Resistors Ratings

Resistors are designed to set a specific current in the circuits of electronic circuits. The main parameter of the resistor is its resistance. Resistance of the resistor can be determined by the following formula (Ohm's law)

The main structural element of a constant resistor is a resistive layer. The resistance of this layer can be calculated using the following formula:

  where ρ is the electrical resistivity of the material,
     l is the length of the resistive layer,
     s is the cross-sectional area of ​​the resistive layer. 

In each case, resistance is required with a certain accuracy. In one case, the deviation of resistance from the nominal value of 20% is acceptable, in the other case - 10%. The highest accuracy of resistors is required when developing active RC filters. Resistors manufactured with a resistance deviation of less than 5% are called precision resistors.

When buying radio components, precision resistors usually make up a significant part of the product cost. Therefore, radio amateurs often try to select resistors for resistance from among resistors with a large resistance deviation. But can it be guaranteed that this resistance will not change with time? What if the temperature changes the resistance will remain the same? What after impacting the resistor its resistance will remain the same? That is why the series of standard values ​​of resistance values ​​were developed. The manufacturer guarantees that under any operating conditions of the equipment the resistance of the resistor will remain within the specified deviation. That is why it is better to buy resistors of a given resistance value than to constantly look for why the developed equipment stopped working!

Resistance values ​​are standardized in accordance with GOST 10318-74, and permissible deviations - in accordance with GOST 9664-74. For general purpose resistors, GOST provides six rows of nominal resistances of various manufacturing accuracy: E6 (resistance deviation from nominal value ± 20%), E12 (± 10%), E24 (± 5%), E48 (± 2%), E96 (± 1%) and E192 (± 0.5%). The number indicates the number of nominal resistance values ​​in the row. The values ​​of standard resistors are given in table 1.

Now let's look at a few examples of determining the resistance values ​​of the resistors in this table. First, consider the series of ratings of resistors E6 (± 20%), E12 (± 10%), E24 (± 5%). This method is mainly suitable for determining import resistors. On the domestic surface mount resistors, the resistance is usually not indicated. On surface-mounted resistors with the above range of resistance, the resistor rating is recorded as the first two digits from table 1, followed by a power of tens (the number of zeros after significant digits). For example, a resistor of 1 kΩ will be recorded 102 (10 × 10 ² ), a resistor of 10 kOhm will be recorded 103 (10 × 10 ³ ), a resistor of 2.2 kOhm will be recorded 222 (22 × 10 ³ )

Resistors with a resistance of less than 10 ohms use the R symbol as a decimal point. For example, a 4.7 ohm resistor will be written 4R7. The inscription 000 is used to designate a resistor with zero resistance (jumper or jumper).

For series of resistors with high precision manufacturing E48 (± 2%), E96 (± 1%), E192 (± 0.5%) significant figures will be three and the resistance will consist of four digits. For example, a 1 kΩ resistor will be recorded 1001 (100 × 10 ¹ ), a 127 ohm resistor will be written 1270 (127 × 10 ⁰ ), a 82.5 kΩ resistor will be written 8252 (825 × 10 ² ).

In resistors with a resistance of less than 100 ohms, the symbol R is used as a decimal point when writing a resistor value. For example, a 24.3 ohm resistor will be written 24R3.

Now consider the writing of resistors in the list of elements. As an example, use resistors firms Bourns [1,3] and Yageo [2].

• CR1206-FX-8252E , where the symbols CR - means a surface-mounted resistor (Chip Resistor); 1206 denotes the size of the resistor; F - sets ± 1% accuracy (if instead of F it is J, then accuracy will be ± 5%); The symbol X means that the TCR of the resistance is ± 100 ppm / ° C (if instead of X there is W, then the TCR is ± 200 ppm / ° C); 8252 - resistor rating; E - type of packaging [1]
• RC0402FR-0756RL , where RC symbols - means a surface-mounted resistor (Resistor Chip); 0402 denotes the size of the resistor; F - sets ± 1% accuracy (if instead of F it is J, then accuracy will be ± 5%); R-07 - type of packaging (7-inch tape); 56R - resistor rating; L - lead free technology (pure tin) [2]