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Basic Electronics 08 – Reading value, tolerance and power rating of resistors

By Nikhil Agnihotri October 15, 2024

In the previous article, we presented a resistor selection reference table. With the help of this table, you can determine which type of resistor will be most suitable for a given circuit or application. Once you’ve determined which type of resistor suits the best, it’s time to pick up the desired resistor. For that, it is essential to identify value, tolerance, and power rating of the resistor.

The value and tolerance of carbon composition, ceramic composition, carbon film, metal film, and metal oxide film resistors are identified by the color codes. Labels identify the value and tolerance of cermet, wirewound, and foil resistors. The power rating of all types of resistors is determined by measuring or observing their physical size. Even before identifying value, tolerance, and power rating of a resistor, it is important to check out if the exact value of the desired resistance is available or not. There are standard values of resistances available for different types of resistors. If the precise value of required resistance is available, then that can be picked up without question. Otherwise, suitable parallel or series combination of resistors need to be figured out.

Standard values of resistors
Resistors come available in standard values or preferred values. These standard values are a recommendation of the International Electrotechnical Commission (IEC) and were first published in standard IEC 60063 in the year 1952. These preferred values are called E-series. These standard values are also applicable to other components like capacitors, inductors, and Zener diodes.

The standardization of resistor values was not only good for electronics engineers, but it was also equally useful for the manufacturers. It was challenging to figure out the manufacturer of resistors as resistors are quite small to denote name or trademark of its manufacturer. In the absence of preferred values, electronics engineers had to identify the manufacturer of the resistor then identify the value of the resistors with the help of datasheets provided by the particular manufacturer. The standardization of the resistor values made it possible to develop color codes and numerical codes for the value and tolerance of the resistors. So, after the adoption of E-series by almost all manufacturers around the world, electronics engineers simply need to decode color-coding or numerical codes printed on the resistors to identify their value and tolerance.

On the other hand, the preferred values or E-series helped manufacturers produce resistors with values that equally space on a logarithmic scale. This limited the different values of resistors that need to be produced and stored. The standardization also helped in maintaining compatibility between resistors produced by different manufacturers. Apart from IEC’s E-series, there are various national standards (like ANSI in the USA) which are generally compatible with the IEC standard.

E-Series
E-Series are preferred values for resistors, capacitors, inductors, and Zener diodes. There were 8 E-series – E1, E3, E6, E12, E24, E48, E96 and E192of which E1 is now obsolete. Each E-series divides the interval of 1 to 10 by the number followed after the letter ‘E’ and rounds off the nominal value to either two or three significant digits. For example, E6-series divides each decade (resistances 1Ω to 10Ω, 10Ω to 100Ω, etc.) into six values as follows:

10(1/6) = 1.5

So, each standard value in E6 is 50% higher than the previous value rounded to two significant digits in case of E-6 Series. Therefore, for 1Ω to 10Ω interval, there are six preferred values in E6 series as follow – 1, 1.5, 2.2, and so on.

By such division, the maximum error in the nominal value of the resistor for E1, E3, E6, E12, E24, E48, E96 and E192 series is divided as 50%, 40%, 20%, 10%, 5%, 2%, 1% and 0.5% respectively.  The E-192 series is also used for tolerances 0.25% and 0.1% and below. The E1 series is now obsolete, and even E3 series is rarely used anywhere as manufacturers now produce resistors with maximum 20% tolerance or less. So each E-series is associated with tolerance of the resistors as follows:

The resistors with tolerance as low as 0.005 are available. Their value is indicated under E192 – Series. The E-series is divided into two groups – one from E3 to E24 and other E48 to E192. The E-series group of E3 to E24 has two significant digits in the resistance value, while the E-series group of E48 to E-198 has three significant digits in the resistance value. The E-series – E6 to E192 are most commonly used. These have the following preferred values of resistances:

E6 Series – E6 Series is used for resistors having 20% tolerance. It has six preferred values, each of two significant digits, for each decade as follows:

10     15     22     33     47     68

E12 Series – E12 Series is used for resistors having 10% tolerance. It has twelve preferred values, each of two significant digits, for each decade as follows:

10     12    15     18     22     27
33     39     47     56     68     82

E24 Series – E24 Series is used for resistors having 5% tolerance. It has twenty-four preferred values, each of two significant digits, for each decade as follows:

10     11     12     13     15     16
18     20     22     24     27     30
33     36     39     43     47     51
56     62     68     75     82     91

E48 Series – E48 Series is used for resistors having 2% tolerance. It has 48 preferred values, each of three significant digits, for each decade as follow –

100   105   110   115   121   127
133   140   147   154   162   169
178   187   196   205   215   226
237   249   261   274   287   301
316   332   348   365   383   402
422   442   464   487   511   536
562   590   619   649   681   715
750   787   825   866   909   953

E96 Series – E96 Series is used for resistors having 1% tolerance. It has ninety six preferred values, each of three significant digits, for each decade as follow –

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

E192 Series – E192 Series is used for resistors having 0.5% tolerance. It has one hundred and ninety two preferred values, each of three significant digits, for each decade as follow –

100   101   102   104   105   106
107   109   110   111   113   114
115   117   118   120   121   123
124   126   127   129   130   132
133   135   137   138   140   142
143   145   147   149   150   152
154   156   158   160   162   164
165   167   169   172   174   176
178   180   182   184   187   189
191   193   196   198   200   203
205   208   210   213   215   218
221   223   226   229   232   234
237   240   243   246   249   252
255   258   261   264   267   271
274   277   280   284   287   291
294   298   301   305   309   312
316   320   324   328   332   336
340   344   348   352   357   361
365   370   374   379   383   388
392   397   402   407   412   417
422   427   432   437   442   448
453   459   464   470   475   481
487   493   499   505   511   517
523   530   536   542   549   556
562   569   576   583   590   597
604   612   619   626   634   642
649   657   665   673   681   690
698   706   715   723   732   741
750   759   768   777   787   796
806   816   825   835   845   856
866   876   887   898   909   920
931   942   953   965   976   988

Determining if the exact value of desired resistance is available
The standard resistances come available in multiples of 10 with the values as mentioned above. Like, under E6-series, the following resistances come available –

1.0    1.5    2.2    3.3    4.7    6.8 (Multiplier 100)
10     15     22     33     47     68 (Multiplier 101)
100   150   220   330   470   680 (Multiplier 102) and So on.

So, now you can quickly determine if the desired value of resistance is exactly available by matching its two or three significant digits with the preferred values of E-series.

Series and parallel combination of resistances
Often, the exact value of the desired resistance is not available. In such case, two or more resistors can be connected in series, parallel, or combination of series and parallel resistances to get an equivalent resistance. In series combination, the equivalent resistance is simply the sum of the connected resistances as follows:

Req = R1 + R2 +….
In parallel combination, the equivalent resistance is given by shunt formulae as follows:
1/Req = 1/R1 + 1/R2 +….

In a series combination of resistances, the voltage drop is divided among the resistors. Therefore, care must be taken when connecting an equivalent resistance by a series of combination of resistors, as there may be an unintentional voltage division network added to the circuit by false connections to the junctions of the connected resistors. Similarly, it is important to take care of a parallel combination of resistors branch current.

Resistor packages
The resistors generally come in three types of termination – Plated Through Hole (PTH), Surface Mount Technology/Device (SMT/SMD) and Metal Electrode Leadless Face (MELF). The PTH is for use on breadboards, prototyping and through-hole mounting on PCBs. The SMT/SMD termination types are meant for soldering over landing pads on PCBs. The MELF style resistors are like SMD/SMT resistors, but they have a cylindrical shape and no leads. They are also meant to be soldered on landing pads on PCBs. MELF resistors have the advantage of lower thermal coefficient and better stability over SMD/SMT resistors while they may be difficult to handle by a mechanized assembly machine. In all the termination types, resistors come in several shapes and sizes. These shapes and sizes are called packages. The most common is the axial package or radial package with PTH termination. The MELF resistors mainly come in three packages – MicroMELF, MiniMELF, and MELF. Similarly, SMD/SMT resistors come in several packages that are standardized through four-digit imperial or metric codes by organizations like JEDEC.

Color codes for resistors
The standard 2-digit and 3-digit preferred values (E-Series) for resistors make it possible to develop color codes and numerical codes for the resistors. The resistors with PTH or MELF termination have their value and tolerance indicated by color codes. The carbon composition, ceramic composition, carbon film, metal film and metal oxide film resistors are generally available in PTH or MELF termination as axial or radial packages. There are 3, 4, 5, and 6-band color codes for the resistors. Reading these color codes has been explained in the following article .3, 4, 5, and 6-band Resistor Color Codes.

Numerical codes for resistors
The resistors with SMD/SMT termination have their value and tolerance indicated by numerical codes. The SMD/SMT resistors are so small that it is not possible to use color codes on them. So, there are two numerical code systems – three-digit and four-digit numerical code system and EIA-96 system to indicate their value and tolerance.

Three and Four-Digit Numerical Code System
In this system, the value of the resistor is indicated by a three or four-digit number. In the three-digit number, the first two digits indicate the significant digits of the value of the resistor, and the third digit indicates the multiplier. In the four-digit number, the first three digits indicate the significant digits of the value of the resistor, and the fourth digit indicates the multiplier. The values of resistances involving decimal point are indicated by inserting the letter ‘R’ at the decimal place in three or four-digit code. For example, a resistor having value 0.01Ω will have 0R01 code printed on it. The following table shows some of the examples of three and four-digit codes.

E96 Code System – E96 system is developed to indicate the value of E-96 Series SMD/SMT resistors having 1% tolerance. In E96 code system, the value of the resistor is indicated by three characters of which first two are numbers, and third is a letter. The numbers indicate three significant digits of the resistance value that can be verified through the following look-up table:

The third character, which is a letter indicates multiplier as per the following look-up table:

Power rating of PTH resistors
The power rating of PTH type resistors can be determined by measuring or observing their physical size. PTH resistors are generally available in an axial or radial package. Their power rating can be determined by measuring body length, body diameter, lead length, or lead diameter. Obviously, measuring body length will be most convenient and accurate because measuring the diameter of the body or lead of a resistor could be a tough task and the lead length remains the same for several power ratings. Also, leads could be cut short due to different reasons. The following lookup table relates a power rating of PTH resistors with their physical dimensions:

Power rating of MELF resistors
MELF resistors too come in axial or radial packages. The following lookup table relates the power rating of MELF resistors with their physical dimensions:

Power rating of SMD/SMT resistors
The power rating of the SMD/SMT resistors can be determined by either measuring their physical dimensions or dimensions of their landing pad on a PCB. The following lookup table relates the power rating of SMD/SMT resistors with their physical dimensions and size of solder landing pad:

Note that some manufacturers particularly of wirewound and foil resistors use their own codes and power rating tables or simply print part numbers. The wirewound resistors usually have wattage printed on them along with their nominal value and tolerance printed or have the part number printed on them. Similarly, manufacturers of foil resistors may have their own coding system according to their national standards. So, in case, there is no color code, numerical code or E96 code printed on a resistor, refer to the datasheet of the particular manufacturer to determine value, tolerance and power rating of the resistor.

Now, you can determine nominal value, tolerance and power rating of any resistor. In the next article, we will discuss variable resistors.


Filed Under: Tutorials
Tagged With: Capacitors, Electronic Part, Electronic Parts, electronics, electronics projects, Resistors
 

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