Surface Mount 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 3 or 4 letters or numbers that may be easier to read with magnifying glass.

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, the last figure will be 0 indicating NO zeros. Therefore 33 ohms would be marked 330 ( i.e. thirty three and no zeros) although some resistors may be marked 33R (to avoid confusion!).

A 330 ohm resistor would be marked as 331 (thirty three followed by one zero).

What if the value is even lower, 4.7ohms for example?

Then the decimal point is replaced with by letter R to give 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, 100 ohms is marked 1000, and 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 (Table 1) below. The letter in the resistor code 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 two numbers of the code are followed by a letter to indicate the multiplier.

Letter	Meaning
S or Y	Multiply the value (ohms) by 0.01
R 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
51S = 51Y	3.32 ohms or 3R32
12R = 12X	13 ohms or 13R
09A	121 ohms = 121R
24B	1740 ohms = 1K74
63C	44200 = 44K2
20D	158000 = 158K
31E	2050000 = 2M05
74F	57600000 = 57M6