Variable resistance wire resistor with drawings and documentation

Contents

Introduction

1. General information about potentiometers

1.1. Appointment

1.2. Potentiometer classification and design features

1.3. Basic parameters of variable resistors

1.4. Materials used in the manufacture of potentiometers

2. Potentiometer calculation

2.1. Linear potentiometer calculation

2.2. Calculation of Frame Profile

3. Substantiation of applied design solutions and selection of applied materials

4. Assembly Drawing Design Description

Conclusion

List of references

Appendix A

Appendix B

Introduction

Variable resistors are one of the most common elements today. They have been widely used in electronic, television, recording and sound reproduction equipment.

Potentiometers have a simple design, a small number of structural units and parts and are one of the most reliable elements of electronic technology. Modern variable resistors are characterized by high technical characteristics and work reliably in harsh operating conditions.

The development of variable resistors is a solution to a set of complex technical problems in the synthesis of conductive and insulating materials, the calculation of structural elements and the determination of optimal technological modes for their manufacture. The correct placement of variable resistors in the equipment and the choice of the optimal version of their attachment are one of the important points in the design of the equipment.

The design and especially the production of potentiometers is associated with great difficulties arising from the requirements for potentiometers. In particular, they must work reliably in severe climatic and temperature conditions, at rotational speeds reaching several thousand revolutions per minute, with the obligatory preservation of stability and high accuracy of electrical characteristics.

Appointment

The potentiometer is designed for smooth or jump-like change of electric voltage and is defined as an adjustable voltage divider built from active resistances (wire, film, liquid, etc.).

Potentiometers are used in computer circuits, in automatic tracking drive systems and measuring bridge circuits.

In most cases, they serve as angular and linear displacement sensors, converting the latter to their corresponding electrical voltages.

Structurally, the simplest potentiometer is an electric conductor (resistive element) with a sufficiently large ohmic resistance, along which a movable contact slides, including any part of a given voltage in the electric network. Therefore, using a potentiometer whose resistance is distributed according to some law, it is possible not only to convert a mechanical value into an electric value, but also to realize the required functional connection between these values.

Basic parameters of variable resistors

To describe the properties of variable resistors, a certain system of parameters and characteristics is adopted, which gives complete information about electrical, structural, operational characteristics, as well as about the reliability of the product.

The rated resistance (RH) of the variable resistor is the electrical resistance indicated on the housing or specified in the regulatory documentation between the extreme terminals 1 and 3 of the resistive element. It is the starting point for the deviation of the actual resistance value from the nominal value.

The impedance of the variable resistor RP is the resistance of the resistive element measured between terminals 1 and 3 when the mobile system is set to the extreme position. Resistor impedance typically differs slightly from the nominal resistance to which resistor production is tuned due to deviations in the product manufacturing process. The difference between nominal and impedance, expressed as a percentage of the nominal value, is called the deviation from the nominal resistance. This deviation must not exceed a certain allowable value.

Permissible deviation of resistance of resistor ΔR - deviation of impedance from nominal one, which is within the limits established by regulatory documentation. The tolerance value is specified as a percentage. Variable resistors can be made with permissible resistance deviations ± 5, ± 10, ± 20 and ± 30%.

Variable resistor resistance is controlled by moving movable contact over surface of resistive element. The electrical resistance between the terminal 1 of the resistive element and the terminal 2 of the movable contact is called the set resistance of the variable resistor. The value of the set resistance changes as the movable resistor system moves. For variable resistors, the following values ​ ​ of the established resistance are normalized at certain positions of the mobile system.

The minimum resistance Rmin of the variable resistor is the set resistance measured when the moving system is brought to the appropriate stop when the axis rotates counterclockwise. For resistors that do not have stops, the minimum resistance corresponds to the lowest value of the established resistance.

The minimum resistance corresponds to the position of the moving system when the contact contacts the surface of the contact pad of the resistive element. When moving the movable resistor system at a moment corresponding to the transition of the movable contact from the contact pad to the current-conducting layer of the resistive element, a sharp change in the established resistance, called the initial resistance jump, is possible.

Initial Rn.s resistance jump. - the value of resistance, starting from which there is a smooth increase in resistance, proportional to the movement of the movable contact. Typically, an initial drag jump occurs when the movable contact is released from the pad. Resistance increases from minimum Rmin to Rn.s. The values ​ ​ of the minimum resistance are usually expressed in ohms, and the initial jump in fractions or percentage of the nominal resistance of the resistor.

Variable resistor resistance is controlled by changing the position of the mobile system. Most variable resistors have a limited rotation angle or limited travel (for propulsion resistors) of the mobile system. Note here that mechanical, electric and working angle of turn or travel of variable resistor moving system are distinguished.

Mechanical rotation angle αp is the full rotation angle of the movable system of the variable resistor from stop to stop.

Electrical rotation angle αe is the rotation angle of the movable system, within the limits of which the electrical resistance of the variable resistor changes.

The working rotation angle αp is the rotation angle of the movable system, within which the given functional characteristic of the variable resistor is reproduced.

In real designs of variable resistors, the condition αp > αe > αp is usually fulfilled.

The functional characteristic - the dependence of the established electrical resistance on the position of the mobile system - is the most important characteristic of the variable resistor. It shows the dependence of the resistance between the movable contact and the terminals 1 or 3 of the resistive element on the angle of rotation or movement of the resistor adjustment member. By the nature of functional dependence, variable resistors are divided into linear and non-linear.

The resolution indicates at which least change in the rotation angle of the movable resistor system the change in the set resistance can be discernible. The resolution of variable wire resistors depends on the number of turns of the resistive element and is determined by the smallest movement of the movable contact at which a discernible change in the value of the set resistance occurs (the more turns, the less the jump in the set resistance and the higher resolution).

Own noises. At the terminals of any resistor, there is always an alternating voltage, characterized by a continuous wide spectrum and approximately the same intensity of all components. In wire resistors, the appearance of noise voltage is associated with the thermal movement of free electrons.

The moving noise of the mobile system is an important variable resistor parameter and largely determines whether a resistor of one type or another can be used in a particular circuit. When moving the movable resistor system, some variable component is superimposed on the regular change of the set resistance in accordance with the given functional characteristic. If some voltage is applied between the terminals 1 and 3 of the resistor, the output voltage removed from the terminals 2 and 3 will comprise an alternating interference component called the moving noise voltage of the mobile variable resistor system. Motion noises are particularly characteristic of non-conductive variable resistors.

Rated power and voltage limit. By rated power Pn is meant the maximum allowable power, which the resistor can dissipate for a long time under continuous electric load under specified conditions, keeping the parameters within the specified limits.

Limiting factors in resistor operation are heating temperature and maximum voltage. With an increase in the ambient temperature, the heat transfer deteriorates, the resistor overheats above the permissible temperature, as a result of which it becomes necessary to reduce the electrical load, that is, reduce the dissipated power. In other words, the actual dissipation power is related to ambient temperature and operating conditions. Typically, the power versus temperature dependencies by which the electrical load is selected depending on the resistor conditions are given.

The power that the resistor can dissipate depends on the design and physical properties of the materials used. The higher the heat resistance of the structural and resistive materials, the higher the dissipated power for a given resistor volume.

Limit operating voltage is the maximum voltage for this type of resistor, which is set based on its design, dimensions and conditions to ensure long-term operability.

At normal and increased atmospheric pressure, the limit voltage on the resistor is limited by the thermal processes in the resistive element and the electrical strength of the resistor, and at reduced pressures, the main limiting factor is electrical strength, which decreases with a decrease in atmospheric pressure. Electrical breakdown or surface overlap is possible here. At very low pressures, electrical strength increases as the number of charge carriers in the environment drops.

The temperature coefficient of resistance (TKS) of the resistor characterizes the relative change in resistance when the external temperature changes by 1 K. TKS of the resistor depends mainly on the temperature coefficient of the material of the resistive element .

Wear resistance - the ability of the resistor to maintain its parameters within certain limits during multiple rotations of the mobile system. This is one of the main performance characteristics of variable resistors.

The wear resistance is quantified by the number of cycles of movement of the movable system during the service life while maintaining stability of the parameters within the specified tolerances and is determined mainly by the structure, material and shape of the movable contact and the resistive element and contact pressure. The cycle is the movement of the mobile system from stop to stop and back.

When the movable system rotates, both the resistive element itself and the movable contact wear. This process is the more intense the greater the contact force. It follows that in order to increase wear resistance, it is necessary to reduce the contact pressure, but in this case, due to a decrease in the rotation moment of the moving system, mechanical resistance is reduced. Therefore, it is very difficult to meet the requirement of high wear resistance while maintaining mechanical resistance.

Accurate potentiometers operating in tracking systems are characterized by low contact pressures, respectively, low moments of rotation. Their wear resistance reaches 105-107 turns, but at the same time the vibration and impact resistance is lower. General use adjustment resistors have good mechanical resistance, but their wear resistance is lower and ranges from 5,000 to 50,000 cycles. Trim resistors have a wear resistance of 500-1000 cycles. [1,3]

Materials used in the manufacture of potentiometers

The main element of the structures of variable wire resistors is a PE consisting of a wire (insulated or non-insulated) wound on an insulating frame. The sliding contact moves along the pre-cleaned (when using an insulated wire) portion of the element so that contact is not disturbed.

The main electrical parameters of the resistor are due to the properties of conductive and contact materials used in its design. Consider the properties of the materials most widely used in variable wire resistor designs.

Conductive materials.

To ensure high operational parameters of the wire resistor, it is necessary that the conductive material has the following properties: high stability of resistivity in time, low TKS, high corrosion resistance, low thermal resistance. relative to copper, the ability to extend into a wire with a diameter of tenths - hundredths of a millimeter. Special alloys based on nickel, chromium, copper, manganese, as well as alloys based on noble metals, have a complex of these properties. In addition, the conductive material must have a small and time stable contact resistance paired with the sliding contact material. This requirement is especially important when selecting materials for low-resistance resistors, therefore, along with alloys of non-noble metals, alloys with palladium, silver and gold began to be used as conductive materials.

For particularly critical structures of variable wire resistors, gold and silver alloys with good contact properties and high corrosion resistance are used; however, such alloys are used limited due to their high cost.

Contact materials.

Resistor resistance is typically controlled by sliding contact springs. Sliding contacts are typically spring parts that directly contact the PE or the current collector ring, providing electrical contact with the middle lead of the resistor. The sliding contact can be made together with a spring of the same material or in the form of a special patch of another material, which is securely attached to the spring; in a number of structures, the sliding contact is metal applied to the spring at the points of contact.

Materials of sliding contacts shall have the following properties: low specific volume resistance, high corrosion resistance at high temperature and humidity, high erosion resistance and resistance against welding, low thermal resistance, resistance to sintering of contacts, high wear resistance paired with PE wire, as well as low, time-stable contact resistance paired with current collector and PE. Some rare metals, as well as noble metals and alloys based on them, satisfy these requirements.

The materials from which the elastic elements of the contact systems of resistors (springs, etc.) are made must have good elastic properties, stable over time, and, when exposed to elevated temperatures, have a low temperature. relative to copper, high corrosion resistance, high wear resistance paired with PE material. As a material for contact springs of wire resistors, tin-phosphorous bronze of the brands BrOF 6.51.5 and BrOF 40.25 and silicon-manganese bronze of the brand BrKMz, copper nickel alloy neisilber, as well as alloys from noble metals are often used.

The frames of PE resistors are made of ceramics, heat-resistant press materials (AG4, K-211-3), ST- and SKM1 steclotextolite, electrical cardboard, etc. Wire attachment on the frames, as well as PE glue to the cases or posts is carried out with varnishes K47, K-57, CLP 5, K5, KP 815,

When sealing resistors, compounds based on resins ED5, ED-6, T10 with various fillers are used. Copper, brass or nickel wire coated with silver, gold or tin-lead solders is used for resistor leads; and wiring wires of various grades.

Fluoroplastic 4, organosilicon rubbers of grades 14p2 and 14p6, polyamide resins and other insulating materials are widely used to manufacture various structural elements. [1,2,5]

Conclusion

In this course design, a single-turn wire resistor of variable resistance was designed and calculated. The frame was designed to implement the condition-defined logarithmic law of change of resistance value.

Texolith on chiffon is chosen as material for carcass, and nichrome is chosen as material of potentiometer winding.

The most effective way to further improve the reliability of variable wire resistors is to create and use new resistive and framework materials that meet constantly increasing requirements.

The materials of the building also play an important role. The use of stronger metal and ceramic materials will lead to increased mechanical and temperature resistance. An important aspect of the reliability of the variable resistor is the tightness of the housing, which means the use of a better adhesive and the search for a more efficient design.