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What is a Potentiometer Voltage Divider?

Author: Apogeeweb
Date: 10 Dec 2020
 2224
types of potentiometers

Introduction

A potentiometer is a three-terminal resistor with a sliding or rotating contact. It is an adjustable voltage divider with two static contacts and one moving contact. The moving terminal is a wiper that travels across the resistance element, usually in an arc controlled by a rotary knob. Rotating the knob gives a ratiometric division of the potential across the resistance element. Potentiometer generally used in speakers and receivers for volume control. In addition, it cannot directly control the motor because its power is too small. When potentiometer acting as a voltage divider, the absolute resistance of the potentiometer will not affect the output voltage, and its output voltage is proportional to the input voltage.

potentiometer structure

Figure 1. Potentiometer Structure

Catalog

Introduction

Ⅰ Potentiometer Wiring

Ⅱ Potentiometer Symbol and Diagram Descriptions

Ⅲ Potentiometer Voltage Divider Circuit Basics

Ⅳ Potentiometer Resistance Measures


Ⅰ Potentiometer Wiring

The potentiometer can be used as a three-terminal component or a two-terminal component. The latter can be regarded as a rheostat. For a general potentiometer (three pins), the slider near the center which is a resistance wire. The two pins at both ends of the resistance wire are connected to the input and the ground (some are not connected), respectively. That is, one pin is connected to the input signal and the other pin is grounded. At this time, the resistance wire has a total resistance value on the two sections. You move the sliding piece to go across this resistance wire to get a variable resistance. If the input and output signals are reversed, the sliding direction of the slide is opposite to the resistance change.

How to Wire a Potentiometer

1) For a potentiometer (or a trimming resistors) with traditional pins (three pins), the resistance at both ends is fixed, and the resistance of the middle pin is variable. That is, the terminal on both sides of the potentiometer are total resistance, and the middle is changing. For example, the power supply is connect from any one contact on resistor sides and output from the middle contact, and the voltage changes with the rotation of the middle contact.

potentiometer voltage divider output depends on the percentage of rotation

Figure 2. Potentiometer Voltage Divider Output


2) Although the resistance can vary with the slider, the total resistance value of pins is fixed. At this time, the potentiometer is equal to a current controller, and the selected current output terminals must be the sliding terminal. Commonly used potentiometers have poor resistance accuracy and poor temperature coefficient, but as long as the resistance of the potentiometer is uniform, so is the output voltage value.

potentiometer as voltage divider

Figure 3. Potentiometer as Voltage Divider

3) If a potentiometer used as a variable voltage divider, one contact connects to the input voltage, the middle contact connects to the output voltage, and the other contact can be grounded. When the rotary handle or sliding handle of the potentiometer take action, the movable contact slides on the resistor. At this time, an output voltage that has a certain relationship with the external voltage, wiper angle and travel stroke.
4) If a potentiometer used as a variable resistor, one end connects to the input voltage, the middle end connects to the output, and the other end can be suspended or connected to the middle end to obtain a smooth and continuously changing resistance value.

potentiometer connected as variable resistor

Figure 4. Potentiometer Connected as Variable Resistor

 

As above mentioned, a potentiometer can be connected as a variable resistor, if you want to know more info, you can get it from The Differences Between Potentiometer and Adjustable Resistor.

 

Ⅱ Potentiometer Symbol and Diagram Descriptions

Name

Potentiometer Symbol

Description

Normal Potentiometer

potentiometer symbol

Use RP (resistor potentiometer) to represent the potentiometer. The symbol in the figure mark the 3 pins of the potentiometer, indicating the chip pins.

Potentiometer Switch

potentiometer symbol

S1 is a switch attached to the RP, and S1 is controlled by the RP rotary handle. When you start to turn the handle, after the switch closes, this circuit is the same as a normal potentiometer. This kind of potentiometer is mainly used in volume control circuits with power switches.

As a Variable Resistor

potentiometer symbol

The potentiometer at this time can be regarded as a variable resistor.

A Potentiometer with dual port

potentiometer symbol

Divide the 3 pins of the potentiometer into 4 ends to form a dual-port circuit. That is, terminals 1 and 2 input signals, terminals 3 and 4 output signals, and terminals 2 and 4 are common terminals, which usually connected to the ground of the line.

Dual Gang Potentiometers

potentiometer symbol

It has two single potentiometer circuit symbols, which are connected by a dotted line to indicate that the resistances of dual gang potentiometer are adjusted simultaneously, that is, their resistance values increase or decrease at the same time.

Half Linear Stroke

potentiometer symbol

This is a special dual gang potentiometer. Two potentiometer travel strokes are synchronized, but their resistance changes are not the same during adjustment. Only one mechanical stroke of this kind of potentiometer changes in resistance, and the resistance value of the silver zone is zero of another one. The silver zone with no resistance is indicated by shading in the circuit symbol. When the moving contact slides up from the middle, the moving piece RP-1 will enter the silver zone, and RP-2 will enter the varistor area; when the moving handle slides down from the middle position, the RP-2 will enter the silver zone, and RP-1 enters the varistor zone. This special potentiometer can be used in a stereo balance controller circuit (a control circuit in stereo equipment).

With Central Tap

potentiometer symbol

It has one more pin than the ordinary potentiometer, that is, the tapping pin. The tapping pin is set at the middle resistance of the potentiometer, and the resistance between the tap and the two fixed contacts is equal. There are also potentiometers whose taps are not set in the middle position.


potentiometer circuit

Figure 5. Potentiometer Circuits

Potentiometer Voltage Divider Circuit Basics

1) The resistor of the potentiometer is mostly made of polycarbonate synthetic resin. The following items should be avoided: ammonia, other amines, aqueous alkali solutions, aromatic hydrocarbons, ketones, lipid hydrocarbons, strong chemicals (excessive pH), etc., otherwise it will affect potentiometer performance.
2) When soldering the potentiometer terminals, avoid using water-capacitive flux, which will cause metal oxidation and mold material. Using inferior flux, poor soldering may cause problems in soldering, resulting in poor contact or open circuit.
3) If the soldering temperature of the terminal is too high or the soldering time is too long, it may cause damage to the potentiometer. The temperature range of the plug-in potentiometer is 235℃±5℃; the wire bonding type is 350℃±10℃, and the soldering point should be more than 1.5mm away from the potentiometer body. In addition, avoid heavy pressure on the terminals, otherwise it is easy to cause poor contact.
4) During soldering, the height of the flux entering the printing machine board should be adjusted properly, and it should be avoided to affect the potentiometer. Because it will cause poor contact between the brush and the resistor, or result in noise.
5) The potentiometer is better in the voltage adjustment structure.
6) Avoid condensation or water droplets on the surface of the potentiometer, and avoid using potentiometer in a humid place to prevent insulation deterioration or short circuit.
7) When fixing the screws of the rotary potentiometer, the strength should not be too strong to avoid poor rotation. For the direct-sliding potentiometer, avoid using too-long screws, otherwise it may hinder the movement of the sliding handle and even damage the potentiometer itself.
8) In the process of putting the potentiometer on the knob, the pushing force should not be too large (don’t exceed the parameter index of the rated pushing and pulling force), otherwise it may cause damage to the potentiometer.
9) The rotary force of the potentiometer will decrease as the temperature increases, and become smaller as the temperature decreases. If the potentiometer is used in a low temperature environment, it needs special low temperature resistant grease.

Potentiometer Voltage Divider Circuit Connections

Figure 6. Potentiometer Voltage Divider Circuit Connections

10) If the shaft or sliding handle of the potentiometer is too long, it is easy to shake and cause the instability of the circuit signal.
11) The carbon film of the potentiometer can withstand the ambient temperature of 70℃, and its function may be lost when the temperature is higher than 70℃.
12) For a adjustable potentiometer, when the DC is allowed to pass through the movable contact, the problem of anodic oxidation may occur. In this case, it is best to connect the component with the negative end and connect the moving contact with the positive end.
13) The load current of the adjustable potentiometer cannot be increased at will to keep circuit safety. And actual current measurement would be to put ammeter in series with the potentiometer in the active circuit.
14) Do not exceed the rated power when using the adjustable potentiometer. For example, when the power dissipation exceeds the rated value, it will cause the potentiometer to overheat.
15) A Potentiometer is sensitive if it is capable of measuring very small potential differences, and shows a significant change in balancing length for a small change in potential difference being measured.
16) A DC potentiometer is created by dropping voltage across a set of resistors in series. Different resistors will produce different values. In AC potentiometer, one can use resistors or even inductors or capacitors as impedances which will drop voltages and provide a voltage less than applied voltage.
17) If positioned the potentiometer wiper on the center of the resistor element then the voltage at the wiper is 50%; if the wiper is positioned 1/4 of the way from the negative node then the wiper voltage is 1/4th the entire voltage.
18) Potentiometer nomenclature: It generally use the direct marking method. Letters and numbers are marked on the potentiometer shell to indicate their model, nominal power, resistance, and the relationship between resistance and rotation angle.

Potentiometer image

If you have any interest, with a deep reading, you can get more info from What is the Voltage Divider Basic and Rule.

 

Ⅳ Potentiometer Resistance Measures

The main checking requirements for the potentiometer are: 
① The resistance value meets the circuit requirements. 
② The connection between the center sliding end and the resistor is good, and the rotation is smooth. For potentiometer with switches, the switch action should be accurate, reliable and flexible. 
Therefore, the performance of the potentiometer must be checked before use.
1) Resistance measurement: First, select the appropriate gear of the multimeter according to the resistance of the measured potentiometer. Whether the resistance between the two ends of the AC is consistent with the nominal resistance. Rotate the sliding contact, and its value should be fixed. If the resistance indicates infinite, the potentiometer is damaged.
2) Then measure the contact between the center end and the resistor, that is, the resistance between the two ends of BC. The method is to set the ohm range of the multimeter in the appropriate range. During the measurement, slowly rotate the shaft and observe the reading of the multimeter. Normally, the reading changes steadily in one direction. If there is a jump, drop, or blockage, it means that the movable contact has failures.
3) When the center end slides to the head or the end, the resistance value of the center end and the coincident end is 0 for an ideal state. In the actual measurement, there will be a certain value (generally determined by the nominal value, generally less than 5Ω), which is normal.

 

Frequently Asked Questions about Potentiometer Voltage Divider

1. How can a potentiometer be used as a voltage divider?
Potentiometers can be used as voltage dividers. To use the potentiometer as a voltage divider, all the three pins are connected. One of the outer pins is connected to the GND, the other to Vcc and the middle pin is the voltage output. Basically, the voltage divider is used to turn a large voltage into a smaller one.

 

2. How does a potentiometer affect voltage?
When the potentiometer resistance is decreased (the wiper moves downwards) the output voltage from pin 2 decreases producing a smaller voltage drop across R2. Likewise, when the potentiometer resistance is increased (the wiper moves upwards) the output voltage from pin 2 increases producing a larger voltage drop.

 

3. What is a dual gang potentiometer?
It means two potentiometer combined on the same shaft, enabling the parallel setting of two channels. Most common are single turn potentiometers with equal resistance and taper.

 

4. How many volts can a potentiometer handle?
100 volts
If the potentiometer is rated at 1 Watt, you can only apply a maximum of 100 volts. I.e 10 mA. That applied to the voltage across the full 10000 ohms. That also means that you cannot pass more than 10 mA into the Wiper.

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pinglun 1 comment

    • pingluntus
    • Joey on 2020/12/14 9:55:04

    A little...what is difference between rheostat and potentiometer?

      • pingluntu
      • author on 2020/12/14 16:45:49
        author

      Re:

      The potentiometer is a three terminal device used for voltage control, while the rheostat is a two terminal device used for current control. Then the potentiometer, trimmer and rheostat are electromechanical devices designed so that their resistance values can be easily changed.

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