Electrical parameters of the potentiometer2021

The potentiometer is a general electromechanical component, which has been widely used in instruments and various electronic equipment. Due to the variety of potentiometers, structures, installation methods and technical parameters, circuit designers first determine the performance indicators according to the role of the potentiometer in the circuit when designing and selecting the type.

Today, let's take a closer look at the definition and standards of the following major indicators of the potentiometer.

1. Total resistance value

It is defined as the resistance value between the two terminals (terminal 1, 3) of the potentiometer. When calibrating any potentiometer, you need to specify the nominal resistance. At the same time, the allowable deviation of the nominal resistance value should be specified, and the total resistance value should be within the allowable deviation range. When measuring the total resistance value, a digital ohmmeter is generally used. When the potentiometer has a mechanical stop, the moving contact should be as close as possible to a terminal lead-out. If it is a continuously rotating potentiometer, the movable contact should be adjusted until it is completely separated from the working section of the resistance element. The standard test also stipulates the maximum (DC) voltage for measuring the total resistance value (see Table 1) to limit the resistance body from not rising significantly in temperature during the measurement process.

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The total resistance value of various types of potentiometers and the typical values of their allowable deviations are shown in Table 2.

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2. Terminal resistance (zero resistance)

It is defined as the minimum resistance value obtained between the leading end of the moving contact and the leading end of the terminal when the moving contact is located at the adjacent stop. The continuously rotating potentiometer has no stop and no terminal resistance is specified. When measuring the terminal resistance, the voltage applied to the potentiometer should make the movable contact current not exceed the limit value specified in the product standard. When the movable contact is at the stop position, the minimum resistance between the terminals 1, 2 is called the front terminal resistance, and the minimum resistance between the terminals 2, 3 is called the rear terminal resistance.

3. Change in contact resistance

It is defined as the change in resistance between the moving contact and the resistor when the moving contact moves at a specified speed. Contact resistance is caused by poor contact between the moving contact and the resistance element. Surface metal oxides, sulfides, etc. can be formed on the surface of the contact or resistance element. These films act as an insulating layer and form contact resistance. The contact resistance will vary with the measured current. The change in contact resistance is also related to the material of the component, the material of the moving contact, the condition of the contact surface, and the contact pressure between the moving contact and the resistance element.

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For a given resistance element, the number of contact resistance measurement points along its contact movement track is unlimited, and it is meaningless for a single contact resistance value. Therefore, the contact resistance cannot be determined by the usual methods or product inspections. The usual technical requirement is the change in contact resistance, which is expressed as a percentage of the total resistance of the potentiometer. The circuit diagram for checking the change of contact resistance is shown in Figure 4. Contact resistance current sensitivity requires limiting current Ib. Typical values of current used to test contact resistance changes are shown in Table 3.

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4. Dynamic noise

It is defined as when the movable contact moves on the resistor body, due to the random change of the contact resistance and the resistance distribution of the resistor body

Inhomogeneity and other factors, the electrical noise that appears in the electrical output of the potentiometer 1, 2 and does not exist in the input is measured in millivolts. The dynamic noise of the potentiometer is related to the moving speed of the moving contact, the contact pressure of the moving contact, the number of the moving contact, and the applied voltage and current.

The measurement of the dynamic noise of the potentiometer adopts one of the following methods:

Method A is used when the current through the moving contact is much lower than the current through the resistor. When the potentiometer is used as a voltage divider, this method is often used for measurement (see Figure 5). The power supply resistance is 10008, a DC voltage of 20V is applied between the terminals 1, 3 of the potentiometer, and the movable contact moves through the total electrical stroke at a speed of 2 to 5 cycles of ????min (for a rotary potentiometer with a switch, The switch corner should be stepped away). Method A is usually used for the detection of dynamic noise characteristics of TV panel potentiometers. This method cannot be used to measure preset potentiometers used as rheostats. Because the preset potentiometer is in actual application, the current passing through the movable contact is the current passing through the resistor, which is completely different from the panel potentiometer used as a voltage divider.

Method B Generally, this measurement is often used when the potentiometer is used as a variable resistor. The movable contact should move at a speed of 2 to 5 cycles per minute. The test principle diagram is the same as the circuit diagram for checking contact resistance changes (see Figure 4).

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5. Resolution

For non-wirewound potentiometers, the output smoothness is used to characterize its resolution, that is, any stray changes that do not exist in the input but appear in the output, usually expressed as a percentage of the total applied voltage. And measure on the specified stroke increment within the effective electrical stroke range. For wirewound potentiometers, the resolution is a measure of the functional relationship between the output increment and the stroke of the moving contact. Theoretical resolution = 1/N&TImes; 100%, N is the effective number of turns of the winding.

6. Compliance

Compliance is the ratio of the maximum deviation between the actual value of the output voltage of the potentiometer and the specified value to the total applied voltage, expressed as a percentage. It characterizes the accuracy of the output characteristics of the potentiometer. The compliance when the prescribed law or voltage ratio is linear is called linearity, and it is a special form of compliance.

7. Temperature coefficient of resistance ΑR

It is defined as the relative change in resistance between two given temperatures within the specified ambient operating temperature range divided by the temperature difference that caused the change and the total resistance before the change (average temperature coefficient), usually 10-6 ℃-1 is the unit. The temperature coefficient of resistance mainly depends on the material of the resistance element and the specific structure of the component itself.

ΑR=(R2-R1)/R1(T2-T1)

The temperature difference between T1 and T2 should be at least 25℃, and there should be enough holding time at each temperature. The resistance temperature coefficients of various types of potentiometers are shown in Table 4.

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8. Rated power

Defined as the maximum power that can be dissipated under specified conditions. It is the maximum power that can ensure continuous and normal operation of the potentiometer within the range of the lowest ambient temperature to the rated ambient temperature. Rated power P=I2R=U2/R. The maximum rated power is the value of the power that the line designer indicates that the potentiometer can be safely dissipated without damage. The use of each specific potentiometer will affect the maximum allowable dissipation value of the rated power.

For most potentiometers, the maximum rated power refers to the situation when the potentiometer is used as a voltage divider. Therefore, when a voltage is applied to the input terminal of the potentiometer, the value of the load current through the movable contact is not large. Potentiometer manufacturers generally use the power reduction curve shown in Figure 6. In the figure, when the ambient temperature is between t1 and tmax, the allowable power dissipation is specified by the load shedding characteristic formed by the straight line connecting the two points A and B. The maximum allowable power dissipation at the ambient temperature between tmin and t1 is the rated power. The line designer should strictly abide by this regulation.

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Allowable value in still air. When the potentiometer needs to be installed close to heating elements such as power transistors, transformers, high-power resistors, etc. or adjacent to another potentiometer, the allowable power dissipation should be reduced.

The rated power specified on the product and on the performance data sheet applies to the situation where the potentiometer is used as a voltage divider. At this time, the dissipated power can be considered to be evenly distributed along the entire resistance element. When the potentiometer is used as a rheostat or two-terminal connection method, corresponding to a given adjustment position of the moving contact, only a part of the resistance element consumes power, and all the current flowing through the resistance element flows through the moving contact circuit , And the pressure contact point between the moving contact and the resistance element is not always able to withstand the current as large as the resistance element alone. The rated power of the voltage divider connection method assumes that the moving contact current is negligible. Therefore, for the rheostat connection method, the maximum allowable value of the moving contact current Im=

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Where P is the maximum power dissipation; RT is the total resistance value. Using this maximum current limit value, you can ensure that the maximum power of the potentiometer is not exceeded. The rated power range of various types of potentiometers is shown in Table 5.

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9. Synchronization error

Defined as the degree of difference between the output voltage and the input voltage ratio of each resistor body of the multi-connected potentiometer at a specified position. According to IEC regulations, the synchronization error is the difference between the attenuation of any potentiometer in decibels and the attenuation of the reference connected potentiometer in decibels To represent. In the normal use state, when the input voltage of each potentiometer is the same to measure the synchronization error, the voltage applied between the terminals 1, 3 is generally a DC voltage (2-15V). The test circuit is shown in Figure 7. Synchronization error is a key indicator for evaluating multi-connected potentiometers. This parameter directly affects the use effect of the whole machine, but it is difficult to accurately control. There are many factors that affect synchronization errors, mainly:

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It can be seen from the definition formula of synchronization error that synchronization error is related to the resistance change law (voltage change law), but has nothing to do with the total resistance value. That is, multi-connected potentiometers with different total resistance values ​​but the same resistance change law can also be used. Achieve good synchronization. However, due to uneven film thickness or uneven distribution of conductive particles, at the same stroke position, different potentiometers have different attenuations, resulting in poor synchronization errors. The mechanical misalignment of the contact between the movable contact of each potentiometer and the resistor body is also an important reason for the synchronization error. Improper matching tolerances of related parts and improper assembly process may cause the synchronization error to increase.

When the potentiometer is used for volume control, it is generally required to be in the range of -40 to 0dB, and the synchronization error is ±3dB. When the potentiometer is used for tone and balance control, it is generally required to have a synchronization error of ±3dB at 50% of the electrical stroke. 3110 The law of resistance change.

10. The law of resistance change

It is defined as the relationship between the ratio of the potentiometer output voltage (V122 or V223) to the input voltage (V123) and the mechanical position of the moving contact (for a rotary potentiometer, it refers to the angle of rotation). The resistance laws of general potentiometers are as follows (see Figure 8):

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a. Straight line law: law A

b. Logarithm law: law B

c. Reversal logarithm law: law C

d. Super logarithm law: law Z

Potentiometer manufacturers can also design and manufacture potentiometers with other resistance changes according to the special requirements of the complete machine factory.

11. Durability

(1) Mechanical durability (wear-resistant life)

Under the specified test conditions, the number of operating cycles obtained by the potentiometer drive mechanism when the reduction of the potentiometer performance is kept within the allowable range of the technical specifications (the moving contact runs back and forth along the working path of the resistance element for one week). Table 6 shows the typical values of wear life of various types of potentiometers.

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(2) Electrical durability The time during which the potentiometer can continue to work normally and its performance remains within the allowable range of the technical specifications under the specified load and the test conditions of the moving contact not moving. According to IEC regulations, the electrical durability of the potentiometer is 1000h.

12. Insulation voltage

It is defined as the maximum peak voltage that can be applied between the lead terminal of the potentiometer and its outer conductor under continuous normal working conditions. For multi-connected potentiometers, the measurement should be made between the lead-out ends of each joint and the lead-out ends of the other joints. Under normal air pressure, the insulation voltage value should not be less than 1142 times the limit voltage of the resistor.

13. The resistor limit voltage is defined as the maximum DC voltage value or the AC effective voltage value that can be applied between the two ends of the potentiometer resistor. When the AC effective value is used, the voltage peak value shall not exceed 1142 times the effective value. The limit voltage of the resistor is usually 20～250V. For specific products, the limit voltage of the resistor is given in the product standard. 3114 The withstand voltage is defined as being added between the lead-out end of the potentiometer and its external conductor; the multi-connected potentiometer should be between the lead-out end of each joint and the lead-out ends of other joints; the switch lead-out end of the switch potentiometer and the potential The AC voltage (frequency 40-60Hz) between the lead-out end of the device and its external conductor shall be applied for 1 min, and no damage, sparks, or insulation damage shall occur. The terminals can be connected together for measurement. The withstand voltage is usually AC 100~1000V. For specific products, the withstand voltage is given in the product standard.