When selecting components, people at different technical levels consider different issues. People engaged in electronic product research and development need to consider many parameters of components, so as to ensure that the electronic products produced have good performance and are not prone to problems; For electronics enthusiasts in maintenance, production and simple design, as long as some important parameters of components are considered, practical problems can be solved.
Examples of resistor selection:
The resistor selection process is as follows:
① Determine the resistance value. The resistance value of the resistor can be obtained by using Ohm's law R=U/I=220V/0.01A=22000Ω=22kΩ.
② Determine the error. For the circuit, the smaller the error, the better. Here, the resistor is selected with an error of ±5%. If it is difficult to find a resistor with an error of ±5%, a resistor with an error of ±10% can also be selected. When selecting a resistor, the resistance value, error, rated power and limit voltage of the resistor are mainly considered. It is required that the current I=0.01A passing through the resistor R. Please choose a suitable resistor to meet the actual requirements of the circuit.
Resistor selection example diagram:
When selecting a resistor, the resistance value, error, rated power and limit voltage of the resistor are mainly considered.
③ Determine the power. According to the power calculation formula, the power of the resistor can be calculated as P=I2R=(0.01A)2×22000Ω=2.2W. In order to allow the resistor to be used for a long time, the power of the selected resistor should be more than twice the actual power . Here the resistor power is chosen to be 5W.
④ Determine whether the limit voltage of the selected resistor meets the needs of the circuit. When a resistor is used in a circuit with high voltage and low current, the power may meet the requirements, but if the limit voltage of the resistor is less than the voltage applied to both ends of the circuit, the resistor will be broken down.
The limiting voltage of the resistor can be obtained. The limit voltage of the resistor here is V≈331V, which is greater than the 220V applied to both ends of the resistor and can be used normally. When the pole of the resistor, in order to ensure that the resistor is not broken down in the circuit, a resistor with a larger resistance value or a higher power can be selected according to the situation. To sum up, in order for the resistor R in the circuit shown in the figure to work normally and meet the requirements, a resistor with a resistance value of 22kΩ, an error of ±5%, and a rated power of 5W should be selected.
Resistor selection skills:
In actual work, it is often encountered that the selected resistor cannot meet the requirements. At this time, the following methods can be used to solve the problem:
① For circuits with low requirements, when selecting a resistor, its resistance value and power should be as close as possible to the required value, and the rated power can only be greater than the required value, otherwise the resistor will be easily burned out.
②If you cannot find a resistor with a certain resistance value, you can solve it by connecting multiple resistors in parallel or in series. Resistance increases when resistors are connected in series, and decreases when resistors are connected in parallel.
③If the power of a certain resistor is not enough, multiple low-power resistors with large resistance can be used in parallel, or multiple resistors with small resistance and low power can be connected in series. Whether in parallel or in series, each resistor can withstand The power will be reduced. As for how much power should be selected for each resistor, it can be calculated by P=U2/R or P=I2R, and then consider the margin of about twice.