# What is a resistor and how to calculate its value?

1. Resistance introduction.

The effect of a substance on an electric current is called the resistance of the substance. Materials with low resistance are called electrical conductors, or conductors. A substance with a large resistance is called an electrical insulator, or insulator for short.

The resistor is represented by the letter R. In the circuit diagram, resistor is represented by the symbols shown in fig. 1. The basic unit of resistance is ohmic (ohm), the symbol is Ω. the larger unit is (kΩ), (MΩ) and so on. 1TΩ=1000GΩ, 1GΩ=1000MΩ, 1MΩ=1000kΩ, 1kΩ=1000Ω

（a）*Common resistor* （b）Resistor array

Fig.1 Resistance symbol

2. Resistance value calculation.

The *resistor commonly used in motherboard* is three-digit patch resistance. Some motherboards also use precision resistors. The common resistance values are calculated as shown in Fig. 2, the first two digits are valid and the third digit is zero. If there is a letter in the middle, the letter indicates the decimal point. As shown in figure 3.

272=27×10²=2700Ω=2.7kΩ 2R7=2.7Ω,R denotes decimal poin

Fig.2 Common resistor value calculation Fig.3 Calculation of resistance valueof intermediate band letter

The precision resistance calculation is shown in figure 4. Precision resistance values are composed of numbers and letters, numbers are codes, corresponding to a number in Table 1, letters represent the multiplier.

Fig.4 Precision resistance calculation

Table 1 Precision resistance reference table

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