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Variable Resistors: Definitions, Symbols And Characteristics

Variable Resistors: Definitions, Symbols And Characteristics

November 30 , 2023 0

This paper will introduce the definition, symbol and characteristics of variable resistors. A Variable Resistor is an electronic component that adjusts the resistance value. It is different from fixed resistors in that its resistance value can be changed by physical operation. The article will first explain the basic principle and structure of variable resistors, including knobs, rotating axes, and rotatable resistance elements. Next, we will explore the symbolic representation of variable resistors in order to correctly identify and use them in circuit diagrams. The article will also cover the characteristics of variable resistors, including adjustment range, resolution, linearity, and temperature coefficient. Understanding these characteristics helps in selecting a variable resistor suitable for a particular application. By reading this article, you will gain a deeper understanding of variable resistors so that you can properly apply and adjust them in electronic circuit design and debugging.

Definition OF Variable Resistance

Variable resistor, a resistor whose resistance value can be adjusted, used in situations where the circuit current needs to be adjusted or the circuit resistance needs to be changed. Variable resistors can change the characteristics of the signal generator, dim the light, start a motor, or control its speed.

Depending on the use, the resistance material of the variable resistor can be a metal wire, a metal sheet, a carbon film or a conductive liquid. For general currents, metal type variable resistors are commonly used. In the case of low current, the carbon film type is used. When the current is large, electrolytic type is most suitable; The electrodes of this variable resistor are immersed in a conductive liquid. Potentiometer is a special form of variable resistor, which makes the unknown voltage or unknown potential phase balance, so as to measure the unknown voltage or unknown potential difference. A more common potentiometer is nothing more than a resistor with two fixed connections, and a third connection connected to an adjustable brush. Another use of potentiometers is for sound control in audio equipment.

Classification OF Variable Resistors

Generally, variable resistors can be divided into film variable resistors and wire-wound variable resistors according to the production material

Film Type Variable Resistor

Film type variable resistor adopts rotary regulation mode, generally used in small signal circuit, signal voltage and so on.

The film type variable resistor is usually composed of a resistance body (synthetic carbon film), a movable contact plate (movable metal reed or carbon contact), a regulating part and three pins (or solder). Two of the fixed pins are connected to both ends of the resistor body, and the other pin (center tap) is connected to the movable contact plate. The resistance value between the center tap and the two fixed pins can be changed by rotating the part and changing the contact position between the moving contact and the resistance body with the small font screw screwdriver.

The film type variable resistor has three kinds of structure: fully sealed, semi-sealed and non-sealed.

a. Fully sealed film type variable resistor, also known as solid type variable resistor, the resistor body is made of carbon black, quartz powder, organic adhesive and other materials mixed, pressed into the matrix of plastic or epoxy resin and other materials, and then heated polymerization. The movable contact plate is made of carbon contact and the adjusting part is made of plastic. The resistance body and the active contact are sealed by a metal housing (with adjustment holes above the metal housing). Its advantage is that it has good dustproof performance and rarely has bad contact faults.

b. The resistance body of the semi-sealed film type variable resistor and the resistance system of the fully sealed variable resistor are basically the same. The movable contact plate is made of metal reed, and the external plastic cover is sealed. When the plastic cover is rotated, the movable contact plate also rotates. The variable resistor is easy to adjust, but its dustproof performance is not as good as that of the fully sealed film variable resistor.

c. Non-sealed film type variable resistor, also known as sheet adjustable resistor, the resistor body is made of carbon black, graphite, quartz powder, organic adhesive, etc., into a suspension, coated in fiberglass board or rubber board. The movable contact plate adopts a metal reed, which is provided with an adjusting hole, and no separate adjusting parts are provided. Its disadvantages are poor dust resistance, easy oxidation of contacts, and poor contact with synthetic carbon film.

Wire-Wound Variable Resistor

The wire-wound variable resistor belongs to the power type resistor, which has the advantages of low noise, high temperature resistance, large current bearing, etc., and is mainly used for voltage or current adjustment of various low-frequency circuits.

High-power wire-wound variable resistor is also known as slip-wire type rheostat, which is divided into axial porcelain tubular wire-wound variable resistor and porcelain disc wire-wound variable resistor.

Small power wire-wound variable resistors include circular vertical wire-wound variable resistors, circular horizontal wire-wound variable resistors and square wire-wound variable resistors, all of which are fully sealed packaging structures.

The Shape Characteristics OF The Variable Special Rent

Variable resistance and ordinary resistance have a great difference in shape, it has the following characteristics, according to these characteristics can be identified in the circuit board variable resistance:

(1) The volume of the variable resistance is larger than the volume of the general resistance, and the variable resistance in the circuit is less, and it can be easily found in the circuit board.

(2) The variable resistor has three pins, the three pins are different, one is the moving pin, the other two are the fixed pin, generally the two fixed pin can be used interchangeably, and the fixed pin can not be used interchangeably with the moving pin.

(3) There is an adjustment port on the variable resistor, and the flat-head screwdriver is extended into the adjustment mouth. Turning the screwdriver can change the position of the moving piece and adjust the resistance value.

(4) The nominal resistance value can be seen on the variable resistance, which refers to the resistance value between the two fixed pins, and is also the maximum resistance value between a fixed pin and the moving pin.

(5) Vertical variable resistance is mainly used in small signal circuit, its three pins are vertically down, vertically installed on the circuit board, resistance value adjustment port in the horizontal direction.

(6) Horizontal variable resistor is also used in small signal circuit, its three pins and resistance plane into 90°, vertical down, horizontal installation on the circuit board, resistance value adjustment port facing up.

(7) The variable resistance of the small plastic shell is smaller and has a circular structure, its three pins are downward, and the resistance value adjustment port is upward.

(8) It is used for variable resistance (wire-wound structure) in large power occasions, and the volume is large, and the moving plate can slide left and right to adjust the resistance value.

Function And Characteristics OF Variable Resistor

Variable resistance is first of all a resistance, it can play the role of resistance in electronic circuits, it is different from the general resistance is that its resistance value can be continuously changed within a certain range, in some require the resistance value change and not often change occasions, variable resistance can be used.

Due to the structure and use 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 cases such as vacuum tube amplifiers.