SMT Resistors
Identifying the Values of Surface Mount Resistors
Surface Mount Technology (SMT) Resistors are available in a range of standard packages (shape and size) agreed by the Electronics Industry Alliance (EIA) through the Solid State Technology Association, formerly known as Joint Electron Device Engineering Council (JEDEC).
These packages are given identifying numbers derived from the (approximate) "footprint" size of the component measured in inches, i.e. the area the component occupies on a printed circuit board. The packages listed below are in common use for resistors and capacitors.
| Package type | Size in inches | Size in mm | Power rating |
|---|---|---|---|
| 0201 | 0.024" × 0.012" | 0.6 mm × 0.3 mm | 1/20W |
| 0402 | 0.04" × 0.02" | 1.0 mm × 0.5 mm | 1/32W 1/16W |
| 0603 | 0.063" × 0.031" | 1.6 mm × 0.8 mm | 1/16W |
| 0805 | 0.08" × 0.05" | 2.0 mm × 1.25 mm | 1/10W |
| 1206 | 0.126" × 0.063" | 3.2 mm × 1.6 mm | 1/8W |
| 1210 | 0.12" × 0.10" | 3.2 mm × 2.6 mm | 1/4W |
| 2020 | 0.20" × 0.20" | 5.08 mm × 5.08 mm | 1/2W |
| 2512 | 0.25" × 0.12" | 6.35 mm × 3.0 mm | 1W |
Because surface mount resistors are so small, there is not enough space for colour code bands. The markings used to give the value of the resistor consist of a 3 or 4 letters or numbers that you will need a magnifying glass to read.
Reading the codes is made more complicated because there are number of different codes in use.
Most common is a 3 number code that works in a similar way to the colour code bands on wire-ended resistors.
The first two numbers give the first two digits of the resistor's value whilst the third digit gives the number of zeros (or multiplying factor).
For example:
A resistor marked 332 is 3300 or 3K3 (3.3 kilohm - the K replaces the decimal point) A resistor marked 475 is 4,700,000 or 4M7 (4.7 Megohm - the M replacing the decimal point).
For resistors less than 100 Ohms, last figure will be 0 indicating NO zeros, Therefore 33ohms would be marked 330 (thirty three and no zeros) although some resistors may be marked 33R (to avoid confusion!).
A 330 ohm resistor would be marked as 331 of course (thirty three followed by one zero)
What if the value is even lower, 4.7ohms for example?
Well then we just replace the decimal point with the letter R to give us 4R7
There is also a 4-digit code in use for resistors with low tolerances of +/-1% or less that gives the 3 digits of the value and uses the fourth digit for the number of zeros (the multiplier).
Using this code a 10 ohm resistor would be marked 10R0, 100ohms is marked 1000, a 1K ohm is 1001 etc.
EIA-96 Coding Scheme
In addition to the 3 and 4 digit codes, the new EIA-96 code uses two numbers to refer to any of the 96 standard values in the look-up table below, (Table 1) and the letter is used to find the multiplier from Table 2.
Examples of the code in use are shown in Table 3.
Table 1.
2 digit number codes for the E96 range of 1% tolerance resistors
| code | ohms | code | ohms | code | ohms | code | ohms | code | ohms | code | ohms |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 01 | 100 | 17 | 147 | 33 | 215 | 49 | 316 | 65 | 464 | 81 | 681 |
| 02 | 102 | 18 | 150 | 34 | 221 | 50 | 324 | 66 | 475 | 82 | 698 |
| 03 | 105 | 19 | 154 | 35 | 226 | 51 | 332 | 67 | 487 | 83 | 715 |
| 04 | 107 | 20 | 158 | 36 | 232 | 52 | 340 | 68 | 499 | 84 | 732 |
| 05 | 110 | 21 | 162 | 37 | 237 | 53 | 348 | 69 | 511 | 85 | 750 |
| 06 | 113 | 22 | 165 | 38 | 243 | 54 | 357 | 70 | 523 | 86 | 768 |
| 07 | 115 | 23 | 169 | 39 | 249 | 55 | 365 | 71 | 536 | 87 | 787 |
| 08 | 118 | 24 | 174 | 40 | 255 | 56 | 374 | 72 | 549 | 88 | 806 |
| 09 | 121 | 25 | 178 | 41 | 261 | 57 | 383 | 73 | 562 | 89 | 825 |
| 10 | 124 | 26 | 182 | 42 | 267 | 58 | 392 | 74 | 576 | 90 | 845 |
| 11 | 127 | 27 | 187 | 43 | 274 | 59 | 402 | 75 | 590 | 91 | 866 |
| 12 | 130 | 28 | 191 | 44 | 280 | 60 | 412 | 76 | 604 | 92 | 887 |
| 13 | 133 | 29 | 196 | 45 | 287 | 61 | 422 | 77 | 619 | 93 | 909 |
| 14 | 137 | 30 | 200 | 46 | 294 | 62 | 432 | 78 | 634 | 94 | 931 |
| 15 | 140 | 31 | 205 | 47 | 301 | 63 | 442 | 79 | 649 | 95 | 953 |
| 16 | 143 | 32 | 210 | 48 | 309 | 64 | 453 | 80 | 665 | 96 | 976 |
Table 2
The number code is followed by a letter to indicate the multiplier.
| Letter | Meaning |
|---|---|
| R or Y | Multiply the value (ohms) by 0.01 |
| S or X | Multiply the value (ohms) by 0.1 |
| A | Add no zeros to value |
| B | Add 1 zero to value |
| C | Add 2 zeros to value |
| D | Add 3 zeros to value |
| E | Add 4 zeros to value |
| F | Add 5 zeros to value |
Table 3
How the EIA–96 code works.
| Code | Value |
|---|---|
| 51R or 51Y | 0.332 ohms |
| 12S or 12X | 1.30 ohms |
| 09A | 121 ohms |
| 24B | 1740 ohms = 1K74 |
| 61C | 44200 = 44K2 |
| 20D | 158000 = 158K |
| 31E | 2050000 = 2M05 |
| 74F | 57600000 = 57M6 |