290

2017/7/13

Fixed resistors have different types of fixed resistors. The resistors of which materials and structures are chosen should be determined according to the specific requirements of the application circuit. High frequency circuits should be used to distribute inductance and distributed capacitance small non-wire-wound resistor, such as carbon film resistor, metal resistor and metal oxide film resistor. High gain small signal amplifier circuit should choose low noise resistor, such as metal film resistor, carbon film resistor, and wire wound resistor, rather than the synthetic carbon film resistor with large noise and organic solid resistor. The power of wire wound resistor is large, the current noise is small, it is high temperature, but it is bigger. Conventional wire-wound resistor is often used in low-frequency circuit or medium - limit flow resistor, partial pressure resistor, discharge resistor or the bias resistor of high power tube. The high precision wire-wound resistor is used in fixed attenuator, resistance box, computer and various precision electronic instruments. The resistors of the selected resistor should be close to a nominal value of the calculated value in the application circuit, and the standard series of resistors should be preferred. The resistor used in the general circuit allows the error to be plus or minus 5% ~ plus or minus 10%. Precision resistors should be used in precision instruments and in special circuits. The rated power of the selected resistor should meet the requirements of the power capacity of the resistor in the application circuit, and the power of the resistor should not be arbitrarily increased or reduced. If the circuit is required to be a power resistor, its rated power can be 1-2 times that of the actual application circuit. There are usually 6 specific cases:

1, the power rating: under the regulation of environmental temperature and humidity, assuming the surrounding air circulation, not in long-term continuous load without damage or basic don't change under the condition of performance, allow the consumption of power resistor. To ensure safe use, the rated power is generally 1 to 2 times higher than the power it consumes in the circuit.

2. Allowable error: the actual resistance value of the resistor is the allowable deviation range of the nominal resistance value, which indicates the precision of the product. The usual accuracy is 5%, 1%, 0.5%, 0.1%, 0.01%

3. Working voltage: it refers to the voltage when the resistor does not overheat or puncture the damage. If the voltage exceeds the regulation value, the internal spark of the resistor can cause noise and even damage.

4. Stability: stability is the measure of resistance of resistors under external conditions.

5. Noise electromotive force: the noise electromotive force of the resistor may not be considered in the general circuit, but it cannot be ignored in the weak signal system. The noise of wire-wound resistor is only concerned with the frequency band of temperature and external voltage. In addition to the thermal noise, the thin film resistor has the current noise, which is approximately proportional to the external voltage.

6. High frequency characteristics: the use of resistors in high frequency conditions to consider the effects of its fixed inductance and intrinsic capacitance. At this point, the distribution of resistor into a dc resistance and inductance in series, and then with the equivalent circuit of capacitance in parallel, the wire wound resistor of LR = 0.01 0.05, CR = 0.1 5 skin method, micro wire wound resistor LR of dozens of heng, CR of dozens of skin method, even the non-inductive winding wire wound resistor, there are still zero JiWei heng LR. Inductive resistance appeared in high frequency when the high impedance, and at the time of the high frequency inductive resistance of resistance will be very big, so it to bear is multiplied by the square of the current resistance of power, far more than its nominal power, resistance to burn out.

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