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Potentiometer

A potentiometer is a manually adjustable resistor.  One terminal of the potentiometer is connected to the power source whilst another goes to ground.  The third terminal runs across a strip of resistive material, this strip tends to have a low resistance at one end and its resistance gradually increases to a maximum resistance at the other end, the third terminal serves as the connection between the power source and the ground and is usually interfaced to the user with some form of control in the form of a lever for example.  The user adjusts the position of the third terminal along the resistive strip in order to manually increse or decrease resistance.  By controlling resistance, the potentiometer determins how much current runs through the circuit.  If it is used to regulate current then the potentiometer is limited by the maximum resistivity of the strip.

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.

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.

3.  High power wirewound preset

Insulated preset with a wirewound track for high voltages (hundreds of volts) and substantial currents.  The connection pins on this potentiometer are designed for soldering directly into a PCB.

4.  High voltage insulated pre−set

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 cathode ray tube display equipment.

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.

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.

7.  Multi−turn pre−set

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 gear wheel at the end.  Provides a simple way of producing an accurately adjustable voltage.

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.

9.  Sub−miniature skeleton preset

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

10.  Miniature skeleton preset

A larger version of 9. Both of these controls are designed for PCB mounting. Upright 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.

My next task will be to design some basic digital circuits using potentiometers.

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