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13th October 2009

 

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Why Log Pots & Lin Pots?

Two types of potentiomers with different tracks are available. These are Linear (Lin) or Logarithmic (Log) tracks.

With linear potentiometers, the resistance between one end of the track and the wiper varies at a constant rate as the slider is moved along the track. In logarithmic types, the change in resistance is much less at one end of the track to the other. A graph of the change in resistance for given equal movements of the slider follows an (approximate) logarithmic curve; this effect is often referred to as "taper".

Logarithmic potentiometers are used as volume controls in audio equipment because the response of the human ear to the loudness of sound is also logarithmic. Using a log pot therefore gives the effect that a setting of full volume on the control sounds twice as loud as a setting of half volume. A linear pot used as a volume control would give large apparent changes in loudness at low volume settings, with little apparent change over the rest of the control´s range.

 

Potentiometers & Variables

Resistive Controls

Controls that produce a varying voltage using resistance are called either potentiometers or variable resistors. Although both types of control may be physically the same, it is the way they are connected that differentiates between the two types.

Skeleton Preset skeleton slider

A common construction is for the control to have three connections. One connected to a sliding contact called the wiper and the other two to either end of a fixed resistor called the track. The wiper is able to be moved along the track either by use of a linear sliding control or a rotary "wiper" contact. Both linear and rotary controls have the same basic operation.

The schematic symbols used for potentiometers are similar to those used for fixed resistors except that they have an arrow to indicate the slider (in potentiometers or variable resistors that are accessible to the user) or a "T" shaped line touching or crossing the fixed resistor (in pre-set controls, available only from within the equipment - for use by technicians)

Potentiometer and variable resistor symbols compared

The name POTENTIOMETER (often abbreviated to "Pot") is used when a variable potential (voltage) is obtained at the wiper terminal that is a fraction of the fixed potential across the track. The control is called a VARIABLE RESISTOR when the wiper is connected to one end of the track, effectively making it a two terminal device having a variable amount of resistance across the two terminals.

In the potentiometer the resistance of the track remains the same as the wiper moves, and only the potential on the wiper changes; in a variable resistor the resistance of the track apparently changes as the wiper moves and short circuits more or less of the track resistance.

Typical potentiometers and presets Potentiometer 1 Potentiometer 2 Potentiometer 3 Potentiometer 4 Potentiometer 5 Potentiometer 6 Potentiometer 7 Potentiometer 8 Potentiometer 9 Potentiometer 10

The construction of variable resistors is very varied as many types are made for specific purposes. Although in much equipment potentiometers have been replaced by digital control systems, a wide variety of resistive controls are still used. Common applications for potentiometers are volume controls in radio or audio equipment and in joystick controls.

The many varied designs are divided into two basic categories, often listed separately in supplier's catalogues as "Potentiometers" and "Pre-set potentiometers"(either of which can also be used as variable resistors).

In this case Potentiometers usually refer to larger types having a control spindle that may be brought to the outside of the equipment it is controlling, normally fitted with a knob or slider for the user to adjust as part of the normal operation of the equipment. The smaller Pre-set types are intended for occasional internal adjustment only during initial set up or servicing by a technician.

Typical user potentiometers and pre−sets

1

Chassis mounting volume on/off control

Volume control with a logarithmic track and double pole mains switch that switches both live and neutral power lines to completely isolate the equipment when switched off.

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2

Dual potentiometer with two pole on/off switch

Two independent potentiometers operated by concentric spindles. Used as volume and tone controls in old mains radios; the rear (volume) control has a logarithmic track and the front (tone) control, a linear track. The volume control also operates a double pole mains switch at the rear.

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3

High power wirewound preset

Insulated preset with a wirewound track for high voltages (hundreds of volts) and substantial currents. Connections pins designed for soldering directly into a printed cirsuit board.

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4

High voltage insulated preset

Using a carbon track for smoother operation than 3 and insulated with p.t.f.e. to withstand hundreds of volts, but at lower current than 3. For use in crt display equipment.

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5

Single, chassis mounting potentiometer

For general user control use. Note the long insulated spindle that may be cut to the required length. Available in a range or resistance values with linear or logarithmic carbon track.

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6

Dual ganged potentiometer

Two potentiometers sharing a single spindle are referred to as being "ganged" (What one does, the other does.) Intended for applications such as stereo audio equipment so both channels may be adjusted simultaneously.

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7

Multiturn preset

Two views of a precision slider preset, the wiper is made to slide slowly along the track by means of a screw thread turned by a small plastic gearwheel at the end. Provides a simple way of producing an accurately adjustable voltage.

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8

PCB mounted potentiometer

Standard user potentiometer for mounting on the edge of a printed circuit board (PCB). Note the small square hole in the case designed to allow cleaning fluid to be sprayed inside the control from an aerosol can. Wiper contacts tend to tarnish over time and carbon tracks become rough with wear, leading to "noisy" (e.g. crackling sound when a volume control is adjusted) operation. Spraying a mixture of alcohol and lubricant inside gives some relief and extends the life of the control.

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9

Sub miniature skeleton preset

Skeleton presets refer to controls without an enclosing case. A basic track and wiper adjusted by small insulated adjusting tool; NOT a screwdriver. Intended for general setting up purposes and only occasional use.

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Ten

Miniature skeleton preset

A larger version of 9. Both of these controls are designed for PCB mounting. Stand up and flat mounting versions are available. Modern types are usually fully enclosed but this example shows construction and operation more clearly. Small presets may have either carbon or "cermet" (a mixture of ceramic and metal) tracks.

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