 # Resistance secrets - Resistance parameters you must know

Resistance is an ordinary component, but there is an unusual way, in doing electronic design for nearly ten years today, I this small electrician just realized some way, here to share with you.

There are many parameters of resistance, usually we generally pay attention to value, accuracy, amount of power, these three indicators are appropriate. Of course, in digital circuits, we don't need to pay much attention to the details. After all, there are only ones and zeros in the number, so we don't care much about the effects. But in analog circuits, small changes in resistance can make a big difference when we use a precise voltage source, or when converting a signal to an a/D, or when amplifying a weak signal. When haggling with the resistance, of course, is in the processing of analog signal occasions, then according to the analog circuit application analysis of the influence of the parameters of the resistance.

Resistance value - Resistance value selection is often fixed by applications, such as limiting the current of an LED lamp, or sampling a current signal, resistance value is basically no other choice. However, in some cases, there are various choices of resistors. For example, to amplify a voltage signal, as shown in the figure, the amplification factor is related to the ratio of R2 and R3, and has nothing to do with the value of R2 and R3. At this time, there are grounds for choosing the resistance value: the higher the resistance value, the greater the thermal noise, the worse the performance of the amplifier; The smaller the resistance value, the greater the current, the greater the current noise, the worse the performance of the amplifier; This is the reason why the resistance of many amplifier circuits is tens of K, there is a need to use a large resistance value, or use the voltage follower, or use T network to avoid. In-phase amplifier

Precision of resistance -- Precision of resistance is easy to understand. The accuracy of resistance is generally 1% and 5%, and the precision is 0.1%, etc. 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,precision code = 0.05%, B = 0.1%, C = 0.25%, D = 0.5%, 1%, G F = = 2%, J = 5%, K = 10%, M = 20%.
The amount of power of resistors - The power of resistors is very simple, but it is easy to use incorrectly. For example, the chip resistance of 2512, the amount of power is 1W, according to the specification of the resistance, when the temperature exceeds 70 degrees Celsius, the resistance will be used in derating. How much power can 2512 SMT resistance use?

At room temperature, if the PCB solder plate has no special heat treatment, when the power of 2512 SMT resistance reaches 0.3W, the temperature may exceed 100 or even 120 degrees Celsius. At 125 degrees Celsius, according to the temperature derating curve, the 2512 power limit needs to be derated to 30%. This situation in any package resistance need to pay attention to, do not believe in the nominal power, the key position is best to double confirm to avoid leaving hidden dangers.Resistance of the voltage value - resistance of the voltage value is generally less mentioned, especially novice, often have no concept, that the capacitor has a voltage value. The voltage that can be applied at both ends of the resistance, one is determined by the amount of power, to ensure that the power does not exceed the amount of power, and the other is the resistance value. Although the power of the resistance body does not exceed the power limit, too high voltage will lead to instability of the resistance, creepage between the resistance pins and other faults. Therefore, choose a reasonable resistance according to the voltage used. Partial package voltage values include: 0603=50V, 0805=100V, 1206 to 2512=200V, 1/4W plug-in =250V. Moreover, in the time application, the voltage on the resistance should be more than 20% smaller than the amount of voltage, or it will be easy to go wrong for a long time. Resistance temperature coefficient - Resistance temperature coefficient is a parameter that describes how resistance changes with temperature. This is mainly determined by the material of the resistance. Generally, thick diaphragm resistance 0603 or more packages can achieve 100ppm/℃, which means that the resistance value may change by 0.25% when the ambient temperature of the resistance changes by 25 degrees Celsius. For a 12bit ADC, a 0.25 percent change is 10 LSB. So like AD620 op-amp, only by a resistor adjustment magnification, many old engineer not to be greedy for the convenience of the use and they will use a regular circuit, through the two in proportion to the resistance to adjust magnification, when resistance is the same type of resistance, temperature resistance caused by the change will not bring the change of the proportion, circuit is more stable. In more demanding precision meters, metal film resistors will be used, their temperature drift to 10 to 20ppm is easy, of course, also expensive point. In a word, in the precision application of instruments, the temperature coefficient is absolutely a very important parameter. If the resistance is not accurate, the parameters can be adjusted on time at school, and the resistance can not be controlled with the change of the external temperature.Structure of resistors -- there are many structures of resistors, and here are some applications that can come to mind. The starting resistance of the machine is generally used to pre-charge large capacity aluminum electrolysis with resistance, and then close the relay to switch on the power supply after it is filled with aluminum electrolysis. This resistance needs impact resistance, it is best to use a large winding resistance, the amount of power resistance is not very important, but the instantaneous power is very high, ordinary resistance is difficult to meet the requirements. For high voltage applications, such as capacitor discharge resistors with actual operating voltages over 500V, it is best to use high voltage glass glaze resistors rather than ordinary cement resistors. Peak absorption applications, such as SCR modules at both ends need parallel RC absorption, DV/DT protection, it is best to achieve non-inductive winding resistance, so as to have good peak absorption performance and not easy to be damaged by impact.