Resistance Selection: principle is used resistor with the desired nominal resistance resistor values ??smaller the difference, the better.
Selection error: error desired time constant RC circuit resistors as small as possible, generally less than 5% optional, for decoupling circuit, the feedback circuit filter circuit load circuit error is not too high, 10% -20% of the optional resistor device.
. b Note resistors limit parameters:
Rated voltage: When the actual voltage exceeds the rated voltage, even to meet the power requirements, the resistors will be breakdown.
Rated power: The selected resistor power rating should be greater than the actual power to bear more than double in order to ensure long-term reliability of the resistor circuit to work in.
. c to the preferred universal Resistors:
Universal resistors more categories, complete specifications, production volume, scope and resistance range, the appearance of shape, volume size has chosen to facilitate the procurement and maintenance.
. d The circuit characteristics selection:
High-frequency circuits: the smaller the better distribution parameters should be used in metal film resistors, metal oxide film resistors and other high frequency resistance.
Low frequency circuits: winding resistance, carbon film resistors are applicable.
Power amplifier, bias circuit, sampling circuit: circuit stability requirements are relatively high, should choose small temperature coefficient resistor.
Decoupling circuit, filter circuit: The change in resistance is not strictly required, any type resistors are applicable.
. e The circuit board size selection of resistance:
Sensitive resistor common sense:
a, thermistor:
Is a kind of temperature-sensitive resistors, divided into positive temperature coefficient and negative temperature coefficient resistors, not only to pay attention to the selection of its rated power, the maximum operating voltage, nominal resistance, but also to pay attention to the maximum operating temperature and resistance temperature coefficient and other parameters, and pay attention to resistance change direction.
b, photoresistor:
With the intensity of light resistance change resistor., Into visible light-sensitive resistors, infrared light sensitive resistor, ultraviolet light-sensitive resistors, first determine the selection of the spectral characteristics of the circuit.
c, varistors:
Is very sensitive to voltage changes nonlinear resistor when the voltage across the resistor on the nominal value, the present value resistors on the infinite state, when the voltage slightly higher than the nominal voltage, its resistance soon decline, the resistance is in the conduction state, when the voltage is reduced to a nominal voltage or less, the resistance began to increase.
Varistors can be divided into non-polar (symmetric) and polar (asymmetric) varistors, the selection, the nominal varistor voltage value should be added to the voltage across the varistor 2 -2.5 times, the other should pay attention to the pressure.
Min temperature coefficient of resistance.
d, humidity resistance:
Is very sensitive to changes in humidity resistor, can be used in various humidity environment, it is converted into electrical signals of the humidity transducer device, the choice should be based on different characteristics of different types of numbers and humidity precision resistors, humidity factor, response speed, humidity range, etc. choose.
NOTE: The main characteristic resistance when in the low frequency is manifested resistance characteristics, but at higher frequencies, not only shows resistance characteristics, also exhibit this side reactance characteristics in terms of radio (RF circuit is particularly important).