Passive Attenuators

Passive Attenuator Basics

An Attenuator is a special type of electrical or electronic bidirectional circuit made up of entirely resistive elements. An attenuator is a two port resistive network designed to weaken or “attenuate” (hence their name) the power being supplied by a source to a level that is suitable for the connected load.

A passive attenuator reduces the amount of power being delivered to the connected load by either a single fixed amount, a variable amount or in a series of known switchable steps. Attenuators are generally used in radio, communication and transmission line applications to weaken a stronger signal.

The Passive Attenuator is a purely passive resistive network (hence no supply) which is used in a wide variety of electronic equipment for extending the dynamic range of measuring equipment by adjusting signal levels, to provide impedance matching of oscillators or amplifiers to reduce the effects of improper input/output terminations, or to simply provide isolation between different circuit stages depending upon their application as shown.

Attenuator Connection

 

Simple Attenuator Networks (also known as “pads”) can be designed to produce a fixed degree of “attenuation” or to give a variable amount of attenuation in pre-determined steps. Standard fixed attenuator networks generally known as an “attenuator pad” are available in specific values from 0 dB to more than 100 dB. Variable and switched attenuators are basically adjustable resistor networks that show a calibrated increase in attenuation for each switched step, for example steps of -2dB or -6dB per switch position.

Then an Attenuator is a four terminal (two port) passive resistive network (active types are also available which use transistors and integrated circuits) designed to produce “distortionless” attenuation of the output electrical signal at all frequencies by an equal amount with no phase shift unlike a passive type RC filter network, and therefore to achieve this attenuators should be made up of pure non-inductive and not wirewound resistances, since reactive elements will give frequency discrimination.

Simple Passive Attenuator

Attenuators are the reverse of amplifiers in that they reduce gain with the resistive voltage divider circuit being a typical attenuator. The amount of attenuation in a given network is determined by the ratio of: Output/Input. For example, if the input voltage to a circuit is 1 volt (1V) and the output voltage is 1 milli-volt (1mV) then the amount of attenuation is 1mV/1V which is equal to 0.001 or a reduction of 1,000th.

However, using voltage, current or even power ratios to determine or express the amount of attenuation that a resistive attenuator network may have, called the attenuation factor, can be confusing, so for the passive attenuator its degree of attenuation is normally expressed using a logarithmic scale which is given in decibels (dB) making it easier to deal with such small numbers.

Degrees of Attenuation

An attenuators performance is expressed by the number of decibels the input signal has decreased per frequency decade (or octave). The decibel, abbreviated to “dB”, is generally defined as the logarithm or “log” measure of the voltage, current or power ratio and represents one tenth 1/10th of a Bel (B). In other words it takes 10 decibels to make one Bel. Then by definition, the ratio between an input signal (Vin) and an output signal (Vout) is given in decibels as:

Decibel Attenuation

Note that the decibel (dB) is a logarithmic ratio and therefore has no units. So a value of -140dB represents an attenuation of 1:10,000,000 units or a ratio of 10 million to 1!.

In passive attenuator circuits, it is often convenient to assign the input value as the 0 dB reference point. This means that no matter what is the actual value of the input signal or voltage, is used as a reference with which to compare the output values of attenuation and is therefore assigned a 0 dB value. This means that any value of output signal voltage below this reference point will be expressed as a negative dB value, ( -dB ).

So for example an attenuation of -6dB indicates that the value is 6 dB below the 0 dB input reference. Likewise if the ratio of output/input is less than one (unity), for example 0.707, then this corresponds to 20 log(0.707) = -3dB. If the ratio of output/input = 0.5, then this corresponds to 20 log(0.5) = -6 dB, and so on, with standard electrical tables of attenuation available to save on the calculation.

Passive Attenuators Example No1

A passive attenuator circuit has an insertion loss of -32dB and an output voltage of 50mV. What will be the value of the input voltage.

 

The antilog (log-1) of -1.6 is given as:

 

Then if the output voltage produced with 32 decibels of attenuation, an input voltage of 2.0 volts is required.

Attenuator Loss Table

Vout/Vin	1	0.7071	0.5	0.25	0.125	0.0625	0.03125	0.01563	0.00781
Log Value	20log(1)	20log (0.7071)	20log (0.5)	20log (0.25)	20log (0.125)	20log (0.0625)	20log (0.03125)	20log (0.01563)	20log (0.00781)
in dB’s	0	-3dB	-6dB	-12dB	-18dB	-24dB	-30dB	-36dB	-42dB

 

and so on, producing a table with as many decibel values as we require for our attenuator design.

This decrease in voltage, current or power expressed in decibels by the insertion of the attenuator into an electrical circuit is known as insertion loss and minimum loss attenuator designs match circuits of unequal impedances with a minimum loss in the matching network.

Now that we know what a passive attenuator is how it can be used to reduce or “attenuate” the power or voltage level of a signal, while introducing little or no distortion and insertion loss, by an amount expressed in decibels, we can begin to look at the different attenuator circuit designs available.

Passive Attenuator Designs

There are many ways in which resistors can be arranged in attenuator circuits with the Potential Divider Circuit being the simplest type of passive attenuator circuit. The potential or voltage divider circuit is generally known as an “L-pad” attenuator because its circuit diagram resembles that of an inverted “L”. But there are other common types of attenuator network as well such as the “T-pad” attenuator and the “Pi-pad” (π) attenuator depending upon how you connect together the resistive components. These three common attenuator types are shown below.

Attenuator Types

The above attenuator circuit designs can be arranged in either “balanced” or “unbalanced” form with the action of both types being identical. The balanced version of the “T-pad” attenuator is called the “H-pad” attenuator while the balanced version of the “π-pad” attenuator is called the “O-pad” attenuator. Bridged T-type attenuators are also available.

In an unbalanced attenuator, the resistive elements are connected to one side of the transmission line only while the other side is grounded to prevent leakage at higher frequencies. Generally the grounded side of the attenuator network has no resistive elements and is therefore called the “common line”.

In a balanced attenuator configuration, the same number of resistive elements are connected equally to each side of the transmission line with the ground located at a center point created by the balanced parallel resistances. Generally, balanced and unbalanced attenuator networks can not be connected together as this results in half of the balanced network being shorted to ground through the unbalanced configuration.

Switched Attenuators

Instead of having just one attenuator to achieve the required degree of attenuation, individualattenuator pads can be connected or cascaded together to increase the amount of attenuation in given steps of attenuation. Multi-pole rotary switches, rocker switches or ganged push-button switches can also be used to connect or bypass individual fixed attenuator networks in any desired sequence from 1dB to 100dB or more, making it easy to design and construct switched attenuator networks, also known as a step attenuator. By switching in the appropriate attenuators, the attenuation can be increased or decreased in fixed steps as shown below.

Switched Attenuator

 

Here, there are four independent resistive attenuator networks cascaded together in a series ladder network with each attenuator having a value twice that of its predecessor, (1-2-4-8). Each attenuator network may be switched “in” or “out” of the signal path as required by the associated switch producing a step adjustment attenuator circuit that can be switched from 0dB to -15dB in 1dB steps.

Therefore, the total amount of attenuation provided by the circuit would be the sum of all four attenuators networks that are switched “IN”. So for example an attenuation of -5dB would require switches SW1 and SW3 to be connected, and an attenuation of -12dB would require switches SW3and SW4 to be connected, and so on.

Attenuator Summary

• An attenuator is a four terminal device that reduces the amplitude or power of a signal without distorting the signal waveform, an attenuator introduces a certain amount of loss.
• The attenuator network is inserted between a source and a load circuit to reduce the source signal’s magnitude by a known amount suitable for the load.
• Attenuators can be fixed, fully variable or variable in known steps of attenuation, -0.5dB, -1dB, -10dB, etc.
• An attenuator can be symmetrical or asymmetrical in form and either balanced or unbalanced.
• Fixed attenuators also known as a “pad” are used to “match” unequal impedances.
• An attenuator is effectively the opposite of an amplifier. An amplifier provides gain while an attenuator provides loss, or gain less than 1 (unity).
• Attenuators are usually passive devices made to from simple voltage divider networks. The switching between different resistances produces adjustable stepped attenuators and continuously adjustable ones using potentiometers.

To simplify the design of the attenuator, a "K" (for constant) value can be used. This "K" value is the ratio of the voltage, current or power corresponding to a given value of dB attenuation and is given as:

We can produce a set of constant values called “K” values for different amounts of attenuation as given in the following table.

Attenuator Loss Table

dB	0.5	1.0	2.0	3.0	4.0	5.0	6.0	10.0	20.0
“K” value	1.0593	1.1220	1.2589	1.4125	1.5849	1.7783	1.9953	3.1623	10.000

and so on, producing a table with as many “K” values as we require.

Fixed value attenuators, called “attenuator pads” are used mainly in radio frequency (Rf) transmission lines to lower voltage, dissipate power, or to improve the impedance matching between various mismatched circuits.

Line-level attenuators in pre-amplifier or Audio Power Amplifiers can be as simple as a 0.5 watt potentiometer, or voltage divider L-pad designed to reduce the amplitude of an audio signal before it reaches the speaker, reducing the volume of the output.

In measuring signals, high power attenuator pads are used to lower the amplitude of the signal a known amount to enable measurements, or to protect the measuring device from high signal levels that might otherwise damage it.

In the next tutorial about Attenuators, we will look at the most basic type of resistive attenuator network commonly called a “L-type” or “L-pad” attenuator which can be made using just two resistive components. The “L-pad” attenuator circuit can also be used as a voltage or potential divider circuit.

Resistor Colour Code

Resistor Colour Code

We saw in the previous tutorial that there are many different types of Resistor available and that they can be used in both electrical and electronic circuits to control the flow of current or to produce a voltage in many different ways. But in order to do this the actual resistor needs to have some form of “resistive” or “resistance” value. Resistors are available in a range of different resistance values from fractions of an Ohm ( Ω ) to millions of Ohms.

Obviously, it would be impractical to have available resistors of every possible value for example,1Ω, 2Ω, 3Ω, 4Ω etc, because literally tens of hundreds of thousands, if not tens of millions of different resistors would need to exist to cover all the possible values. Instead, resistors are manufactured in what are called “preferred values” with their resistance value printed onto their body in coloured ink.

4 Coloured Bands

The resistance value, tolerance, and wattage rating are generally printed onto the body of the resistor as numbers or letters when the resistors body is big enough to read the print, such as large power resistors. But when the resistor is small such as a 1/4W carbon or film type, these specifications must be shown in some other manner as the print would be too small to read.

So to overcome this, small resistors use coloured painted bands to indicate both their resistive value and their tolerance with the physical size of the resistor indicating its wattage rating. These coloured painted bands produce a system of identification generally known as a Resistors Colour Code.

An international and universally accepted Resistor Colour Code Scheme was developed many years ago as a simple and quick way of identifying a resistors ohmic value no matter what its size or condition. It consists of a set of individual coloured rings or bands in spectral order representing each digit of the resistors value.

The resistor colour code markings are always read one band at a time starting from the left to the right, with the larger width tolerance band oriented to the right side indicating its tolerance. By matching the colour of the first band with its associated number in the digit column of the colour chart below the first digit is identified and this represents the first digit of the resistance value.

Again, by matching the colour of the second band with its associated number in the digit column of the colour chart we get the second digit of the resistance value and so on. Then the resistor colour code is read from left to right as illustrated below:

The Standard Resistor Colour Code Chart.

 

The Resistor Colour Code Table.

Colour	Digit	Multiplier	Tolerance
Black	0	1
Brown	1	10	± 1%
Red	2	100	± 2%
Orange	3	1,000
Yellow	4	10,000
Green	5	100,000	± 0.5%
Blue	6	1,000,000	± 0.25%
Violet	7	10,000,000	± 0.1%
Grey	8		± 0.05%
White	9
Gold		0.1	± 5%
Silver		0.01	± 10%
None			± 20%

Calculating Resistor Values

The Resistor Colour Code system is all well and good but we need to understand how to apply it in order to get the correct value of the resistor. The “left-hand” or the most significant coloured band is the band which is nearest to a connecting lead with the colour coded bands being read from left-to-right as follows;

Digit, Digit, Multiplier = Colour, Colour x 10 colour  in Ohm’s (Ω’s)

For example, a resistor has the following coloured markings;

Yellow Violet Red = 4 7 2 = 4 7 x 102 = 4700Ω or 4k7.

The fourth and fifth bands are used to determine the percentage tolerance of the resistor. Resistor tolerance is a measure of the resistors variation from the specified resistive value and is a consequence of the manufacturing process and is expressed as a percentage of its “nominal” or preferred value.

Typical resistor tolerances for film resistors range from 1% to 10% while carbon resistors have tolerances up to 20%. Resistors with tolerances lower than 2% are called precision resistors with the or lower tolerance resistors being more expensive.

Most five band resistors are precision resistors with tolerances of either 1% or 2% while most of the four band resistors have tolerances of 5%, 10% and 20%. The colour code used to denote the tolerance rating of a resistor is given as;

Brown = 1%, Red = 2%, Gold = 5%, Silver = 10 %

If resistor has no fourth tolerance band then the default tolerance would be at 20%.

It is sometimes easier to remember the resistor colour code by using mnemonics or phrases that have a separate word in the phrase to represent each of the Ten + Two colours in the code. However, these sayings are often very crude but never the less effective for remembering the resistor colours. Here are just a few of the more “cleaner” versions but many more exist:

• Bad Booze Rots Our Young Guts But Vodka Goes Well
• Bad Boys Ring Our Young Girls But Vicky Goes Without
• Bad Boys Ring Our Young Girls But Vicky Gives Willingly — Get Some Now (This one is only slightly better because it includes the tolerance bands of Gold, Silver, and None).

The British Standard (BS 1852) Code.

Generally on larger power resistors, the resistor colour code systems is not required as the resistance value, tolerance, and even the power (wattage) rating are printed onto the actual body of the resistor instead of using the resistor colour code system. Because it is very easy to “misread” the position of a decimal point or comma especially when the component is discoloured or dirty. An easier system for writing and printing the resistance values of the individual resistance was developed.

This system conforms to the British Standard BS 1852 Standard and its replacement, BS EN 60062, coding method were the decimal point position is replaced by the suffix letters "K" for thousands or kilohms, the letter "M" for millions or megaohms both of which denotes the multiplier value with the letter "R" used where the multiplier is equal to, or less than one, with any number coming after these letters meaning it’s equivalent to a decimal point.

The BS 1852 Letter Coding for Resistors.

BS 1852 Codes for Resistor Values

0.47Ω = R47 or 0R47

1.0Ω = 1R0

4.7Ω = 4R7

47Ω = 47R

470Ω = 470R or 0K47

1.0KΩ = 1K0

4.7KΩ = 4K7

47KΩ = 47K

470KΩ = 470K or 0M47

1MΩ = 1M0

Sometimes depending upon the manufacturer, after the written resistance value there is an additional letter which represents the resistors tolerance value such as 4k7 J and these suffix letters are given as.

Tolerance Letter Coding for Resistors.

Tolerance Codes for Resistors (±)

B = 0.1%

C = 0.25%

D = 0.5%

F = 1%

G = 2%

J = 5%

K = 10%

M = 20%

Also, when reading these written codes be careful not to confuse the resistance letter k for kilohms with the tolerance letter K for 10% tolerance or the resistance letter M for megaohms with the tolerance letter M for 20% tolerance.

Tolerances, E-series & Preferred Values.

Hopefully by now we understand that resistors come in a variety of sizes and resistance values but to have a resistor available of every possible resistance value, literally hundreds of thousands, if not millions of individual resistors would need to exist. Instead, resistors are manufactured in what are commonly known as Preferred values.

Instead of sequential values of resistance from 1Ω and upwards, certain values of resistors exist within certain tolerance limits. The tolerance of a resistor is the maximum difference between its actual value and the required value and is generally expressed as a plus or minus percentage value. For example, a 1kΩ ±20% tolerance resistor may have a maximum and minimum resistive value of.

Maximum Resistance Value

1kΩ or 1000Ω + 20% = 1,200Ω’s

Minimum Resistance Value

1kΩ or 1000Ω – 20% = 800Ω’s

 

Then using our example above, a 1kΩ ±20% tolerance resistor may have a maximum value of1200Ω’s and a minimum value of 800Ω’s resulting in a difference of some 400Ω’s!! for the same value resistor.

In most electrical or electronic circuits this large 20% tolerance of the same resistor is generally not a problem, but when close tolerance resistors are specified for high accuracy circuits such as filters or oscillators etc, then the correct tolerance resistor needs to be used, as a 20% tolerance resistor cannot generally be used to replace 2% or even a 1% tolerance type.

The five and six band resistor colour code is more commonly associated with the high precision 1% and 2% film types while the common garden variety 5% and 10% general purpose types tend to use the four band resistor colour code. Resistors come in a range of tolerances but the two most common are the E12 and the E24 series.

The E12 series comes in twelve resistance values per decade, (A decade representing multiples of 10, i.e. 10, 100, 1000 etc), while the E24 series comes in twenty four values per decade and the E96series ninety six values per decade. A very high precision E192 series is now available with tolerances as low as ± 0.1% giving a massive 192 separate resistor values per decade.

Tolerance and E-series Table.

E6 Series at 20% Tolerance – Resistors values in Ω’s
1.0, 1.5, 2.2, 3.3, 4.7, 6.8
E12 Series at 10% Tolerance – Resistors values in Ω’s
1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2
E24 Series at 5% Tolerance – Resistors values in Ω’s
1.0, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2.0, 2.2, 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.2, 8.2, 9.1
E96 Series at 1% Tolerance – Resistors values in Ω’s
1.00, 1.02, 1.05, 1.07, 1.10, 1.13, 1.15, 1.18, 1.21, 1.24, 1.27, 1.30, 1.33, 1.37, 1.40, 1.43, 1.47, 1.50, 1.54, 1.58, 1.62, 1.65, 1.69, 1.74, 1.78, 1.82, 1.87, 1.91, 1.96, 2.00, 2.05, 2.10, 2.15, 2.21, 2.26, 2.32, 2.37, 2.43, 2.49, 2.55, 2.61, 2.77, 2.74, 2.80, 2.87, 2.94, 3.01, 3.09, 3.16, 3.24, 3.32, 3.40, 3.48, 3.57, 3.65, 3.74, 3.83, 3.92, 4.02, 4.12, 4.22, 4.32, 4.42, 4.53, 4.64, 4.75, 4.87, 4.99, 5.11, 5.23, 5.36, 5.49, 5.62, 5.76, 5.90, 6.04, 6.19, 6.34, 6.49, 6.65, 6.81, 6.98, 7.15, 7.32, 7.50, 7.68, 7.87, 8.06, 8.25, 8.45, 8.66, 8.87, 9.09, 9.31, 9.53, 9.76

Then by using the appropriate E-series value for the percentage tolerance required for the resistor, adding a multiplication factor to it, any ohmic value of resistance within that series can be found. For example, take an E-12 series resistor, 10% tolerance with a preferred value of 3.3, then the values of resistance for this range are:

Value x Multiplier = Resistance

3.3 x 1 = 3.3Ω

3.3 x 10 = 33Ω

3.3 x 100 = 330Ω

3.3 x 1,000 = 3.3kΩ

3.3 x 10,000 = 33kΩ

3.3 x 100,000 = 330kΩ

3.3 x 1,000,000 = 3.3MΩ

 

The mathematical basis behind these preferred values comes from the square root value of the actual series being used. For example, for the E6 20% series there are six individual resistors or steps (1.0 to 6.8) and is given as the sixth root of ten ( 6√10 ), so for the E12 10% series there are twelve individual resistors or steps (1.0 to 8.2) and is therefore given as the twelfth root of ten (12√10 ) and so on for the remaining E-series values.

The tolerance series of Preferred Values shown above are manufactured to conform to the British Standard BS 2488 and are ranges of resistor values chosen so that at maximum or minimum tolerance any one resistor overlaps with its neighbouring value. For example, take the E24 range of resistors with a 5% tolerance. It’s neighbouring resistor values are 47 and 51Ω’s respectively.

   47Ω + 5% = 49.35Ω’s, and 51Ω – 5% = 48.45Ω’s, an overlap of just 0.9Ω’s.

Surface Mount Resistors

4.7kΩ SMD Resistor

Surface Mount Resistors or SMD Resistors, are very small rectangular shaped metal oxide film resistors designed to be soldered directly onto the surface, hence their name, of a circuit board. Surface mount resistors generally have a ceramic substrate body onto which is deposited a thick layer of metal oxide resistance.

The resistive value of the resistor is controlled by increasing the desired thickness, length or type of deposited film being used and highly accurate low tolerance resistors, down to 0.1% can be produced. They also have metal terminals or caps at either end of the body which allows them to be soldered directly onto printed circuit boards.

Surface Mount Resistors are printed with either a 3 or 4-digit numerical code which is similar to that used on the more common axial type resistors to denote their resistive value. Standard SMD resistors are marked with a three-digit code, in which the first two digits represent the first two numbers of the resistance value with the third digit being the multiplier, either x1, x10, x100 etc. For example:

“103” = 10 × 1,000 ohms = 10 kiloΩ´s

“392” = 39 × 100 ohms = 3.9 kiloΩ´s

“563” = 56 × 1,000 ohms = 56 kiloΩ´s

“105” = 10 × 100,000 ohms = 1 MegaΩ

 

Surface mount resistors that have a value of less than 100Ω’s are usually written as: “390”,
“470”, “560” with the final zero representing a 10 xo multiplier, which is equivalent to 1. For example:

“390” = 39 × 1Ω = 39Ω´s or 39RΩ

“470” = 47 × 1Ω = 47Ω´s or 47RΩ

 

Resistance values below ten have a letter “R” to denote the position of the decimal point as seen previously in the BS1852 form, so that 4R7 = 4.7Ω.

Surface mount resistors that have a “000” or “0000” markings are zero-Ohm (0Ω) resistors or in other words shorting links, since these components have zero resistance.

Then we have seen that the resistor colour code system is used to identify the resistive value of a resistor. In the next tutorial about Resistors, we will look at connecting resistors together in a series chain and prove that the total resistance is the sum of all the resistors added together and that the current is common to a series circuit.

Resistor

Types of Resistor

The Different Types of Resistor

Resistors, ( R ) are the most fundamental and commonly used of all the electronic components, to the point where they are almost taken for granted. There are many different Types of Resistor available for the electronics constructor to choose from, from very small surface mount chip resistors up to large wirewound power resistors.

The principal job of a resistor within an electrical or electronic circuit is to “resist” (hence the nameResistor), regulate or to set the flow of electrons (current) through them by using the type of conductive material from which they are composed. Resistors can also be connected together in various series and parallel combinations to form resistor networks which can act as voltage droppers, voltage dividers or current limiters within a circuit.

A Typical Resistor

Resistors are what are called “Passive Devices“, that is they contain no source of power or amplification but only attenuate or reduce the voltage or current signal passing through them. This attenuation results in electrical energy being lost in the form of heat as the resistor resists the flow of electrons through it.

Then a potential difference is required between the two terminals of a resistor for current to flow. This potential difference balances out the energy lost. When used in DC circuits the potential difference, also known as a resistors voltage drop, is measured across the terminals as the circuit current flows through the resistor.

Most types of resistor are linear devices that produce a voltage drop across themselves when an electrical current flows through them because they obey Ohm’s Law, and different values of resistance produces different values of current or voltage. This can be very useful in Electronic circuits by controlling or reducing either the current flow or voltage produced across them.

There are many thousands of different Types of Resistor and are produced in a variety of forms because their particular characteristics and accuracy suit certain areas of application, such as High Stability, High Voltage, High Current etc, or are used as general purpose resistors where their characteristics are less of a problem.

Some of the common characteristics associated with the humble resistor are; Temperature Coefficient, Voltage Coefficient, Noise, Frequency Response, Power as well as a resistorsTemperature Rating, Physical Size and Reliability.

In all Electrical and Electronic circuit diagrams and schematics, the most commonly used symbol for a fixed value resistor is that of a “zig-zag” type line with the value of its resistance given in Ohms, Ω. Resistors have fixed resistance values from less than one ohm, ( <1Ω ) to well over tens of millions of ohms, ( >10MΩ ) in value.

Fixed resistors have only one single value of resistance, for example 100Ω’s but variable resistors (potentiometers) can provide an infinite number of resistance values between zero and their maximum value.

Standard Resistor Symbols

The symbol used in schematic and electrical drawings for a Resistor can either be a “zig-zag” type line or a rectangular box.

All modern fixed value resistors can be classified into four broad groups;

• Carbon Composition Resistor – Made of carbon dust or graphite paste, low wattage values
• Film or Cermet Resistor – Made from conductive metal oxide paste, very low wattage values
• Wire-wound Resistor – Metallic bodies for heatsink mounting, very high wattage ratings
• Semiconductor Resistor – High frequency/precision surface mount thin film technology

There are a large variety of fixed and variable resistor types with different construction styles available for each group, with each one having its own particular characteristics, advantages and disadvantages compared to the others. To include all types would make this section very large so I shall limit it to the most commonly used, and readily available general purpose types of resistors.

Composition Types of Resistor

Carbon Resistors are the most common type of Composition Resistors. Carbon resistors are a cheap general purpose resistor used in electrical and electronic circuits. Their resistive element is manufactured from a mixture of finely ground carbon dust or graphite (similar to pencil lead) and a non-conducting ceramic (clay) powder to bind it all together.

Carbon Resistor

The ratio of carbon dust to ceramic (conductor to insulator) determines the overall resistive value of the mixture and the higher the ratio of carbon, the lower the overall resistance. The mixture is moulded into a cylindrical shape with metal wires or leads are attached to each end to provide the electrical connection as shown, before being coated with an outer insulating material and colour coded markings to denote its resistive value.

Carbon Resistor

 

The Carbon Composite Resistor is a low to medium type power resistor which has a low inductance making them ideal for high frequency applications but they can also suffer from noise and stability when hot. Carbon composite resistors are generally prefixed with a “CR” notation (eg,CR10kΩ ) and are available in E6 ( ± 20% tolerance (accuracy) ), E12 ( ± 10% tolerance) and E24 ( ± 5% tolerance) packages with power ratings from 0.125 or 1/4 of a Watt up to 5 Watts.

Carbon composite resistor types are very cheap to make and are therefore commonly used in electrical circuits. However, due to their manufacturing process carbon type resistors have very large tolerances so for more precision and high value resistances, film type resistors are used instead.

Film Type Resistors

The generic term “Film Resistor” consist of Metal Film, Carbon Film and Metal Oxide Film resistor types, which are generally made by depositing pure metals, such as nickel, or an oxide film, such as tin-oxide, onto an insulating ceramic rod or substrate.

Film Resistor

The resistive value of the resistor is controlled by increasing the desired thickness of the deposited film giving them the names of either “thick-film resistors” or “thin-film resistors”.

Once deposited, a laser is used to cut a high precision spiral helix groove type pattern into this film. The cutting of the film has the effect of increasing the conductive or resistive path, a bit like taking a long length of straight wire and forming it into a coil.

This method of manufacture allows for much closer tolerance resistors (1% or less) as compared to the simpler carbon composition types. The tolerance of a resistor is the difference between the preferred value (i.e, 100 ohms) and its actual manufactured value i.e, 103.6 ohms, and is expressed as a percentage, for example 5%, 10% etc, and in our example the actual tolerance is 3.6%. Film type resistors also achieve a much higher maximum ohmic value compared to other types and values in excess of 10MΩ (10 Million Ω´s) are available.

Film Resistor

 

Metal Film Resistors have much better temperature stability than their carbon equivalents, lower noise and are generally better for high frequency or radio frequency applications. Metal Oxide Resistors have better high surge current capability with a much higher temperature rating than the equivalent metal film resistors.

Another type of film resistor commonly known as a Thick Film Resistor is manufactured by depositing a much thicker conductive paste of CERamic and METal, called Cermet, onto an alumina ceramic substrate. Cermet resistors have similar properties to metal film resistors and are generally used for making small surface mount chip type resistors, multi-resistor networks in one package for pcb’s and high frequency resistors. They have good temperature stability, low noise, and good voltage ratings but low surge current properties.

Metal Film Resistors are prefixed with a “MFR” notation (eg, MFR100kΩ) and a CF for Carbon Film types. Metal film resistors are available in E24 (±5% & ±2% tolerances), E96 (±1% tolerance) andE192 (±0.5%, ±0.25% & ±0.1% tolerances) packages with power ratings of 0.05 (1/20th) of a Watt up to 1/2 Watt. Generally speaking Film resistors are precision low power components.

Wirewound Types of Resistor

Another type of resistor, called a Wirewound Resistor, is made by winding a thin metal alloy wire (Nichrome) or similar wire onto an insulating ceramic former in the form of a spiral helix similar to the film resistor above.

Wirewound Resistor

These types of resistor are generally only available in very low ohmic high precision values (from 0.01 to 100kΩ) due to the gauge of the wire and number of turns possible on the former making them ideal for use in measuring circuits and Whetstone bridge type applications.

They are also able to handle much higher electrical currents than other resistors of the same ohmic value with power ratings in excess of 300 Watts. These high power resistors are moulded or pressed into an aluminium heat sink body with fins attached to increase their overall surface area to promote heat loss and cooling.

These special types of resistor are called “Chassis Mounted Resistors” because they are designed to be physically mounted onto heatsinks or metal plates to further dissipate the generated heat. The mounting of the resistor onto a heatsink increases their current carrying capabilities even further.

The non-inductive resistance wire is wound around a ceramic or porcelain tube covered with mica to prevent the alloy wires from moving when hot. Wirewound resistors are available in a variety of resistance and power ratings with one main use of power wirewound resistor is in the electrical heating elements of an electric fire which converts the electrical current flowing through it into heat with each element dissipating up to 1000 Watts, (1kW) of energy.Another type of wirewound resistor is the Power Wirewound Resistor. These are high temperature, high power non-inductive resistor types generally coated with a vitreous or glass epoxy enamel for use in resistance banks or DC motor/servo control and dynamic braking applications. They can even be used as low wattage space or cabinet heaters.

Because the wire is wound into a coil inside the resistors body, it acts like an inductor causing them to have inductance as well as resistance and this affects the way the resistor behaves in AC circuits by producing a phase shift at high frequencies especially in the larger size resistors. The length of the actual resistance path in the resistor and the leads contributes inductance in series with the “apparent” DC resistance resulting in an overall impedance path of Z Ohms.

Impedance ( Z ) is the combined effect of resistance ( R ) and inductance ( X ), measured in ohms and for a series AC circuit is given as, Z 2 = R 2 + X 2.

When used in AC circuits this inductance value changes with frequency (inductive reactance,XL = 2πƒL) and therefore, the overall value of the resistor changes. Inductive reactance increases with frequency but is zero at DC (zero frequency). Then, wirewound resistors must not be designed or used in AC or amplifier type circuits where the frequency across the resistor changes. However, special non-inductive wirewound resistors are also available.

Wirewound Resistor

 

Wirewound resistor types are prefixed with a “WH” or “W” notation (eg WH10Ω) and are available in the WH aluminium clad package (±1%, ±2%, ±5% & ±10% tolerance) or the W vitreous enamelled package (±1%, ±2% & ±5% tolerance) with power ratings from 1W to 300W or more.

Resistor Types Summary

Then to summarise, there are many different types of resistor available from low cost, large tolerance, general purpose carbon type resistors through to low tolerance, high cost, precision film resistors as well as high power, wirewound ceramic resistors. A resistor regulates, impedes or sets the flow of current through a particular path or it can impose a voltage reduction in an electrical circuit.

The resistive value of a resistor, its ability to limit current flow is measured in Ohm’s ( Ω ) ranging from less than one Ohm each to many millions of Ohm’s, (Mega-Ohm’s). Resistors can be of a fixed value, for example: 100 Ohms, (100Ω) or variable as in 0 to 100Ω’s.

A resistor will always have the same resistance value no matter what the frequency of the supply from DC to very high frequencies and all resistors have one thing in common, their resistive value in Ohm’s in a circuit will ALWAYS be positive in nature and never negative.

The uses and applications of a resistor within an electrical or electronic circuit are vast and varied with virtually every electronic circuit ever designed using one or more types of resistor. Resistors are commonly used for purposes such as current limiting, providing appropriate control voltages to semiconductor devices, such as bipolar transistors, protecting LEDs or other semiconductor devices from over current damage, as well as adjusting or limiting the frequency response in an audio or filter circuit.

In digital circuits different types of resistors can be used for pulling up or pulling down the voltage at the input pin of a digital logic chip or by controlling a voltage at a point in a circuit by placing two resistors in series to create a voltage divider network, the list is endless!.

In the next tutorial about Resistors, we will look at the different ways of identifying the resistive value of the different types of fixed resistors with the most common method of identification being the use of Colour Codes and colour bands around the body of the resistor.