The LM317 is a widely used adjustable positive voltage regulator integrated circuit. Here is an introduction to the LM317 adjustable power supply circuit12:
Circuit Components
- LM317 Chip: It is the core component of the circuit, which has the functions of voltage regulation and current limiting. It can provide a stable output voltage within a certain range according to the external resistance settings12.
- Input Capacitor: Usually, a relatively large electrolytic capacitor is connected in parallel at the input end of the LM317 to filter out the ripple of the input DC voltage and provide a relatively stable DC input for the chip, such as a 100μF capacitor.
- Output Capacitor: A capacitor is also connected in parallel at the output end to further filter the output voltage and improve the stability of the output voltage. Commonly, a 100nF ceramic capacitor is used.
- Resistors: There are mainly two resistors, one is the current limiting resistor connected between the adjustment terminal and the output terminal , and the other is the adjustable resistor connected in series with . These two resistors are used to set the output voltage according to the formula 1.
Circuit Operation Principle
- Voltage Regulation: The LM317 adjusts the output voltage by changing the voltage drop across the resistors and . When the resistance value of changes, the voltage at the adjustment terminal also changes, and the internal circuit of the LM317 adjusts the output voltage accordingly to maintain a stable output1.
- Current Limiting: The LM317 has an internal current limiting circuit. When the output current exceeds the set current limit value, the chip will automatically reduce the output voltage to limit the current, thereby protecting the circuit components from being damaged by excessive current12.
Design Considerations
- Input Voltage Range: The input voltage of the LM317 should be within the specified range. Generally, the maximum input voltage can be up to 40V, but it is necessary to ensure that the input voltage is always higher than the output voltage by a certain value to ensure the normal operation of the regulator1.
- Heat Dissipation: When the LM317 is working, it may generate a certain amount of heat, especially when the output current is relatively large. Therefore, it is necessary to consider adding an appropriate heat sink to ensure that the chip operates within a safe temperature range.
- Output Voltage and Current Requirements: According to the actual application requirements, select the appropriate resistance values of and to set the required output voltage range. At the same time, it is also necessary to ensure that the maximum output current of the circuit meets the load requirements1.
- Filtering: In order to obtain a more stable output voltage, in addition to the input and output capacitors, additional filtering circuits can be added as needed to further reduce the ripple and noise of the power supply.
Example Circuit
- A simple LM317 adjustable power supply circuit consists of an LM317 chip, a 100μF input capacitor , a 100nF output capacitor , a 240Ω fixed resistor , and a 5kΩ adjustable resistor . The input voltage is applied to the input terminal of the LM317, and the output voltage is taken from the output terminal . By adjusting the resistance value of , the output voltage can be continuously adjusted within the range of approximately 1.25V to 37V1.
In conclusion, the LM317 adjustable power supply circuit is a very practical and flexible circuit, which can be widely used in various electronic circuits and experimental circuits that require adjustable power supplies.