved.

The resistance value of a trimmer resistor is usually printed on its housing or surface in an obvious place, and the marked resistance value is its maximum resistance value. Trimmer resistors are mostly used in low-current circuits with small power ratings, mostly synthetic carbon film resistors, which have the WH symbol in their model number. If the trimmer resistor is used in high-current circuits, such as power filter circuits, it is necessary to use a wire-wound semi-adjustable resistor (model number with the WX symbol).

(2) Potentiometer

Potentiometer is also an Variable Resistor, which is used in circuits where it is often necessary to change the resistance value, to carry out some kind of control or adjustment, such as radio volume adjustment, stable power supply output voltage adjustment, etc. are done through the potentiometer. Commonly used potentiometers are ordinary rotary potentiometers, potentiometers with switches, small potentiometers with switches, straight slide potentiometers, etc. Their physical objects are as follows:

Potentiometers and trimmer resistors are similar in construction, they generally have three lead pins (as shown below), where the resistance between the two fixed resistor lead pins on either side is maximum, while the resistance between the middle sliding contact lead pin and the left and right lead pins can be changed by the movement of the reed type contact connected to the rotary axis, but the sum of the two resistance values is always equal to the maximum resistance (nominal resistance value). Compared to trimmer resistors, potentiometers have longer spindles and housings and are more compactly manufactured, with some potentiometers having a separate power switch. Potentiometers can be used in place of trimmer resistors in amateur electronic productions or temporary lap line electronic experiments, as long as the size allows.

In addition to the nominal resistance, allowable deviation and power rating of the potentiometer, there is another important parameter - the resistance change characteristic, which refers to the relationship between its resistance value and the rotation angle of the shaft (or the sliding stroke of the moving arm) and the change. Common potentiometer resistance change law are linear, exponential and logarithmic 3, respectively, with the letters X, Z, D (or B, A, C) to represent the change characteristics curve as follows, specific features and uses are as follows:

Rotation angle (percentage of maximum angle of rotation) (%) 3 types of potentiometer group value change characteristics curve

1. linear (X-type) potentiometer. Its resistance value according to the rotation angle of the axis and uniform change, suitable for some requirements of uniform adjustment, such as voltage divider, crystal transistor bias current adjustment and other circuits.

2. exponential (Z-type) potentiometer. Its resistance value in the rotation of the shaft through the angle is small, the change is slow, later as the rotation of the shaft through the angle increases, the change in resistance value is gradually accelerated. Simply put, the resistance of this potentiometer changes exponentially as the shaft is rotated. This potentiometer is suitable for volume control circuit because the human ear is sensitive to a slight increase in sound, but the sound is large to a certain value, even if the sound power has a large increase, the human ear does not feel much change, using this potentiometer for volume control, you can get the volume and potentiometer angle of approximately linear relationship.

3. logarithmic (D-type) potentiometer. Its resistance value in the rotation of the shaft through the angle of a small, fast change, and later, with the increase in the angle of rotation, the change will gradually become slower, which is exactly the opposite of the Z-type potentiometer. Simply put, the resistance of this potentiometer changes logarithmically as the shaft is rotated. This type of potentiometer is suitable for feedback tone control circuits, etc. It allows the potentiometer's spindle to be turned to the center position (rotation angle of 50%) when the high and bass control circuits are in a state of neither boost nor decay.

Potentiometers must be fitted with suitable manual insulated knobs or dials for practical applications. For example, the WH15-K type small synthetic film potentiometer with switch, which is widely used in portable radios, tape recorders, etc., can only be operated smoothly if it is equipped with a special plastic dial. The plastic dial controls the rotation of the knob to control the power switch and change the resistance value.

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