Resistors are the most common components in the circuit, and there are always resistors on all kinds of boards, whether they are patch resistors, in-line resistors or power resistors. What is the function of resistance in a circuit? The more commonly used things, the more difficult to say clearly, this paper tries to discuss with you the role of resistance.
Upright resistance
Partial voltage action of resistance
Ohm's law is well known, U=IR, when a current flows through a resistor, it creates a voltage drop across the resistor, where the resistor acts as a voltage divider. What functions can be achieved by using the partial voltage of resistors? For example, the microcontroller establishes a corresponding relationship between the voltage at both ends of the resistance and the actual physical quantity when sampling, and the change of the physical quantity can be obtained by calculating the voltage value. Here, the NTC temperature detection is used as an example to explain
Partial voltage action of resistance
In the figure above, a resistor with fixed resistance R1 is connected in series with a variable resistance NTC. When the temperature changes, the resistance of NTC changes, resulting in a change in the voltage of R1. By calculating the voltage of R1, the current resistance of NTC can be obtained, and the corresponding temperature value can be calculated.
Current limiting effect of resistor
The function of resistance is to limit the current. In a series loop, the greater the resistance, the smaller the current in the loop. Therefore, this function can be used to limit the current in the loop, so as to prevent the current from being too large to burn out the load equipment. Take the light-emitting diode LED as an example, as shown below:
Current limiting effect of resistor
Leds require a current of 2-10mA to light, and too much will burn out. In the figure above, a resistor R3 is connected in series in the LED loop to limit current and prevent the LED from burning out due to excessive current. Take VCC=12V as an example, assuming that the LED voltage drop is 2V and the required current is 2mA, then the resistance R3=(12-2)/2= 5kω, that is, the resistance of 5kω will limit the current in the loop to 2mA.
Pull up and pull down action of resistors
When learning the course of single-chip microcomputer, I encountered pull-up resistance and pull-down resistance. The so-called pull-up is to pull the interface up to the power supply through the resistance, so that the level of the interface is high in the initial state; The so-called pull-down is to pull down the interface to GND through the resistance, so that the level of the interface is low in the initial state. Can LET LEVEL MAINTAIN STABLE SO, UNAPT MISTAKE action, CAUSE UNNECESSARY result.
Pull up of resistance
The PNP triode is used to control the LED, and the LED is expected to be extinguished under normal conditions. The microcontroller is used to control the base pin of the triode by the resistor R7 to pull up to a high level, and the triode is cut off when it is not working.
Pull-down of resistance
In the figure above, NPN triode is used to control the LED, and the LED is expected to be off under normal conditions. The triode base is pulled down to the bottom by resistor R5 to prevent misoperation during program initialization at the moment of powerup.
The resistor acts as a jumper
In the design of the circuit, in order to facilitate later debugging, in the pin or chip through a resistance connection, when not used to remove the resistance, to short welding a 0 ohm resistance. In particular, a 0 ohm resistor can be used for single/multipoint grounding when laying ground cables
Single point grounding action of resistance
The filtering effect of resistance
Resistors and capacitors can form filters. Common ones are high-pass and low-pass filters.
RC low-pass filter
The figure above shows the filter composed of resistance and capacitance. The capacitive reactance of the capacitor is inversely proportional to the input frequency. The higher the frequency, the smaller the capacitive reactance, and the less easy the low-pass filter is to pass through.