The emitter type resistors (SMD), like other components, require marking. You can use it to get information about the rating of the resistor and its accuracy. But in the case of SMD components, the dimensions become a problem. It is not possible to apply a complete alphanumeric designation in a limited space. Marking in the form of color bars is also not a solution - there is not enough space to place the required number of labels. Determining the first place (where to start reading) will also be a problem: a thicker line or displacement of the marking to one of the sides will also require additional space. That's why a special designation system is adopted for leadless elements.
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What is the marking of SMD resistors
Resistors for surface mounting are marked by placing three or four digits on top of the housing. These symbols are only enough to indicate the nominal resistance and, in certain cases, the accuracy.
There is no space on the surface of the element to indicate the power, so this characteristic can only be determined visually, by the dimensions of the resistor. However, in most cases this also applies to lead elements, especially color-coded ones.
Three-digit numbering of resistors with tolerances of 2%, 5% and 10%
If there are three symbols on the body of the device, it means that the resistor has an accuracy of 2% to 10%. There are two options for three-digit marking of electronic components - fully numeric and numeric-letter designation.
Three digits
In most cases, the marking consists of three digits XYZ. They represent resistance in the form XY⋅10Z. An example of such a notation is 332. The first two digits represent 33 Ohms, and the third is the power to which you must add 10 and then multiply by 33. Simply put, it means the number of zeros to be added to the right of the first two numbers. In this case, the marking means 3300 ohms = 3.3 kOhms. If the third digit is a zero, you don't have to assign anything either (10=1). So, a marking of 100 means 10 ohms (10×1). There are no decimal multipliers less than one (0.1 or 0.01) in this system.
Two digits and letter R
If the letter R is used in the marking, it means that its resistance is less than 10 ohms, and the value is not equal to a whole number of ohms. The letter symbol indicates the position of the decimal point. The general type of marking may be 3R3=3.3 ohms or 0R5=0.5 ohms.
Four-digit numbering of resistors
Three symbols are not always sufficient for marking electronic components. In some cases, additional symbols have to be applied. For devices with an accuracy of 1% or higher, a mantissa of two digits is not enough. They are designated by a numerical code as WXYZ and the resistance value is WXY⋅10Z. Here Z also means how many zeros should be added to the right side. For example, to mark 7992 you must add two zeros to the number 799. The result is 79900 ohms=79.9 kOhms.
For values less than 1 ohm.
If the rating of a one-percentage resistor is 1 ohm or less, three characters are also insufficient to mark its resistance. Therefore, a four-digit designation is used. Zero is not marked to save space, a decimal point symbol comes first, followed by three digits representing resistance. If R100 is marked on the case, it means that it is a one percent resistor with a rating of 0.1 ohm.
EIA-96 SMD resistor marking
Four-digit notation of parameters resistors is not the optimal method. Still, there is not enough space for four characters on the small-sized cases. Therefore, devices with 1% accuracy for form factors below 0805 use another marking system consisting of two digits and a letter symbol. This designation has been established by the EIA-96, according to which the two digits represent the ohms rating and the letter represents the multiplier.
Table of resistor codes and values
The EIA-96 standard has no direct correspondence between the digits of the marking and the rating. The actual resistance value is compared to a code. To determine the resistance value, refer to the table:
Table 1: EIA-96 table of resistor marking codes and values.
So, code 20 corresponds to a value of 158 ohms, and code 69 corresponds to a value of 511 ohms. Obviously, it is very difficult to remember the correspondence between the code and the value. Therefore, it is recommended to use a table or an online calculator.
Multiplier table
The table of multipliers is smaller, but also non-obvious and difficult to remember:
Table 2: Table of letter multiplier values in EIA-96 resistor markings.
| Code | Multiplier |
|---|---|
| Z | 0.001 |
| Y or R | 0.01 |
| X or S | 0.1 |
| A | 1 |
| B or H | 10 |
| C | 100 |
| D | 1000 |
| E | 10000 |
| F | 100000 |
This means that the full rating of the resistor labeled 22A is 165×1=165 ohms, and 44B is 280×10=2800 ohms = 2.8 kOhms.
Examples of deciphering of numerical-letter marking of SMD resistors
It is not necessary to memorize tables of values to determine the parameter of resistors. There are many online calculators on the Internet, and many offline programs are also available for download. But if you understand the principles of marking, it is possible to determine resistance values and accuracy without resorting to reference books, after a little training it is possible at a glance. To consolidate the understanding of the basics we need to break down some practical examples.
Resistors 101, 102, 103, 104
In all of these examples, the numerical value of resistance is the same and is 10, but the multipliers in each case are different:
- 101 - 10 ohms must be multiplied by 101, that is, by 10, or add one 0 to the value - the total will be 100 ohms;
- 102 - 10 ohms must be multiplied by 102102 - 10 ohms must be multiplied by 10, i.e. by 100, or by adding two zeros to the value, resulting in 1000 ohms (=1 kOhm);
- 103 - 10 ohms should be multiplied by 103103 - 10 Ohms must be multiplied by 10, i.e. by 1000, or by three zeros to obtain 10000 Ohms (=10 kOhms);
- 104 - 10 ohms should be multiplied by 104or add four zeros to the value, you get 100000 ohms (=100 kOhms).
It is easy to remember that for three-character coding, the last digit of 3 stands for kilohms and 6 for megaohms - this will make the marking even easier to read visually.
Resistors 1001, 1002, 2001
If an electronic component is marked with 4 digits, it means that its accuracy is at least 1%. And the rating also consists of the mantissa and the multiplier, which is given by the last character:
- 1001 - 100 ohms must be multiplied by 101that is, by 10, which is equivalent to adding one zero to the mantissa - the result will be 1000 ohms (1 kOhm);
- 1002 - the mantissa is also 100 ohms, but the multiplier is 102=100 (it is necessary to add two zeros), and the nominal will be 10000 Ohm = 10 kOhm;
- 2001 - in this case, 200 ohms must be multiplied by 101=10, the rating is 2000 ohms=2 kOhms.
In principle, reading this marking is not different from the three-character marking.
Resistors r100, r020, r00, 2r2
If a resistor is labeled with the letter R, you can immediately mentally replace it with a decimal point:
- R100 means ".100" - by adding a zero before the decimal point, you get the value 0.100 Ohm = 0.1 Ohm (resistor with 1% accuracy).
- R020 - by the same principle ".020" turns into 0.020 Ohm=0.02 Ohm;
- R00 means a resistor with zero resistance - such elements are used as jumpers on the board (it is often more technologically advanced in production);
- 2R2 - three symbols mean accuracy of 2% and below, the nominal value is 2.2 Ohms.
If the resistance value of a 2%, 5% or 10% element is less than 1 ohm, a zero is placed in front of the letter R (for example, 0R5 would mean 0.5 ohms).
Resistors 01b, 01c
To determine the rating, refer to mantissa and multiplier tables:
- 01B - code 01 denotes a resistor with a "base" resistance of 100 ohms, multiplier B=10, the final resistance of 100x10=1000 ohms=1kOhm;
- 01C - this variant differs from the previous one only by the multiplier (C is equivalent to 100), and the full rating is 100x100=10000 Ohm = 10 kOhm.
The above examples show that the same resistor rating can be labeled differently depending on its design. For example, a 1 kOhm resistor can be coded:
- 102 - for 2-10% series;
- 1001 - for 1% of the series;
- 01B - for small-sized resistors of 1% series.
This coding system is used on 90+ percent of lead-free devices manufactured around the world. But there is no guarantee that any manufacturer does not use their own labeling system. Therefore, when in doubt, the most reliable way is to is to measure the real resistance value with a multimeter .. After a little practice this will not be difficult. The same method is the only one for the smallest SMD elements - they are not marked at all.
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