potentiometer lesson
When a voltage is applied across a potentiometer, it can deliver a variable fraction of that voltage. It is often used to adjust sensitivity, balance, input, or output, especially in audio equipment and sensors such as motion detectors.
A potentiometer can also be used to insert a variable resistance in a circuit, in which case it should really be referred to as a variable resistor, although most people will still call it a potentiometer.
It can be used to adjust the power supplied to a circuit, in which case it is properly known as a rheostat, although this term is becoming obsolete.
Massive rheostats were once used for purposes such as dimming theatrical lighting, but solid-state components have taken their place in most high-wattage applications.
A full-size, classic-style potentiometer is shown in Figure 11-1.
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| Figure 11-1 classic-style potentiometer |
The correct symbols for a variable resistor or rheostat are shown at center, although a potentiometer symbol may often be used instead. A preset variable resistor is shown at the bottom, often referred to as a trimmer or Trimpot. In these examples, each has an arbitrary rated resistance of 4,700Ω. Note the European substitution of K for a decimal point.
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| Figure 11-2. potentiometer Schematic symbols |
How potentiometer Works
A potentiometer has three terminals. The outer pair connect with the opposite ends of an internal resistive element, such as a strip of conductive plastic, sometimes known as the track. The third center terminal connects internally with a contact known as the wiper (or rarely, the pickoff), which touches the strip and can be moved from one end of it to the other by turning a shaft or screw, or by moving a slider.If an electrical potential is applied between opposite ends of the resistive element, the voltage “picked off” by the wiper will vary as it moves. In this mode, the potentiometer works as a resistive voltage divider. For example, in a potentiometer with a linear taper (see “Variants,” coming up), if you attach the negative side of a 12V battery to the right-hand end terminal and the positive side to the left-hand end terminal, you will find an 8V potential at the center terminal when the potentiometer has rotated clockwise through onethird
of its range. In Figure 11-3, the base of the shaft (shown in black) is attached to an arm (shown in green) that moves a wiper (orange) along a resistive element (brown). The voltages shown assume that the resistive element has a linear taper and will vary slightly depending on wire resistance and other factors.
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How potentiometer Works |
potentiometer types
Classic-style Potentiometer
To achieve greater precision, a track inside a potentiometer may be manufactured in the form of a helix, allowing the wiper to make multiple turns on its journey from one end of the track to the other. Such multiple-turn potentiometers typically allow 3, 5, or 10 turns to move the wiper from end to end. Other multiple-turn potentiometers may use a screw thread that advances a wiper along a linear or circular track. The latter is com parable with a trimmer where multiple turns of a screwdriver are used to rotate a worm gear that rotates a wiper between opposite ends of a circular track. |
| classic potentiometer |
Multiple-Turn Potentiometer
To achieve greater precision, a track inside a potentiometer may be manufactured in the form of a helix, allowing the wiper to make multiple turns on its journey from one end of the track to the other. Such multiple-turn potentiometers typically allow 3, 5, or 10 turns to move the wiper from end to end. Other multiple-turn potentiometers may use a screw thread that advances a wiper along a linear or circular track. The latter is com parable with a trimmer where multiple turns of a screwdriver are used to rotate a worm gear that rotates a wiper between opposite ends of a circular track.
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| Multiple-Turn Potentiometer |
Switched Potentiometer
In this variant, when the shaft is turned clockwise from an initial position that is fully counterclockwise,
it flips an internal switch connected to external terminals. This can be used to power-up associated components (for example, an audio amplifier). Alternatively, a switch inside a potentiometer may be configured so that it is activated by pulling or pushing the shaft.
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| Switched Potentiometer |
Slider Potentiometer
Also known as a slide potentiometer. This uses a straight resistive strip and a wiper that is moved to and fro linearly by a tab or lug fitted with a plastic knob or finger-grip. Sliders are still found on some audio equipment. The principle of operation, and the number of terminals, are identical to the classic-style potentiometer. Sliders typically have solder tabs or PC pins. In Figure 11-7, the large one is about 3.5” long, designed for mounting behind a panel that has a slot to allow the sliding lug to poke through. Threaded holes
at either end will accept screws to fix the slider behind the panel. A removable plastic finger-grip (sold separately, in a variety of styles) has been pushed into place. Solder tabs underneath the slider are hidden in this photo. The smaller slider is designed for through-hole mounting on a circuit board.
The classic-style potentiometer was once used universally to control volume, bass, and treble on audio equipment but has been replaced increasingly by digital input devices such as tactile switches (see “Tactile Switch” (page 34)) or rotational encoders (see Chapter 8), which are more reliable and may be cheaper, especially when assembly costs are considered. Potentiometers are widely used in lamp dimmers and on cooking stoves (see Figure 11-10).
In these applications, a solid-state switching device such as a triac (described in Volume 2) does the actual work of moderating the power to the lamp or the stove by interrupting it very rapidly. The potentiometer adjusts the duty cycle of the power interruptions. This system wastes far less power than if the potentiometer controlled the lighting or heating element directly as a rheostat.
Since less power is involved, the potentiometer can be small and cheap, and will not generate significant heat.
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| Slider Potentiometer |
Trimmer Potentiometer
Often referred to as Trimpots, this is actually a proprietary brand name of Bourns. They are usually mounted directly on circuit boards to allow fine adjustment or trimming during manufacturing and testing to compensate for variations in other components. Trimmers may be singleturn or multi-turn, the latter containing a worm gear that engages with another gear to which the wiper is attached. Trimmers always have linear taper. They may be designed for screwdriver adjustment or may have a small knurled shaft, a thumb wheel, or a knob. They are not usually accessible by the end user of the equipment, and their setting may be sealed or fixed when the equipment is assembled. In Figure 11-8, the beige Spectrol trimmer is a single-turn design, whereas the blue trimmer is multi-turn. A worm gear inside the package, beneath the screw head, engages with an interior gear wheel that rotates the wiper. |
| Trimmer Potentiometer |
How to Use potentiometer
The classic-style potentiometer was once used universally to control volume, bass, and treble on audio equipment but has been replaced increasingly by digital input devices such as tactile switches (see “Tactile Switch” (page 34)) or rotational encoders (see Chapter 8), which are more reliable and may be cheaper, especially when assembly costs are considered. Potentiometers are widely used in lamp dimmers and on cooking stoves (see Figure 11-10).
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| lamp dimmers schematic |
In these applications, a solid-state switching device such as a triac (described in Volume 2) does the actual work of moderating the power to the lamp or the stove by interrupting it very rapidly. The potentiometer adjusts the duty cycle of the power interruptions. This system wastes far less power than if the potentiometer controlled the lighting or heating element directly as a rheostat.
Since less power is involved, the potentiometer can be small and cheap, and will not generate significant heat.
