There are many parameters of resistors. Usually we usually pay attention to value, accuracy, and rated power. These three indicators are suitable. Admittedly, in digital circuits, we don't need to pay attention to too many details. After all, there are only 1 and 0 numbers, and we don't care much about the tiny impact. But in analog circuits, when we use a precise voltage source, or convert the signal to analog-to-digital, or amplify a weak signal, a small change in resistance will have a great impact. When we are fussing with resistors, of course, it is when processing analog signals. Later, we will analyze the influence of various resistor parameters based on the application of analog circuits.

Rated resistance of resistors - The choice of rated resistance of resistors is often fixed by the application. For example, when limiting the current of an LED lamp or sampling a current signal, there is basically no other choice for the resistance value of the resistor. However, in some occasions, there are many choices for resistors, such as amplifying a voltage signal. As shown in the figure, the amplification factor is related to the ratio of R2 to R3, and has nothing to do with the values of R2 and R3. At this time, there is still a basis for choosing the resistance value of the resistor: the larger the resistance value, the greater the thermal noise, and the worse the performance of the amplifier; the smaller the resistance value, the greater the working current, the greater the current noise, and the worse the performance of the amplifier; this is the reason why the resistance of many amplifier circuits is tens of K. There are places where large resistance values are needed, or voltage followers, or T-type networks are used to avoid it.

Resistor accuracy - The accuracy of resistors is easy to understand. The accuracy of resistors is generally 1% and 5%, and the precision is 0.1%. The price of 0.1% is about ten times that of 1%, and the price of 1% is about 1.3 times that of 5%. Generally, the accuracy code is A=0.05%, B=0.1%, C=0.25%, D=0.5%, F=1%, G=2%, J=5%, K=10%, M=20%.

The rated power of resistors - the power of resistors is originally very simple, but it is often easy to use inappropriately. For example, the rated power of a 2512 chip resistor is 1W. According to the resistor specification, when the temperature exceeds 70 degrees Celsius, the resistor must be derated. How much power can a 2512 chip resistor use? At room temperature, if the PCB pad does not have special heat dissipation treatment, when the power of the 2512 chip resistor reaches 0.3W, the temperature may exceed 100 or even 120 degrees Celsius. At a temperature of 125 degrees Celsius, according to the temperature derating curve, the rated power of 2512 needs to be derated to 30%. This situation needs to be paid attention to in any packaged resistor. Don't be superstitious about the nominal power. It is best to confirm the key position again and again to avoid hidden dangers.

The withstand voltage value of the resistor - the withstand voltage value of the resistor is generally rarely mentioned, especially for novices, who often have no concept and think that only capacitors have withstand voltage values. The voltage that can be applied to both ends of the resistor is determined by the rated power. To ensure that the power does not exceed the rated power, the other is the withstand voltage value of the resistor. Although the power of the resistor body does not exceed the rated power, excessive voltage will cause resistor instability, creepage between resistor pins and other faults. When using it, you need to choose a reasonable resistor according to the voltage used. The withstand voltage values of some packages include: 0603=50V, 0805=100V, 1206 to 2512=200V, 1/4W plug-in=250V. Moreover, in time applications, the voltage on the resistor should be more than 20% less than the rated withstand voltage value, otherwise it will easily cause problems over time.

Temperature coefficient of resistor - The temperature coefficient of resistor is a parameter that describes the change of resistor with temperature. This is mainly determined by the material of the resistor. Generally, thick film chip resistors with packages above 0603 can achieve 100ppm/℃, which means that when the ambient temperature of the resistor changes by 25 degrees Celsius, the resistance value may change by 0.25%. If it is a 12-bit ADC, a change of 0.25% is 10 LSB. Therefore, many experienced engineers will not use op amps like AD620, which only rely on one resistor to adjust the amplification factor, for convenience. They will use conventional circuits to adjust the amplification factor through the ratio of two resistors. When the resistors are of the same type, the change in resistance caused by temperature will not bring about a change in ratio, and the circuit will be more stable. In instruments with higher requirements for precision, metal film resistors will be used. It is easy for them to achieve a temperature drift of 10 to 20ppm, but of course they are more expensive. In short, in precision applications of instruments, the temperature coefficient is definitely a very important parameter. If the resistor is not accurate, the parameter can be adjusted during calibration, but the change of resistance with the external temperature cannot be controlled.

Resistor structure - There are many resistor structures, and here are the applications that can be thought of. The starting resistor of the machine is generally used to pre-charge the large-capacity aluminum electrolytic capacitor with a resistor. After the aluminum electrolytic capacitor is fully charged, the relay is closed to connect the power supply. This resistor needs to be impact-resistant, and it is best to use a large winding resistor. The rated power of the resistor is not very important, but the instantaneous power is very high, and ordinary resistors are difficult to meet the requirements. For high voltage applications, such as capacitor discharge resistors, the actual working voltage exceeds 500V. It is best to use high voltage glass glaze resistors instead of ordinary cement resistors. For spike absorption applications, such as the need to connect RC in parallel at both ends of the thyristor module for absorption and dv/dt protection, it is best to use non-inductive winding resistors, so that they can have good absorption performance for spikes and are not easily damaged by impact.