Six points to note when selecting resistors

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Six points to note when selecting resistors [Copy link]

Selection of fixed resistors There are many types of fixed resistors. The choice of which material and structure of resistors should be determined according to the specific requirements of the application circuit. High-frequency circuits should use non-wirewound resistors with small distributed inductance and distributed capacitance, such as carbon film resistors, metal resistors, and metal oxide film resistors. High-gain small signal amplifier circuits should use low-noise resistors, such as metal film resistors, carbon film resistors, and wirewound resistors, instead of synthetic carbon film resistors and organic solid resistors with large noise. Wirewound resistors have high power, low current noise, and high temperature resistance, but they are large in size. Ordinary wirewound resistors are often used in low-frequency circuits or as current limiting resistors, voltage dividing resistors, bleeder resistors, or bias resistors for high-power tubes. Wirewound resistors with higher precision are mostly used in fixed attenuators, resistor boxes, computers, and various precision electronic instruments. The resistance value of the selected resistor should be close to a nominal value calculated in the application circuit, and standard series resistors should be preferred. The allowable error of resistors used in general circuits is ±5%~±10%. Resistors used in precision instruments and special circuits should be precision resistors. The rated power of the selected resistor must meet the requirements of the resistor power capacity in the application circuit. Generally, the power of the resistor should not be increased or decreased at will. If the circuit requires a power resistor, its rated power can be 1~2 times higher than the power required by the actual application circuit. Usually, it is selected in 6 specific situations: 1. Rated power: Under the specified ambient temperature and humidity, assuming that the surrounding air is not circulating, the maximum power allowed to be consumed by the resistor under long-term continuous load without damage or basically no change in performance. To ensure safe use, the rated power is generally selected to be 1-2 times higher than the power it consumes in the circuit. 2. Allowable error: The maximum allowable deviation range of the actual resistance value of the resistor from the nominal resistance value, which represents the accuracy of the product. Commonly used accuracies are 5%, 1%, 0.5%, 0.1%, 0.01% 3. Maximum working voltage: It refers to the voltage when the resistor works for a long time without overheating or electrical breakdown damage. If the voltage exceeds the specified value, sparks will be generated inside the resistor, causing noise and even damage. 4. Stability: Stability is a measure of the degree of resistance change of the resistor under the influence of external conditions (temperature, humidity, voltage, time, load properties, etc.) 5. Noise electromotive force: The noise electromotive force of the resistor can be ignored in general circuits, but it cannot be ignored in weak signal systems. The noise of wirewound resistors is limited to thermal noise (caused by molecular disturbances) and is only related to resistance value, temperature and the frequency band of external voltage. In addition to thermal noise, thin film resistors also have current noise, which is approximately proportional to the applied voltage. 6. High-frequency characteristics: When resistors are used under high-frequency conditions, the influence of their fixed inductance and inherent capacitance must be considered. At this time, the resistor becomes an equivalent circuit of a DC resistor (R0) connected in series with a distributed inductor, and then in parallel with a distributed capacitor. The LR of a non-wirewound resistor is 0.01-0.05 microhenry, and the CR is 0.1-5 picofarads. The LR of a wirewound resistor is tens of microhenries, and the CR is tens of picofarads. Even for a wirewound resistor with no inductive winding, the LR is still a few tenths of a microhenry. Inductive resistors will show high impedance at high frequencies, and the resistance (inductive reactance plus impedance) of inductive resistors at high frequencies will be very large, so the power it bears is the resistance (inductive reactance plus impedance) multiplied by the square of the current, which is far more than its nominal power. The resistor is easy to burn out.