Practical Resistors

Materials

Resistors can be made from a wide variety of materials. Here are some examples of the most common types produced, along with a brief description of how they are made.

Tolerances

As good as modern manufacturing techniques are. They are not perfect and the final value rolling off the end of the production line may not be precise and there will also be some slight variance between each resistor produced.

Since out of specification equates to manufacturing waste and ultimately extra cost, a process known as binning is used. This process assigns a range for allowable values to each bin. That is any resistor within some defined percentage of the target value is acceptable for that bin.

That is why resistors are sold as conforming within some specified tolerance of the specified value. For example, common tolerances are 5% and 1% (there are others too, but these are the most common).

Preferred Values

There is another potential issue for manufacturing costs, and that is the number of possible resistor values that would need to be produced to cover all scenarios (it would huge, arguably infinite).

To address the two issues of waste due to manufacturing tolerances and the sheer quantity of possible resistor values a series of standards have been created. These standards allow for a finite (and relatively small) range of values to be targeted for manufacture, while still covering all/most possible values.

Where more specific values are required, they can easily be created using series/parallel combinations of values from the range.

There are several of these standards which are sub-sets of each other. In which for a given standard, each value in a decade range is on the edge of the tolerance for the previous value in that decade. Each value specified is known as a preferred value.

Here is a table of the set of standards showing the tolerances for each standard and how many values that standard specifies for each decade range:

Standard	Tolerance	# Values in a Decade	Preferred Values
E3	36%	3	1, 2.2, 4.7
E6	20%	6	1, 1.5, 2.2, 3.3, 4.7, 6.8
E12	10%	12	1, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2
E24	5%	24	1, 1.1, 1.2, 1.3, 1.5, 1.6, 1.8, 2, 2.2, 2.4, 2.7, 3, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6, 6.2, 6.8, 7.5, 8.2, 9.1
E48	2%	48	1, 1.05, 1.1, 1.15, 1.21, 1.27, 1.33, 1.4, 1.47, 1.54, 1.62, 1.69, 1.78, 1.87, 1.96, 2.05, 2.15, 2.26, 237, 2.49, 2.61, 2.74, 2.87, 3.01, 3.16, 3.32, 3.48, 3.65, 3.83, 4.02, 4.22, 4.42, 4.64, 4.87, 5.11, 5.36, 5.62, 5.9, 6.19, 6.49, 6.81, 7.15, 7.5, 7.87, 8.25, 8.66, 9.09, 9.53
E96	1%	96	E96 & E192 standards do exist, but they are rarely used.
E192	0.5% or better

When designing electronic circuits, it is rare to need precise values for ‘R’. Rather than specifying, for example, 1.375kΩ. Ask yourself if you can get away with using one of the values from a preferred range. Start by seeing if a resistor from the E3 range could be used, if not how about from the E6 range etc.

The benefit as a hobbyist is two-fold: Firstly you don’t need to keep as many resistors in stock; Secondly, values exclusive to the higher standards are both rarer to find, and more costly.

Colour Codes

You may have noticed that many of the axial, through-hole resistors colour bands on them. These bands are used to let you know some important parameters for that resistor:

• It’s value.
• The tolerance to which the resistor has been manufactured.
• If there are 6 bands (very rare), then the temperature coefficient of the resistor

You should take the time to learn the value and tolerance colour codes as you will spend a lot of your time looking for resistors of the correct value.

Colour	Value	Multiplier	Tolerance	Temperature coefficient
	0	x 100	N/A	N/A
	1	x 101	±1%	100 ppm/℃
	2	x 102	±2%	50 ppm/℃
	3	x 103	±3%	15 ppm/℃
	4	x 104	±4%	25 ppm/℃
	5	x 105	±0.5%	N/A
	6	x 106	±0.25%	10 ppm/℃
	7	x 107	±0.1%	5 ppm/℃
	8	x 108	±0.05%	N/A
	9	x 109	N/A	N/A
Gold	N/A	N/A	±5%	N/A
Silver	N/A	N/A	±10%	N/A

There are three formats for the colour banding on through hole resistors:

• 4 Band lines – 2 bands for the digits in the number, 1 band signifying what power of 10 to multiply the digits by, and 1 band denoting the tolerance, If the tolerance band is missing then the resistor has a 20% tolerance.
• 5 Band lines – same as the 4 band version, but 3 bands for the digits.
• 6 Band lines – Same as the 5 band version, with an added band denoting the temperature coefficient.

4 Colour Bands

First two bands are the digits of the value (12 in this case) and the third band is the power of 10 multiplier (10² in this case) giving a value of 1200Ω (or 1.2kΩ usually written as 1k2 on schematic diagrams). The tolerance colour band is gold indicating a 5% tolerance for the value. This means that the value could be anywhere between 1140Ω and 1260Ω.

5 Colour Bands

First three bands are the digits of the value (122 in this case) and the third band is the power of 10 multiplier (10³ in this case) giving a value of 122000Ω (or 122kΩ usually written as 122k on schematic diagrams).

The tolerance colour band is gold indicating a 5% tolerance for the value. This means that the value could be anywhere between 115900Ω and 128100Ω.

6 Colour Bands

First three bands are the digits of the value (122 in this case) and the third band is the power of 10 multiplier (10³ in this case) giving a value of 122000Ω (or 122kΩ usually written as 122k on schematic diagrams).

The tolerance colour band is gold indicating a 5% tolerance for the value. This means that the value could be anywhere between 115900Ω and 128100Ω.

The temperature coefficient band is brown indicating a coefficient of 100 ppm/℃. This means that for every 1MΩ, the value will change by 100Ω for every 1℃ change in temperature. Assuming that our resistor is actually the value marked (122kΩ), then in our case the value will change by:

\(100 * \frac{122000}{1000000} = 12.2\Omega/ ^{\circ}C\)

So for example if the resistor had a value of 122000Ω at 20℃, then at 30℃ the resistor would have a value of: 122000 + (10 * 12.2) = 122122Ω.

Surface Mount (SMD) Resistor Codes

Due to the small size of surface mount resistor devices colour codes are really practical. So a few different alpha-numeric code standards were created. The most common three being:

3 Digit Code

Very similar to the way that the resistor colour code works. The first two digits provide the number part of the value, and the third digit is the power of 10 that number must be multiplied by. So for example given the code “472” the value would be interpreted as 47 * 10² = 4700Ω or 4.7kΩ.

4 Digit Code

Works just like the 3 digit code, but the number is defined by the first three digits and the fourth digit is the power of 10 multiplier. So for example the code “1224” would indicate a value of 122 * 10⁴ = 1220000Ω or 1.22MΩ.

EIA-96 System

This code is another 3 character code, but is based on the E96 preferred values standard and consists of two digits and a letter.

The two digits give a 1 based index in to the preferred values of the E96 range and the letter on the end indicates the multiplication factor. So to find the value you have to look it up in two different tables (or use one of the many online calculators).

100	102	105	107	110	113	115	118	121	124	127	130	133	137	140	143
147	150	154	158	162	165	169	174	178	182	187	191	196	200	205	210
215	221	226	232	237	243	249	255	261	267	274	280	287	294	301	309
316	324	332	340	348	357	365	374	383	392	402	412	422	432	442	453
464	475	487	499	511	523	536	549	562	576	590	604	619	634	649	665
681	698	715	732	750	768	787	806	825	845	866	887	909	931	953	976

Code	Multiplier
Z	0.001
Y / R	0.01
X / S	0.1
A	1
B / H	10
C	100
D	1000
E	10,000
F	100,000

So given a code of “04C” we would find the 4th entry in the E96 table (107 in our case), then lookup ‘C’ in the EIA-96 multiplier table (100 in our case) and then multiply them. So in our case we get 107 * 100 = 10700Ω or 10k7Ω.

Warning

Be aware that for resistors with a tolerance other than 1% (which E96 is specifically for) there are other tables of multiplier codes. However, not all manufacturers follow these stand codes so it is important to check the manufacturer notes to find what multiplier codes they are using.

Variable Resistors

So far we have only looked at the, admittedly wide variety of, fixed value resistors. However, there is another very important type of resistor who’s value can be adjusted (either by the end user, or at build time). These are available with either a linear or logarithmic progression. Some examples are:

Speciality Resistors

One final category of resistor are the speciality resistors. These are a type of variable resistor, but unlike those looked at so far which are adjusted by a human. These are operated automatically by some other kind of external changing condition. Some examples are: