RESISTORS AND THEIR USES

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This article is not intended to be a definitive treatise on the subject, but is intended to give a general over-view, which I hope will be particularly interesting and helpful to the newcomer to Amateur Radio.

Some people think that a resistor is a resistor and whilst there are different values and different power ratings, they will all do any job.  Nothing could be further from the truth, as I will attempt to show in this article.   The different types of resistor have different circuit symbols, as shown below.  The old zig-zag symbol is still used on many circuit diagrams and each of the variants shown can be applied to it.  The symbol for a non-inductive resistor is not now in general use but is sometimes found in older literature.

Resistors may be either fixed or variable.  There are many basic types and numerous variations of each type.  Many are available with axial or radial leads or are primarily intended for through-hole printed circuit board mounting or are fitted with solder tags.  There are types with no leads at all that are intended for surface mounting and use metalled ends intended to be directly soldered onto copper tracks on a PCB.  Larger wirewound resistors are often fitted with metal end caps affixed to the ceramic substrate.

There are encapsulated variants for use in adverse environmental conditions, high power types intended to be mounted on a heat-sink, sub-miniature surface mounting types, low self-inductance types, high voltage types, the list goes on and on.

Resistance values are stated in ohms (R), using the normal decimal multipliers of kilo (K) and mega (M).  Thus, for example, 0.1 ohm is normally written as 0R1, 1.0 ohm as 1R0, 1.5 ohms as 1R5, 10 Kilo-ohms as 10K, 1.0 megohms as 1M and 1.8 megohms as 1M8. Standard colour coding or alpha-numeric markings are used to indicate component values.   The latter method is self evident, but two methods of colour coding have been used over the years.  The "body-tip-spot" system was used for older components, whereas modern components use a series of coloured bands to indicate value and tolerance.  In the modern system, the first band is located nearer one end of the component and is usually wider than the other bands.  In the older system, the body colour was equivalent to the 1st band, the tip was equivalent to the 2nd band and the spot was equivalent to the 3rd band.  The tolerance, which was only ever 5%, 10% or 20%, was indicated by the colour of the other tip.

The Standard EIA Colour Code Table per EIA-RS-279 is as follows:-
     Colour   1st band   2nd band      3rd band       4th band      Temperature
(Multiplier) (tolerance) Coefficient
Black 0 0 1 1%
Brown 1 1 10 2% 100 ppm Red 2 2 100 50 ppm
Orange 3 3 1000 15 ppm Yellow 4 4 100000 25 ppm
Green 5 5 1000000 0.5% Blue 6 6 10000000 0.25%
Violet 7 7 100000000 0.1% Grey 8 8 1000000000 0.05%
White 9 9 10000000000
Gold 0.1 5% Silver 0.01 10%
None 20%

Although it is theoretically possible to manufacture resistors having any nominal value of resistance, they are normally produced with "standard" values.  These values are normally in the E12, also known as the 10% series (10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82) or the E24, also known as the 5% series (10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91) series.  Other series are the E3 (50% series no longer used), the E6 (20% series now seldom used), the E48 (2% series), the E96 (1% series) and the E192 (0.5%, 0.25% and 0.1% series).  The nominal values for these other series are not included in this article, as they are seldom found in Amateur Radio equipment.  Should any reader require to know these values, they are available in the appropriate literature or on the Internet.

Before continuing, let us consider power and voltage ratings.  Most people understand the power rating of a resistor but many do not appreciate that resistors also have maximum voltage ratings.  When a current passes through a resistor, a voltage is developed across it according to Ohms Law and power is therefore dissipated, which causes a rise in temperature.  Modern resistors tend to be smaller than older types for any given wattage rating.  This does not mean that modern watts are smaller than old-fashioned watts but it does mean that the modern component will safely run at a considerably higher surface temperature than its older counterpart.  For any given enclosure and internally dissipated power, the final temperature at equilibrium will be the same, regardless of the physical size of resistor generating the heat.  Adequate ventilation and heat dissipating surface area must always be provided.

The maximum allowable voltage across a resistor may be considerably lower than that required to result in maximum power dissipation.  Consider a 1M, 0.5W resistor.  It would be necessary to apply 707V across such a resistor in order to dissipate 0.5W.   A modern metal or carbon film resistor has a maximum working voltage of about 300V and so is unsuitable for high voltage applications, even though its power rating may not be exceeded.  A typical example is when a resistor is used in series with an EHT supply to provide smoothing or current limiting in case of flash-over.  Under normal operating conditions, the voltage across the resistor will be negligible.  However, if a flash-over occurs, the full EHT voltage will appear, albeit momentarily, across the resistor, which would almost certainly result in failure if the wrong type of component had been used.

One other point to note is that all resistors generate electrical noise.  This does not necessary imply a faulty component but is a matter of physics.  Total noise consists of three types, namely thermal noise, shot noise and contact noise, added together.  Thermal noise is related to resistor value, resistor temperature and the bandwidth over which the noise is measured.  This thermal noise is essentially "white noise" and has a gaussian distribution over any given bandwidth.  The RMS noise voltage may be calculated from the following formula.  Note that normal room temperature is approximately 300 degrees Kelvin.

Note that this noise is generated regardless of whether or not any current flows in the resistor.  If a voltage is applied across the resistor, two additional types of white noise are generated, referred to as "shot noise" and "contact noise".  Shot noise is related to the current flowing through the resistor and the bandwidth, whereas contact noise is related to current flowing, bandwidth and the physical form of the resistor and the material from which it is made.  It is very difficult to quantify these types of noise.  Note also that noise is related to the bandwidth of the circuit, rather than to the absolute frequency.  In other words, all other things being equal, a resistor will produce the same amount of noise at 1MHz as it does at 100MHz.  Wirewound resistors are the quietest, as they essentially produce only thermal noise, followed by metal film, metal oxide, carbon film, and lastly, carbon composition.

Generally speaking, the noise generated by resistors is unimportant, except where they are used in low noise, high gain applications, such as the input stages of audio amplifiers and oscilloscopes, or where they form part of low noise, UHF or microwave amplifiers.

Let us now consider the different types of resistor that are available and their applications.

FIXED RESISTORS

Carbon Composition

Solid carbon rod types, also sometimes referred to as carbon composition types, such as the RMA8, where the rod is enclosed in a ceramic tube, are now obsolete, as are the various bare rod types, although this form of construction is still used in specialised high voltage types and low inductance resistors intended to be used in RF applications and attenuators.

The high wattage, bare types intended for use as RF loads are often tubular in construction and have metalled ends.

None of these types is generally very stable by modern standards and they are seldom found with value tolerances better than 5% but are available with wattage ratings ranging from 0.25W to several hundred watts.  Resistance values between 10R and 10M are normally available.

Carbon Film

Most modern carbon resistors use a technique whereby a thin carbon film is deposited on a substrate, usually ceramic.  These types are very stable and are available with tolerances between 0.25% and 5% although wattage ratings greater than 1W are rare.  They are often laser trimmed in manufacture and this often leads to relatively high self-inductance, which makes them unsuitable for use above about 30MHz.  They are also very intolerant of voltage or current "spikes".  Resistance values between 1R0 and 1M are normally available, although higher and lower values are sometimes found as "specials".

Thin Metal Film

This is also a very popular type of construction, where a thin metal film is deposited on a substrate in a similar manner to the carbon film types.  Most of the comments made about the carbon film types also apply to metal film resistors.  Low inductance versions are available, where laser trimming is not used, the wanted resistance being obtained by adjusting the constituents of the metal alloy used to produce the film.

Thick Metal Film

In this type of resistor, a thick metal film is deposited on a substrate, which is mounted such that the device may be fixed to a heat sink.  This type of resistor is intended for high power applications and many types exhibit low series inductance and have low capacity to the mounting surface, making them suitable for use in RF power attenuators and loads.

Multiple Resistors

Metal film resistors are sometimes supplied in multiple units housed in DIL packages similar to those used for digital integrated circuit packages.  These are used where a large number of identical resistors are required in a given physical area of a PCB, for example pull-up resistors.

Wirewound

The simplest type of "wirewound" resistor is a short length of resistance wire supported between two terminals.  This type of resistor is normally used for meter shunts and similar applications, where very low values of accurately determined resistance and good temperature stability are required.

The two widely available types of resistance wire are Eureka and Constantan.  They are both metallic alloys but Eureka is solderable with normal soft solder, whereas Constantan is not and must be either silver-soldered, welded, crimped or clamped.  There are several other types of resistance wire but they are all similar alloys and most will not soft solder.  Most resistance wires are available in a range of gauges between 0.025mm and 4.0mm, or in strip form.

A variation of the simple meter-shunt is the four terminal shunt, which is normally used for very high current applications.  Here the resistive element has two end connections and two tapping points very close to the end connections.  The meter is connected between the two tapping points, with the end terminals being used to connect the assembly into the external circuit.  The resistance between the tapping points is the actual meter shunt and is accurately controlled.  The purpose of this arrangement is to remove the ill-defined connection resistance of the terminals from the measuring circuit.

Other applications of "bare wire" resistors are in old style radiant electric fires and as the heating element in electric toasters, the latter usually employing strip rather than wire, which is supported on a sheet mica former.  Infra-red heaters, as used in bathrooms, usually consist of a long coil of resistance wire enclosed in a silica-glass tube.  The resistance of all these devices is chosen such that the wire glows dull to bright red when connected to the power source, usually the 230V mains supply.

The incandescent lamp is basically a length of resistance wire, usually tungsten, enclosed within a glass envelope from which all air has been removed.  Incandescent lamps are only about 50% efficient and therefore produce considerable amounts of heat.  They are often used as the active elements in devices such as green-house heaters, propagators and incubators, where they can also be used for lighting.  The incandescent lamp is sometimes used as the sensing element in RF power meters and as a crude RF load resistor.  It is important to note that the "cold" resistance of the filament will be considerably less than the resistance when hot.  Consequently, lamps cannot be used as RF loads where the maintenance of a good VSWR is required.  Furthermore, lamps have intrinsic self inductance, which precludes their use as RF loads at high frequencies unless this unwanted inductive reactance is "tuned out".  In days gone by, a small torch bulb or "pea lamp" was often coupled to the output circuit of a transmitter to indicate power output, where it was used to optimise the tuning, maximum brilliance indicating maximum power output.

As mentioned above, the "cold" resistance of a lamp filament is much less than its resistance when glowing brightly.  This results in a switch-on current surge, which shortens the life of the lamp.  This is not really a problem with domestic lamps or torch bulbs but is a problem when a number of low voltage lamps are connected in a series string that is then connected across the mains, as is done, for example, in Christmas tree lights.  A considerable increase in life expectancy can be achieved if an appropriately rated, negative temperature coefficient thermistor is connected into the series string, usually replacing one bulb.

Another application of resistance wire, thankfully no longer used, is "line cord".  Line cord is essentially insulated resistive cable that was used as the mains lead supplying some mains powered radio receivers.  Receivers often required ballast resistors in the mains input line to their power supplies.  In the better quality receivers, this took the form of either a wire wound resistor or a barretter but cheaper receivers often used the resistance of line cord instead.  There were a number of potential hazards associated with this practice.  Firstly, if a fault occurred in the receiver that would normally blow a fuse, when line cord was being used, the fuse often did not blow but the line cord heated up, sometimes to the point of combustion.  Secondly, if the line cord became damaged, replacement of the entire lead with an identical length of similar lead was necessary.  This was not always appreciated by repairers, who sometimes replaced the damaged lead with ordinary mains lead or shortened the original lead to remove the damaged section.  The first "repair" would almost certainly result in catastrophic damage to the receiver, as there would now be no ballast resistor.  The second "repair" would result in a lower value ballast resistor, which would probably result in premature failure in use.  Thirdly, line cord would normally run warm, which would have a detrimental, long term effect on the insulation, bearing in mind that sophisticated modern materials such as silicone rubber, were not available.

There are many styles of normal wirewound resistors, the two most common being the vitreous type, where the wire element is wound on a ceramic tube and then covered in a glass coating and the ceramic cement type where a ceramic based "cement" replaces the glass covering.  The latter process is cheaper, although less environmentally robust, than the vitreous process.

Power ratings between 2W and 200W are available, although higher power versions are sometimes encountered.  The normal resistance range is 0R1 to 100K, although higher and lower values are available as "specials".  The value tolerance is normally 5%, although closer tolerances may sometimes be encountered.

These types of resistor have high self-inductance and are not suitable for use at frequencies much above a few hundred kHz.  These components attain very high surface temperatures when run at their maximum dissipation.  

An important variant of this type is the metal-clad version where the resistor element is mounted in a metal tube, usually fitted with a mounting flange for fixing to a heat sink.  For any given wattage rating, this type is smaller than its normal vitreous equivalent but MUST be mounted on a heat sink.  Not only do these components exhibit high self inductance but there is also appreciable capacitance between the resistance element and the case.  These components are therefore not suitable for use at frequencies above about 1MHz.

Specially wound low inductance types using Ayrton-Perry windings, are available but these tend to be manufactured for specific purposes and are normally close tolerance, low wattage components.  This type of component is often found in measuring bridges where they are used as the comparison standards and bridge ratio "arms".

Another variation on the theme is the fusible resistor, which is designed to go open circuit when its dissipation is exceeded.  This is usually achieved by incorporating a spring-loaded switching device in series with the resistive element itself, which is held in place by a soft soldered joint.  If the dissipation is exceeded, the surface temperature is high enough to melt the solder, causing the spring-loaded switch to open.  The component can be returned to "normal" by re- soldering the joint.  This type of component is usually only found in domestic apparatus, as they are not intended for use in severe environmental conditions.

It is worth noting that an ordinary fuse is nothing more than a wirewound resistor using a single, usually copper, wire that is intended to melt when a given current is exceeded.  Obviously, this type of "fusible resistor" is not repairable if it is a cartridge fuse but replacing the fuse wire element is possible in the older type of mains fuses employing plug-in ceramic carriers.  There are many different types of fuse, namely fast acting, ultra fast acting, time delay, surge resistant or "slow-blow", high voltage and high rupture capacity.  It is beyond the scope of this article to deal with these different types in detail but it does show that the operation of a device as apparently simple as a fuse is much more complicated than it appears.

Precision and Special Purpose Types

Most styles, with the exception of the solid carbon rod types, are available in close tolerance, high stability variants but these are normally manufactured for specific purposes.  Extremely high value resistors intended for use in, EHT control circuitry, electrometers and similar applications, are often hermetically sealed in glass tubes, which are sometimes filled with an inert liquid.  This is to prevent the resistance of surface moisture or other contamination appearing in parallel with the actual resistive element.

Temperature Dependent Types

This type of resistor, often referred to as a thermistor, is designed to change its value as its temperature changes.  They are normally available as either rod or disc types, which may, or may not be fitted with leads.  Very small types, often encapsulated in glass tubes, are also available.  Nominal values at room temperature are usually in the range 1R0 to 100K and the temperature coefficient can be either positive or negative.  This type of resistor is used for temperature measurement, temperature compensation, RF power measurement, oscillator output amplitude stabilisation, heater control, surge current limiting and certain specialised uses such as controlling the current in the automatic degaussing coils of colour TV receiver CRTs.

As mentioned above, thermistors are often used as the sensing element in RF power meters and when used in this application they are usually referred to as bolometers or, occasionally, as barretters.  The device originally referred to as a barretter was a demodulating detector invented in 1902 by Reginald Fessenden that had limited use in early radio receivers.  It was a highly sensitive but mechanically delicate hot-wire thermistor, enclosed in an evacuated glass envelope.  In some ways it was similar in construction to an incandescent lamp and had the advantage of being able to demodulate amplitude modulated signals, something that the coherer could not do.  This device had several shortcomings and was rapidly superseded by another one of Fessenden's inventions, the chemical detector or electrolytic barretter.

Mechanically robust, high power, low sensitivity, barretters were later developed for use in the power supplies of early mains powered radio receivers, particularly those intended to be powered from DC mains supplies.  In these applications, the glass envelope was usually filled with hydrogen and they were often referred to as ballast resistors.  Essentially, these were incandescent lamps, although they were designed for use with the filament barely glowing, rather than brightly lit.

Although not a temperature or voltage dependent resistor in the normal sense, the coherer mentioned above is a device where the resistance across its terminals will vary depending upon the amount of RF energy applied to it.  It is not a linear device but behaves more like a crude switch than a variable resistor and was used to detect the presence of RF signals such as those emitted by spark transmitters.  A coherer consisted of an evacuated glass tube, with a contact at each end, loosely filled with metal filings.   With no applied RF, the resistance across the device was quite high.  Applying RF resulted in the filings "sticking together", which caused this resistance to fall.  Obviously, such a device can only detect the presence or absence of a signal and requires a relatively large signal to operate it.  However, the coherer was the preferred detector in the early days of radio when only Morse Code from spark transmitters was in use.  In use, coherers suffered from a decrease in sensitivity caused by the filings failing to become "unstuck" after reception of signals.  A light tap on the glass tube was necessary to unstick the filings and restore sensitivity.  The more sophisticated coherers actually incorporated a small rubber hammer, driven by a mechanism similar to that used in electric bells, to accomplish this.  Provided that the "tap rate" was much faster than the keying rate of the signal being received, no information would be lost.  Although the tapping would modulate the incoming signal, this was not a problem, as the received signal was not the pure tone associated with CW today, but was a very rough and rasping sound.  The tapping rate modulation may have actually improved the received "tone".

Voltage Dependent Types

This type of resistor is designed to change its value as the voltage across it changes.  They are normally available as either rod or disc types and are usually encapsulated and fitted with leads.  Nominal values at room temperature and low voltage are usually in the range 10K to 10M and the voltage coefficient is usually negative, with a sharply defined "knee" at a specified voltage.

This type of resistor is used for surge voltage suppression and for peak voltage clipping.  They are sometimes used as crude voltage stabilisers, although a zener diode is much better in this application and about the same price as a VDR.  High voltage versions of the VDR were developed by Metropolitan Vickers in the mid 1950s and were used in TV receivers of the period as crude EHT voltage regulators and surge suppressors.  They were made in the form of rods, about 15cms long and 1cm diameter and were known as "Metrosils".  Similar devices, still using the same trade-name but made in disc form, are now manufactured by M&I Materials Group.  The original types are no longer used in television receivers and are no longer made.  However, the modern disc devices can absorb huge amounts of transient pulse power and can withstand potentials of many kilovolts.  These devices are often connected in series/parallel combinations and are used in industrial applications.

Light Dependent Types

This type of resistor is also called a photo-resistive cell and is designed to change its value depending on the incident light falling upon it.  They are usually encapsulated, such that light can impinge upon the sensitive area of the device.  Photo-resistive cells must not be confused with photo-voltaic cells, which generate a voltage proportional to the incident light, or with photo-transistors, which operate as normal transistors except that the photons of the incident light replace the normal base current input.  These two devices are not really resistors at all.  All three types are designed to respond to light of specific wavelengths, some responding only to infra-red, others to only ultra-violet, whilst others will respond to a wide range of visible light.

All types of photo-cell are used for measuring or detecting light in devices such as photographic exposure meters, general purpose light meters and alarms.

Pressure Dependent Types

This type of resistor is sometimes called a strain-gauge and is designed to change its value depending on the pressure or stress applied to it.  These devices are used to measure pressure or stress.

Thermo-couples

Strictly speaking, thermo-couples are not resistors in the normal sense, although they do employ resistance wire.  A thermo-couple is often constructed by welding the end of a length of resistance wire (usually Eureka) to the end of a similar length of copper wire.  If this junction is heated with respect to the other ends of the wires, a voltage of a few milli-volts will be generated between the "cold" ends of the wires.  This voltage may be used to measure temperature or to actuate control or safety devices.  Applications of thermo-couples include fuel control and safety shut off valves on domestic and industrial boilers, specialised biological thermometers where the thermo-couple is incorporated into the point of a hypodermic needle, laboratory temperature measuring devices where the temperature of a clearly defined point on a test-piece needs to be measured and in the pyrometers used in kilns and furnaces.

Extremely small and sensitive thermocouples, incorporating equally minute heating elements, are used in old style RMS reading meters, RF ammeters and for monitoring the output of RF signal generators.  This type of thermocouple is mechanically very delicate and is extremely intolerant of electrical overload.

Fixed RF Attenuators

These devices are normally fitted with coaxial connectors and represent a very specialised use of resistance elements.  Full details are given in the "RF Connector" section.

 

VARIABLE RESISTORS

Many of the methods of construction used for fixed resistors may also used for variable types, although certain variable resistor types require specialised construction methods.  Variable resistors fall into two main categories, namely rheostats and potentiometers.  Both types are available as either pre-set or operator controlled types and may be either carbon, metal film or wirewound.

Rheostats

Rheostats are usually two terminal devices, where the resistance between the terminals is varied by turning a shaft or moving a lever.  They are usually high power wirewound devices and are used for adjusting currents in battery charging or for machine and theatre lighting control applications.  Their use in the latter two applications has now been largely superseded by semiconductor control systems.  This type of rheostat usually has a linear mechanical movement versus resistance value characteristic, although non-linear devices are occasionally found.

Another type of rheostat is the carbon pile type, although these are now obsolete and are no longer manufactured.  They consist of a series of carbon blocks in a holder, pressed against each other by a spring.  Increasing the spring pressure decreases the resistance of the "pile" and vice-versa.  This type of rheostat is inherently electrically noisy and the setting stability is very poor.  This type of device was used mainly in machine control and battery charging applications.

Generally speaking, the resistance values normally encountered vary from 1R0 to 500R.  Normally, the tolerance is unimportant and is usually about 20%.

The carbon microphone cell is a variation of the carbon pile rheostat.  Here the carbon blocks are replaced by carbon granules and the spring by a thin metal diaphragm, which, provides a very light pressure that prevents free movement of the granules.  When a sound wave impinges on the diaphragm, causing it to vibrate in unison with the amplitude and frequency of the sound, the resistance of the carbon granules varies, also in unison with the audio frequency.  If a voltage is applied across the cell, the resulting current will vary in unison with the audio amplitude and frequency and is used as the audio output signal.  The audio quality is poor and this type of microphone was normally used in telephones and other applications where quality of reproduction was not the primary requirement.  Carbon microphones have now been replaced by dynamic inserts in telephone applications.

Potentiometers

The word "potentiometer" implies a device for measuring potential, rather than the device normally referred to as a potentiometer.  This derives from the fact that the original potentiometer, invented by Johann Christian Poggendorff (1796-1877) in 1841, was a three terminal device primarily intended to be used in the measurement of potential.  The device consisted of a straight length of resistance wire mounted adjacent to an accurate scale calibrated in units of length.  A sliding knife edge contact could be slid along the length of the wire and a corresponding pointer traversed the calibrated scale.  If a known potential was applied across the ends of the wire, an accurate proportion of this potential could be tapped off by the sliding contact, the exact proportionality being determined by accurate and easily made measurements of physical length.  Unknown potentials could be compared to this known potential by techniques already well known in the mid-nineteenth century, enabling an unknown potential to be determined with great accuracy.

Potentiometers used in radio and electronics are also three terminal devices, where the resistance between two of the terminals is fixed and the third terminal is a tapping point on the resistive element, which may be connected at any point between the two fixed terminals.  They are usually low power devices and are mainly used for general control purposes.  These components may be either carbon, carbon film, metal film or wirewound and are available with linear, log, antilog, or special purpose mechanical movement versus resistance value characteristics.  Some variants have intermediate fixed tapping points brought out to additional terminals.

Potentiometers intended to be used as audio gain controls usually have logarithmic characteristics, as this gives an apparent linear increase in volume as the control is turned clockwise.  In some applications an increase in volume by turning the control anti-clockwise is required.  In these applications, the characteristic should be anti-logarithmic.  Use of logarithmic or anti-logarithmic characteristics are necessary as the human ear responds logarithmically to sound amplitude.

Potentiometers having linear characteristics are used for general purpose control applications where they are often configured to act as rheostats.  Characteristics, such as sine or cosine, are used for specialised applications often found in test equipment or process control.

Components having values between 1R0 and 1M are manufactured as standard but some special purpose devices having resistance values higher or lower than this range are occasionally encountered.  Resistance tolerances are not normally stated but are usually in the range 10% to 20%.  Wattage ratings are not normally stated as potentiometers are regarded as low power devices intended for low-level signal applications.  Power rating and tolerance are not normally important criteria.

Most potentiometers are "single-turn" devices where the slider moves from one end of the track to the other with a single turn of the shaft.  However, some are "multi-turn", where up to ten turns of the shaft are required to traverse the track completely.

Potentiometers are often supplied as double or even triple units, being operated by a common shaft, called ganged units, or by separate, concentric, shafts.  It is not uncommon to find switches attached to potentiometers, which are operated by the same shaft as the potentiometer itself, or by a separate concentric shaft.

Potentiometers are either equipped with threaded bushes to enable them to be panel mounted or are fitted with tags to facilitate direct PCB mounting.  Potentiometers are also available as small pre-set devices for mounting on printed circuit boards.  These components are used for adjustment purposes, where they are used in initial setting up procedures but are not normally adjusted thereafter.  Trim-pots may be either single-turn or multi-turn devices.

Many specialised types of potentiometer are available, such as screened, low inductance versions for use as RF variable attenuators, or slider types as used in audio mixing panels.

 

This article is intended to give an overview of the subject and none of the items has been dealt with in depth.  Further information is available from resistor manufacturer's literature or from the Internet.


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