The resistor is an ordinary component, but it has uncommon ways. There are many parameters of resistance. Usually, we only need to pay attention to value, accuracy, and rated power. These three indicators are suitable.
It is true that in digital circuits, we don’t need to pay too much attention to the details, after all, in the numbers with only 1s and 0s, we don’t care about the insignificant effects.
But in analog circuits, when we use a precise voltage source, or perform analog-to-digital conversion on a signal, or amplify a weak signal, a small change in resistance will have a big impact.
When comparing with resistors, of course, it is in the case of processing analog signals. Later, the influence of various parameters of resistors will be analyzed according to the application of analog circuits.
01 The rated resistance of the resistor
The selection of the resistance value of the resistor is often fixed by the application, such as limiting the current of an LED lamp, or sampling a certain 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 Figure 1, the magnification is related to the ratio of R2 and R3, and has nothing to do with the values of R2 and R3.
At this time, the resistance value of the resistor is still based on:
The larger the resistor value, the greater the thermal noise and the worse the amplifier’s performance;
The smaller the resistance value of the resistor, the greater the current during operation, the greater the current noise, and the worse the performance of the amplifier;
This is the reason why the resistance of many amplifying circuits is tens of K. Where a large resistance value is required, either a voltage follower or a T-type network is used to avoid it.
02 The accuracy of the resistance
The accuracy of the resistor is well understood. I won’t go into it here. The accuracy of the resistor is generally 1% and 5%, and the precision is 0.1%. 0.1% is about ten times the price of 1%, and 1% is about 1.3 times the price of 5%.
Generally, the precision codes are A=0.05%, B=0.1%, C=0.25%, D=0.5%, F=1%, G=2%, J=5%, K=10%, M=20%.
03 The rated power of the resistor
The power of the resistor is very simple, but it is often used inappropriately. For example, the 2512 chip resistor has a rated power of 1W. According to the resistor’s specification, when the temperature exceeds 70 degrees Celsius, the resistor must be derated.
How much power can the 2512 chip resistors be used for?
At room temperature, if the PCB pads have no 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 the 2512 needs to be derated to 30%.
In this case, you need to pay attention to the resistance of any package. Don’t be superstitious about the nominal power. It is best to confirm the key position again and again to avoid hidden dangers.
04 Transistor’s withstand voltage
The withstand voltage value of resistors is generally less mentioned, especially for novices, who often have no concept, thinking that capacitors have withstand voltage values.
The voltage that can be applied across the resistor, one is determined by the rated power, to ensure that the power cannot exceed the rated power, and the other is the withstand voltage value of the resistor.
Although the power of the resistor body does not exceed the rated power, too high voltage will lead to failures such as unstable resistance and creepage between the resistance pins. In use, a reasonable resistance should be selected 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 the application of time, the voltage on the resistor should be more than 20% lower than the rated withstand voltage value, otherwise it will be prone to problems after a long time.
05 Temperature coefficient of resistance
The temperature coefficient of resistance is a parameter that describes the change of resistance with temperature. This is mainly determined by the material of the resistance. Generally, the package of thick film chip resistance above 0603 can achieve 100ppm/℃.
It 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 12bit ADC, the 0.25% change is 10 LSBs.
Therefore, op amps like AD620 only rely on one resistor to adjust the magnification. Many old engineers will not use it for convenience. They will use conventional circuits to adjust the magnification through the ratio of the two resistors. When the resistors are of the same type When the resistance value changes caused by temperature will not bring about proportional changes, the circuit is more stable.
For more precise instruments, metal film resistors are used. It is easy for them to have a temperature drift of 10 to 20 ppm, but of course they are expensive.
In short, in the precision application of instruments, the temperature coefficient is definitely a very important parameter. If the resistance is not accurate, the parameters can be adjusted during calibration, and the change of the resistance with the external temperature cannot be controlled.
06 The structure of the resistor
There are many structures of resistors. Here is the application that can be remembered. The starting resistance of the machine is generally used to pre-charge the large-capacity aluminum electrolysis. After the aluminum electrolysis is fully charged, the relay is closed and the power is turned on to work.
This kind of resistance needs impact resistance, it is best to use a large wire wound resistance, the rated power of the resistance is not very important, but the instantaneous power is very high, and the ordinary resistance is difficult to meet the requirements.
For high voltage applications, such as capacitor discharge resistors, where the actual working voltage exceeds 500V, it is better to use high voltage glass glaze resistors instead of ordinary cement resistors.
For the application of peak absorption, for example, both ends of the thyristor module need parallel RC for absorption and dv/dt protection.
