Surface Mount Resistor: codes & markings

Surface mount resistors often have small codes on them to indicate their value - there are several different coding schemes that may be seen.

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Although not all SMD resistors, or SMT resistors are marked with their values, some are, and in view of the lack of space the SMD resistor code systems may not always provide an obvious indication of the resistor value.

The surface mount resistor code systems provide are mainly used to enable service, repair and fault-finding. During manufacture the resistors are held either in tapes that are reeled, or in hoppers used for the surface mount machines. The SMD resistor markings can be used as a check to ensure the correct values are being fitted, but normally the reels or hoppers will be suitable marked and coded.

SMD resistors on a printed circuit board
SMD Resistors on a PCB along with other components
SMD resistors are the small components with figures on a dark background

SMD resistor coding schemes

Many SMD resistors do not have any markings on them to indicate their value. For these devices, once they are loose and out of their packaging it is very difficult to tell their value. Accordingly SMD resistors are typically used within reels or other packages where there is no chance of different values being mixed.

Many resistors do have markings on them. There are three systems that are used:

  • Three figure SMD resistor coding system
  • Four figure SMD resistor coding system
  • EIA96 SMD resistor coding system

3 figure SMD resistor code system

A three figure SMT resistor coding system is the one that is normally used for standard tolerance resistors.

As the name indicates this SMD resistor marking system uses three figures. The first two figures in the code indicate the significant figures, and the third is a multiplier. This is the same as the coloured rings used for wired resistors, except that actual numbers are used instead of colours.

Therefore an SMD resistor with the figures 472 would have a resistance of 47 x 102 ohms, or 4.7kΩ. However beware of resistors marked with figures such as 100. This is not 100 ohms, but it follows the scheme exactly and it is 10 x 100 or 10 x 1 = 10 Ω.

SMD surface mount resistors resistor marking code with three figures
Three figure SMD resistor marking code

Where resistance values less than ten ohms are used, the letter "R" is used to indicate the position of the decimal point. As an example, a resistor with the value 4R7 would be 4.7Ω.

4 figure SMD resistor code system

The four digit or four figure SMT resistor marking scheme is used for marking high tolerance SMD resistors. Its format is very similar to the three figure SMT resistor making scheme, but expanded to give the higher number of significant figures needed for higher tolerance resistors.

In this coding scheme, the first three numbers will indicate the significant digits, and the fourth is the multiplier.

Therefore an SMD resistor with the figures 4702 would have a resistance of 470 x 102 ohms, or 47kΩ.

SMD surface mount resistors resistor marking code with four figures
Four figure SMD resistor marking code

Resistors with values of less than 100 ohms are marked utilise the letter 'R', as before, to indicate the position of the decimal point.

EIA96 SMD resistor code system

A further surface mount resistor code scheme or SMD resistor coding scheme has started to be used, and it is aimed at 1% tolerance SMD resistors, i.e. those using the EIA96 or E-96 resistor series. As higher tolerance resistors are used, further figures are needed. However the small size of SMT resistors makes the figures difficult to read. Accordingly the new system seeks to address this. Using only three figures, the actual characters can be made larger than those of the four figure system that would otherwise be needed.

The EIA SMD resistor coding scheme uses a three character code: the first 2 numbers indicate the 3 significant digits of the resistor value. The third character is a letter which indicates the multiplier. In this way this SMD resistor marking scheme will not be confused with the 3 figure markings scheme as the letters will differentiate it, although the letter R can be used in both systems.

To generate the system the E-96 resistor series has been taken and each value or significant figure set has been numbered sequentially. As there are only 96 values in the E-96 series, only two figures are needed to number each value, and as a result this is a smart way of reducing the number of characters required.

The details for the EIA SMD resistor code scheme are tabulated below:

EIA SMD Resistor Code Scheme
Code Multiplier
Z 0.001
Y or R 0.01
X or S 0.1
A 1
B or H 10
C 100
D 1 000
E 10 000
F 100 000

EIA SMD Resistor Code Scheme Multipliers
Code Sig Figs   Code Sig Figs   Code Sig Figs   Code Sig Figs
01 100   25 178   49 316   73 562
02 102   26 182   50 324   74 576
03 105   27 187   51 332   75 590
04 107   28 191   52 340   76 604
05 110   29 196   53 348   77 619
06 113   30 200   54 357   78 634
07 115   31 205   55 365   79 649
08 118   32 210   56 374   80 665
09 121   33 215   57 383   81 681
10 124   34 221   58 392   82 698
11 127   35 226   59 402   83 715
12 130   36 232   60 412   84 732
13 133   37 237   61 422   85 750
14 137   38 243   62 432   86 768
15 140   39 249   63 442   87 787
16 143   40 255   64 453   88 806
17 147   41 261   65 464   89 825
18 150   42 267   66 475   90 845
19 154   43 274   67 487   91 866
20 158   44 280   68 499   92 887
21 162   45 287   69 511   93 909
22 165   46 294   70 523   94 931
23 169   47 301   71 536   95 953
24 174   48 309   72 549   96 976

For example a resistor that is marked 68X can be split into two elements. 68 refers to the significant figures 499, and X refers to a multiplier of 0.1. Therefore the value indicated is 499 x 0.1 = 49.9Ω.

Sometimes it may appear that it might be difficult to distinguish between the different SMD resistor marking codes. Fortunately they are sufficiently different that they can be distinguished from one another and there is no change of confusion.

When the SMD resistors are marked with their value it certainly helps with fault finding - referring to a parts list can be more difficult as one may not be to hand. Accordingly knowing how to read the different SMD resistor marking codes can be very useful.

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