Resistors in Practice V3

7/31/2019 Resistors in Practice V3

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LISTOF CONTENTS

Section Topic Page

1 Objectives and Summary 2

2 Resistors - Features of the Component 3

3 Resistor Circuit Symbols - British Standards 4

4 Resistor Colour Code System 5

5 Tolerance and Preferred Values 6

6 Resistor Letter Code System - BS1852 7

7 Fixed Value Linear Resistor Construction 8

8 Fixed Value Linear Resistor Types 9

9 Variable Resistor Types 11

10 Variable Resistor Extended Features 13

11 Non-Linear Resistor Types 14

12 Resistor Power Rating 15

13 Linear Resistor Types and Features Table 16

Nick Brackenbury Electronics Notes September 2004 page 1

Correct selection of componentdesign and materials at the start

gives best long term use


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1 OBJECTIVESAND SUMMARY

All electronic circuits include a variety of resistor types. The circuit designercalculates each resistor value required. However, there are many other aspects toselecting the correct type of resistor, often these aspects are taken into account bya Production Engineer, as opposed to the Circuit Designer.

The standard resistor is a two-wire component, generally with four coloured rings

painted on it to identify its value. These resistors are Ohmic or Linear. That meansthe resistor value does not change with possible signal voltage or waveform.

There are a number of ways of manufacturing Linear resistors, and these designmethods are explained. Understanding the construction and materials usedenables you to select a resistor type that meets other design criteria, over andabove the resistor value in Ohms. Other factors include frequency range of appliedsignal, heat dissipation required, electrical noise level, physical size and cost.

Most analogue electrical equipment, particularly audio-visual, will have UserControls for volume, tone, brightness, colour, etc. These controls are effected usingVariable Resistors. These also manufactured in a variety of forms, which are

explained as well, to enable correct component selection.

There are three types of resistor that are not Linear or Ohmic. They are thus callednon-linear. Their resistive value changes with the application of a second energysource, the first or primary energy source being the electric current passingthrough the component:

LDR (Light Dependant Resistor), whose value of resistance varies withambient light intensity

Thermistor (Thermal Resistance), whose value of resistance varieswith ambient temperature

Varistor (Voltage Dependant Resistor), whose value of resistancevaries with the applied signal (source) voltage

These resistor types are used in Sensor circuits and component protection circuits.

The Chapter ends with a Summary Table of Linear Resistor Types & Features.



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2 RESISTORS - FEATURESOFTHE COMPONENT

All materials have some resistance to the flow of an electric current.Materials are classified as:

Conductors for very low resistance and high currentSemi-conductors in the middle resistance rangeInsulators for no current flow at all

The term RESISTOR describes a passive electronic component thatis designed to have a resistance value somewhere between aconductor and a semi-conductor.

A standard fixed value linear resistor is a two-terminal or two-wirecomponent whose value remains constant regardless of:

The level of applied voltageThe direction of the applied voltage

Whether AC or DC currentThe waveform shapeThe waveform frequency

Resistors are used in electronic circuits to:Limit current flow through another componentProvide voltage divisionProvide current division

Resistance values vary with temperature. This is an undesirablefeature in most cases of electronic circuit use.

Some resistors create electrical noise. This is also an undesirablefeature, particularly in analogue amplifiers.

A variety of resistor types are manufactured using different materialsand methods of manufacturing. Resistor types are selectedaccording to specific application, i.e. switching circuit, amplifiercircuit, transmitter circuit, in-vehicle circuit, and so on.Nick Brackenbury Electronics Notes September 2004 page 3

RESISTOR FEATURES - CONT

Some resistors are designed to be variable by User adjustment,in some forms these are called rheostats or potentiometers. In

normal use they are called volume controls or brightness controlsA principal aspect of circuit design is to calculate the resistor val-

ue in Ohms () that is required. However, when selecting a re-sistor component there are other features that also need to beconsidered:

Fixed Resistors - Linear propertiesVariable Resistors - Preset or User AdjustableFixed Resistors - Non-linear propertiesPreferred values - the range of allowable valuesTolerance - how accurately resistors are made

Power rating - the maximum voltage that should be usedFrequency response - high frequencies can cause problems

Fixed Linear Resistors are normally marked in one of two ways:Colour code system to identify valueBS1852 letter code system to identify value

Resistors are not polarised like electrolytic capacitors or semi-conductors, so it does not matter which way round they are con-nected in a circuit.

Resistors should not be reactive. This means they should nothave any element of inductance or capacitance in them - they

should be purely resistive. However, some resistor designs dohave inductance due to the winding nature of their designs. Inparticular any fixed or variable resistors that are wire wound willbe unsuitable for high frequency use. This is not a problem at DCor low frequency AC, but is a problem at higher frequencies.


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3 RESISTOR CIRCUIT SYMBOLS

The British Standards Institution, BSI

for short, have specified a series ofsymbols for use in circuit diagrams.

The Standards Document ReferenceNumber for electronic circuits used tobe called BS3939.

The scope of these symbolsexpanded considerably with manynew component types beingdesigned. As a result BS3939 hasbeen superseded with a new

document reference number BS60617.

The most commonly used resistortypes and their British Standardsymbols are shown on the right.

The British Standards web site iswww.bsi.org.uk.


Old symbol New symbol

Fixed resistors are the usu-al colour coded componentsas soldered onto Printed Cir-cuit Boards.

General Variable Resistor

Normally fitted with User ad-justable dial for applicationssuch as volume and tonecontrol, brightness control.

Pre-Set Variable Resistor

Soldered onto PCBs and locat-ed inside equipment enclosure.The resistance value is adjust-ed with a screwdriver by a pro-duction or service engineer.

Moving contact variableresistor

Historically known as arheostat

Variable resistor as volt-age divider

Historically known as apotentiometer

Thermistors

Derived from thermal resistorsGive a large change in re-sistance for a small change intemperature.

PTCPositive Temp Coefficient

NTCNegative Temp Coefficient

LDR - LightDependant Resistor

The resistance reduceswhen the light gets stronger;used in light detector circuits

Varistor - VoltageDependant Resistor

The actual resistance valuechanges with applied volt-age; also called VDR

+t0 PTC

+t0 NTC


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4 RESISTOR COLOUR CODE SYSTEM

Fixed value resistors are tubular in shape and often very small -perhaps only 5 mm long. Often there is not enough space to putreadable values on the resistor body. To overcome this problem

a colour code system was introduced, as shown in the tablebelow.

The first Band is the one that is nearest one end of the resistor,as indicated on the drawing above. On very small resistors only5 mm long, it may not be obvious which end to start. In thesecases one can use the preferred values list overleaf which canassist in guessing the first two colours, since there are only afew combinations allowed.

There are only three bands for a resistor with a tolerance of20%. This is because Band 4, the normal tolerance band

position, is no colour for 20%.There are four bands for resistors with tolerances of 10%, 5% or2%, with the fourth band being silver, gold or red respectively.

There are five bands for resistors with a tolerance of 1% or less.In these cases the first three colours are integers, with thefourth colour being the number of zeros or decimal pointposition. The fifth colour is the tolerance.


Colour Band 1 Band 2 Band 3 Band 4

Black 0 0 none -

Brown 1 1 0 1%

Red 2 2 00 2%

Orange 3 3 000 3%

Yellow 4 4 0 000 -

Green 5 5 00 000 0.5%

Blue 6 6 000 000 0.25%

Violet 7 7 0 000 000 0.1%

Grey 8 8 - -

White 9 9 - -

Gold - - 0.1 5%

Silver - - 0.01 10%

No colour - - - 20%

Band 1 gives the first digitBand 2 gives the second digitBand 3 gives the number of following 0sBand 4 gives the tolerance of the value

12K 100K 3.9K 2.2K 3.9K 470


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5 RESISTOR TOLERANCEAND PREFERRED VALUES

If a resistor has a 22 value and a 10% tolerance band, then its value can vary from22-10% to 22+10%

= 22-2.2 to 22+2.2 = 19.8 to 24.2

The tolerance value is 2.2 .The tolerance range is 24.2-19.8 = 4.4 or 20%

Example: What is the tolerance value of an 82 K resistor with a red tolerance band?

Answer: red is 2% of 82K = 1640

One can use a Digital Multi Meter (DMM) to measure a resistors actual value and see if it iswithin the manufacturer specified tolerance value.

Example: The above 82 K resistor was measured with a DMM to be 81,230 .Is it within tolerance?

Answer: The difference between manufacturer specified value and measured value is:

82,000 - 81,230 = 770 770 is less than the 1640 (2% of 82K) allowed, so this resistor is well within tolerance.

Resistors are not manufactured to arbitrary values. The values available originate from three

sets of preferred values as per the table on the right. This is in accordance with BS2488 (UK)and IEC Puc63 (USA).

The E6 steps are about 40%. This is because E6 originates from 20% tolerance resistors.

The E12 steps are about 20%. This is because E12 originates from 10% tolerance resistors

The E24 steps are about 10%. This is because E24 originates from 5% tolerance resistors.

These days most resistors are in the E24 column with 5% tolerance as a consequence ofmodern improvements in manufacturing technology in the last 10 years. 50 years ago thestandard item was E6 at 20% with E12 10% being a slight premium on price. 25 years ago thestandard was E12 with E24 being a slight premium. Now there is little cost benefit in notbuying E24.

When visually identifying a resistor it is quite possible to read one of the colours incorrectly.This could be as a result of discolouration due to heat, or the use of a dark body colour.However, reference to the Preferred Value table enables correction of the error. For example,you may see the first two colours as yellow and brown, being 41, for a 10% tolerance resistor.

The only digit following a 4 is a 7, so the colours are yellow and violet for 47 .


E6 Series20%

tolerance

E12 Series10%

tolerance

E24 Series5%

tolerance

10 10 10

11

12 12

1315 15 15

16

18 18

20

22 22 22

24

27 27

30

33 33 3336

39 39

43

47 47 47

51

56 56

62

68 68 68

75

82 82

91

Table of Preferred Values


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6 RESISTOR LETTER CODE SYSTEM - BS1852

The Letter Code system is an alternative to the Colour Code system inmarking components with their values.

It is not always possible or ideal to print colour bands on a component.Particular examples are resistors that get very hot (colours change), verysmall components, and variable resistors.

This code consists of letters and numbers printed on the resistor bodyindicating the resistor value.

In the value the following letter abbreviations are used:R means x1K means x1,000M means x1,000,000

Examples are:a 4.7 K resistor is 4K7 a 5.6 resistor is 5R6

a 470K resistor is 470K a 56 resistor is 56R

a 6.8 M resistor is 6M8 a 12 M resistor is 12MThe Letter Code System purposely avoids using a decimal point, whichcould easily be visually missed or rubbed off.

Tolerances may be specified by adding a letter at the end of the code:F means 1% J means 5%G means 2% K means 10% M means 20%

Examples are:

4R7F is 4.7 1% 16MG is 16 M 2%5K6J is 5.6 K 5% 82KK is 82 K 10%

330RK is 330 10% 3M9G is 3.9 M 2%The letter code typically takes half the number of digits of the fullmathematical expression, so the Letter Code is a good short form to useon small or awkwardly shaped components.


Pre-set Wirewound 100RK8 means 100 also 10% 8 Watt

Ceramic Heatsink Body over Wirewound Resistor

also it is 10% 3 Watt

270R means 270

1KF11 means 1K 1% 11W

3R3J means 3.3 5% 11W

Wire wound over ceramic tube


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7 FIXED VALUE LINEAR RESISTOR CONSTRUCTION

The word resistor generally refers to fixed value linear resistors.Resistors are the normal component as plentifully used in practicallyevery electronic circuit, analogue and digital.

There are many manufacturing methods and variations in commonuse, which can be summarised to:

Carbon rod made from crushed clay and carbon compoundCeramic tube filled with clay and carbon resin compoundHigh resistivity metal wire wound onto a ceramic tubeMetal or Metal Oxide film sprayed onto an insulator tubeMetal or Metal Oxide film helix sprayed onto an insulator tube

Resistors are fixed value because they have no means of adjustment,mechanical or otherwise. They are linear because oncemanufactured, their value of resistance is constant regardless ofapplied voltage or other conditions such as heat and light.

However, they are not perfect, which is why a number of different

types are made. The winding and helix techniques produceinductance and so these types are unsuitable at higher frequencies.The metal thick film technique overcomes these problems, but isrestricted in power consumption and cannot reach high resistancevalues. Films of carbon-boron and metal oxides give high values butare electrically noisier.


embeddedconnection

lead

siliconlacqueroff whitemixture

of carbonand claycrushed

into a rodCarbon Rod Resistor

ceramic orclay former

metallicend capwith leadWire Wound Resistor

highresistance

wire

winding

vitreousenamel coating


metallicend capwith lead

Metal or Metal Oxide Film Resistor Helix

spiral

film ofmetal ormetal oxide



metallicend capwith lead

Metal or Metal Oxide Film Resistor Full Coat

Thick filmOver surface of

metal ormetal oxide



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8 FIXED VALUE LINEAR RESISTOR TYPES

Carbon Composition

The carbon-clay compound is bonded together with a resin. Ahigher proportion of carbon will give a lower resistive value.Early volume production resistors were the simple crushedrod type, but they break easily. A ceramic tube filled with

carbon-clay compound is much stronger. Carbon contentresistors are electrically noisy, particularly with high currentsand/or when hot. They are very low cost to make.

Full Coat Thick Film Resistors

The thick film can be a resistive metal, or more likely a metaloxide or carbon-boron. A thick film is made over a tubularbobbin such as fibreglass for low power devices or ceramicfor higher power devices. This construction technique enableslow to medium values, reasonable power dissipation and highfrequency use because there is no coil or winding. Wires are

welded to metal caps which are pressure fixed over each end,giving rise to the rim bulge at each end.

Resistor Packs

Resistor Packs conform to Integrated Circuit style packaging.One type is the Dual In Line (DIL) package containing anumber of equal valued resistors. The picture shows a 16-pin

DIL with eight 270 metal oxide W resistors. A furtherspace saving option is the Single In Line (SIL) packagingwhich contains a similar number of resistors but all wiredtogether at one end (for ground) to save board space.

Resistor Packs are used to terminate the data lines from aperipheral device (disc drive) to the motherboard (controller)in a computer system to eliminate signal reflection.


Carbon Rod 10 K 5% Watt

Carbon 1.5 K 5% 1 Watt Metal Oxide 4.7 M 5% Watt

Metal Film 470 5% Watt

SIL Resistor Array, 8 resistors

Pin 8 7 6 5 4 3 2 1

Pin 9 10 11 12 13 14 15 16Pin 10 9 8 7 6 5 4 3 2 1


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8 FIXED VALUE LINEAR RESISTOR TYPES - CONT

Metal Wire Wound

Nichrome, an alloy of nickel and chromium, is popular dueto its high resistivity and physical resilience. The centralformer is made from a ceramic and the outer coat a hardvitreous enamel. Wirewound resistors are low value high

wattage and can be safely run at constant hightemperatures. Tolerances down to 0.1% are easilyachieved. Wirewound are the most expensive to make andare unsuitable at high frequencies due to the inductiveeffect.

Metal and Metal Oxide Helix Film

Similar in construction to the full coat thick film, but with ahelical groove cut into the film coating leaving a resistivespiral track from one end to the other. This enables higherresistance values and physically smaller sizes. Being very

low cost they are popular as general purpose low powerlow to medium frequency use. The helix has an inductiveeffect at high frequencies.

Heatsink Encapsulation

Heatsink resistors are designed to dissipate from 5 to over30 Watts. 5 to 10 Watt resistors are made using a ceramicenclosure as the heatsink. 10 to 30 Watt or more resistorsare built into an aluminium extrusion which can be screwedto the chassis. Typically they are very low value, less than

10. A wirewound bobbin is inserted through a tube and

sealed in with a ceramic paste and baked dry. Thisconstruction can safely dissipate a large amount of heat,supporting very high currents.

Characteristic Graph of all Linear Resistors

All Linear Resistors have a straight line graph when plottingcurrent Amps against Volts applied.


Enamel wirewound on ceramic tube Wirewound on fibreglass bobbin

Heatsink Wirewound 4.7 10% 4 W Heatsink Wirewound 1 1% 25 W

AluminiumCeramic enclosure

10 5% 5 W 100 1% 1 W

0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

40

30

20

10

0Amps

Volts R1

R2

R3

Graph of current reading for increasing voltagesetting across three linear resistors R1, R2 & R3

high resistance

medium resistance

low resistance

Wirewound on fibreglassbobbin

3K 1% 10 W


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9 VARIABLE RESISTOR TYPES

There are a large variety of variable resistor typescovering many options. Variable resistors combineLinear resistor materials with a mechanicalconstruction that facilitates User variations.

Carbon or Wirewound

Just as with fixed linear resistors, the mainchoices for variable resistors are carbon based orwirewound. The carbon based are made from athick film of carbon onto an insulator base such asmica, like PCB material. Long term reliability ispoor due to the carbon wearing off. The goodpoints are low cost, small and light. Wirewoundutilise nichrome wire wound round a small strip ofmica or a plastic tube. The resistance valuechanges in small steps as the slider moves fromone turn to the next. Due to the inductive effectwirewound are not suitable over 500KHz, and cancause inductive problems well below thatfrequency in many circuit designs.

Resistance Range

Manufacturing methods are such that best

reliability is achieved below 10K with carbon and

wire. Up to 25K is readily availlable in carbon.

Up to 250K is available with carbon but can poselong term reliability problems with wiper wear, over-current use and electrical noise.

Slide or Rotate

The mechanical movement can be eitherrotational (less than 3600)or a longitudinal slide.


Rotary variable carbon

10 K All plastic pot

Rotary variable

carbon 10K pot

Rotary variable, dual

carbon 10K ganged pot

Slider variable carbon 10K pot

Rotary variable wire-

Wound 10 K potRotary variable pots - one wirewound andtwocarbon - carbon are much smaller in size


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9 VARIABLE RESISTOR TYPES - CONT

Rheostats and Potentiometers

See Section 3 on Resistor circuit symbols.

A Rheostat is a 2-terminal device whose resistance between thosetwo terminals can be mechanically varied from zero to the maximum.

A Potentiometer (pot) is a 3-terminal device where a full circuitcurrent is continuously flowing from one outer terminal through to theother outer terminal. A sliding centre tap provides a third terminal witha signal strength somewhere between 0% and 100% of the fullapplied signal.

Mounting and Connecting Methods

One popular method of combining mounting with connecting is tosolder the 3-termal device onto the edge of a PCB using stronglydesigned tags. Before PCBs, the most popular method was to drill ahole through the equipment outer casing and use screw nuts and

thread to fix the pot through the hole. Wiring up would be doneseparately.

Pre-Set Potentiometers

The lowest cost variable resistor is the pre-set skeleton pot for PCBmounting. Adjustment is made with a small screwdriver rotating thedial from zero to almost a complete turn on a carbon-mica surface.

Power Rating

Standard variable and pre-set pots forget to display maximum powerrating information. Generally, small carbon pre-sets are and Watt,whilst larger carbon variables may go up to Watt. It is not advisable

to design circuits with a high ambient DC current passing through apot, since it leads to electrically noisy and carbon wear.

Wirewound are designed to be more robust, combining lower noiseand higher ambient DC current to handle from 1 Watt to 10 Watts. Thedownside is wirewound are physically larger and more expensive.


Pre-set carbon filmskeleton pot

Pre-set carbon filmenclosed pot

Pre-set wirewoundCermet Trimmer

Cermet trimmer pre-setwirewound enclosed pot

1 K W

1K W

4.7KW

1KW

Pre-set wirewound pot, 100 8 Watt, high current use


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10 VARIABLE RESISTOR EXTRA FEATURES

Ganged Potentiometers

Ganged potentiometers are where two or more pots areadjusted in unison by the rotary shaft. Typical use are for anyadjustments in stereo systems or quadraphonic systems.

Mid-Range Click

A small click in the centre of the range is useful with bass andtreble controls where the centre position represents the Norm,neither attenuated nor amplified. Also useful in remote controlof moving motorised vehicles.

Integral On/Off Switch

Quite often a pot used as the volume control is also used asthe On/Off switch

Linear or Logarithmic

Most variables are linear which means the resistance isproportional to the movement of the dial. Linear are used forvolume and brightness controls. Some are logarithmic whichhas a low resistive value at the start building up to a high

resistive value at the end of the turn. Log is used particularlywhere the User outcome is frequency related such as tonecontrols or radio tuning.

Preferred Values

Above 250 most variable resistors are made to preferredvalues of:

250 2.5K 25K 250K 2.5M500 5K 50K 500K1K 10K 100K 1M

Miniature 4-ganged carbonPot with mid-range click

Miniature 4-ganged carbonPot with On/Off switch


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11 NON-LINEAR RESISTORS

Light Dependant Resistor - LDR

LDRs are sensitive to light - as the light level increases the resistance reduces. The 1cm diameter component consists of a clear circular window with a cadmium sulphideetched looped track underneath it providing about 12 cm of light sensitive track.

Thermal Resistors - Thermistors

Thermistors are designed to maximise resistance change with change in temperature.They have a controlled temperature coefficient with the heat sensitive material beingmade from a semi-conductor. The coefficient is positive when resistance increaseswith temperature increase, and negative when the resistance reduces withtemperature increase. Old Thermistors looked like a 1 cm diameter disc or rectangularplate. Modern designs are a much smaller bead which is sometimes enclosed in aglass tube. The component may use the standard resistor colour code (bead), theBS1852 Letter Code (glass) or a single colour dot (rod). The dot has the followingvalues for a resistance reading at 250C:

3 K - red 5 K - orange 10 K - yellow

30 K - green 100 K - violetVoltage Dependant Resistors - Varistors

Varistors are made from a semi-conductor family material. The resistance increasesas the applied voltage increases. They are principally used as voltage surgesuppressors.


Light Dependant Resistor

Thermister bead

Thermister bead in glass tube

Varistorvoltage dependant resistor


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12 RESISTOR POWER RATING

Resistors have a Power Ratingthis is the maximum power thatshould be dissipated by the resistor. If the power rating is exceededthe resistor will heat up, and in some cases explode. The PowerRating is generally known from its physical size, particularly withcarbon-clay resistors. It is good practice to operate a resistor belowhalf its rated power value.

There are a range of standard or preferred power ratings, all inWatts:

0.125 0.25 0.5 1.0 2.0 3.0 5.0 10 20

The formula for Power in Watts consumed by a resistive load is:

P = I2 R or V2 / R Watts

Thus the maximum voltage a particular resistor should be operatedat is:

- the square root of P Watts times R

The maximum voltage that can be applied to a resistor will dependon its Power Rating AND its resistance value. This has to be workedout for each and every resistor value under consideration.

Low value resistors are prone to heating because they pass a highercurrent. This is often a cause of unreliability in electronic equipment.

Example: What is the maximum voltage that can beapplied to a 5.6 K 0.25 Watt resistor

Answer: Vm= PR = 0.25x5600 = 1400 = 37.4 volts.

Example: What is the maximum voltage that can be

applied to a 33 M 0.5 Watt resistor?Answer: Vm= PR = 0.5x33,000,000 = 16,500,000 = 4,062 volts


Avoid component overheating toavoid unexpected component

mortality

mV PR


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13 LINEAR RESISTOR TYPESAND FEATURES TABLE

Selection of the type of resistor manufacturing method can ensure a circuit works reliably; the wrong selection leads to early circuit failure.


FIXED VALUE LINEAR RESISTORS VARIABLE RESISTORS

RESISTOR PROPERTIES Carbonrod

Carbonfilm

Metalfilm

Metaloxide

Ceramicwirewound

Vitreouswirewound

Carbonfilm

Wirewound

Typical resistance range 10 to 10M 1 to 10M 1 to 1M 10 to 1M 0.1 to 20K 0.1 to 20K 1K to 1 M 10 to 100K

Typical tolerance % 10 5 1 2 5 5 20 5

Typical min-max power rating -2 W -2 W - W - W 4 to 20 W 2 to 4 W - W -1 W

Temperature stability pass pass Very good Very good good good poor Very good

Temperature coefficient (ppm/C) -250 -250 50 to 100 250 250 75 -250 250

Electrical noise issue poor pass good good Very good Very good poor good

Relative size (per) standard standard Very small small large large standard large

Stability at high frequencies good pass poor poor Very poor Very poor pass poor

Stability over time poor pass good good Very good Very good poor Very good

Physical wear with use - - - - - - Very poor Very good

Applications Audio, toys, general Amps, test equip,transmitters, receivers

Power amp output, pre-setlighting switch

Controls tone, volumeColour, brightness

Documents

Resistors in Practice V3