1.5 A Adjustable DC Variable Voltage Regulator Using LM317 ic

lm317 voltage regulator working

The LM317T is an adjustable three-terminal positive-voltage regulator. This regulator IC capable of supplying more than 1.5 A over an output-voltage range of 1.25 V to 37 V. To set the output voltage it requires only two external resistors.

Resistors R1 and R2 set the output to any desired voltage over the adjustment range 1.2v to 37v. It has current limiting, thermal overload protection, and safe operating area protection. Overload protection remains functional even if the ADJUST terminal is disconnected. This device can also be used as a programmable output regulator, or by connecting a fixed resistor between the adjustment and output, the LM317 can be used as a precision current regulator.

the LM317 develops and maintains a nominal 1.25 V between its output and adjustment terminals called reference voltage (Vref). This reference voltage is converted to a programming current (IPROG) by R1, and this constant current flows through R2 to the ground. The output voltage desired can be calculated using.

(I_Adj) represents an error Current from the adjustment terminal, With a Typical LM317 IC value
Iadj is to less than 100μ A and keeps it constant. Vref= 1.25V. If the load current is less than this minimum, the output voltage will rise. Since the LM317 is a floating regulator, it is only the voltage differential across the circuit which is important to performance, and operation at high voltages with respect to ground is possible.

I_adj is very small amount and it can be ignored in practical application

Example

Determine the Regulated output voltage of a circuit which haveR1=240 Ω and R2=2.4KΩ

Solution= We know that Vout= 1.25(1+2.4KΩ/240Ω)

= 13.75V

For 12v output- R1=220Ω, R2=1.9k

For 8v output- R1= 220Ω, R2= 1.2k

For 9v output – R1 = 220Ω, R2 = 1.3k

For 5v output- R1= 220Ω, R2=680Ω

For 3v output- R1=220Ω , R2= 330Ω

Features of LM317

• Output Voltage Range Adjustable From 1.25 V to 37 V
• Output Current Greater Than 1.5 A
• Internal Short-Circuit Current Limiting
• Thermal Overload Protection
• Output Safe-Area Compensation

Disadvantages of the LM317

The main disadvantage of LM317 is the voltage is that as much as 2.5 volts is dropped or lost as heat across the regulator. for example, if we need to 9v at output then the input voltage will need to minimum 12 volts or more if the output voltage is to remain stable under maximum load conditions. This voltage drop across the regulator is called “dropout”. A heatsinking is required to keep the regulator cool.

FAQ(Frequently asked questions)

Q: What is an LM317 IC?

A: The LM317 is a popular adjustable voltage regulator integrated circuit (IC) that is widely used in electronic circuits. It is a three-terminal device capable of providing a regulated output voltage that can be adjusted based on the desired application requirements.

Q: What are the main features of the LM317 IC?

A: The LM317 IC offers several key features, including:

Adjustable output voltage: The output voltage can be adjusted by using external resistors.
Current limiting: The IC has built-in current limiting capability to protect the circuit and prevent excessive current flow.
Thermal overload protection: It includes internal thermal shutdown circuitry to protect the IC from overheating.
Wide input voltage range: The LM317 can accept a wide range of input voltages, typically up to 40 volts.
High ripple rejection: It has good ripple rejection characteristics, making it suitable for applications with AC ripple on the input.

Q: How does the LM317 IC work?

A: The LM317 IC operates as a voltage regulator by using a reference voltage (1.25 volts) between its output and adjustment terminals. By connecting external resistors between these terminals, the output voltage can be set based on the desired voltage value. The IC adjusts its internal resistance to maintain a constant voltage difference between the output and adjustment terminals.

Q: What is the output voltage range of the LM317 IC?

A: The LM317 can provide an adjustable output voltage range from 1.25 volts to approximately 37 volts. The exact range depends on the input voltage and the values of the external resistors used for voltage adjustment.

Q: How do you adjust the output voltage of the LM317 IC?

A: The output voltage of the LM317 can be adjusted by connecting external resistors between the output and adjustment terminals. The formula Vout = Vref (1 + R2/R1) is typically used, where Vout is the desired output voltage, Vref is the reference voltage (1.25 volts), R1 is the resistance connected between the output and adjustment terminals, and R2 is the resistance connected between the adjustment terminal and the ground.

Q: What are the typical applications of the LM317 IC?

A: The LM317 IC is commonly used in various applications, including:

Power supply circuits: It is used to regulate and stabilize the output voltage in power supply circuits.
Battery chargers: The LM317 can be used to control the charging voltage for batteries.
Adjustable voltage regulators: It is employed in circuits requiring a variable output voltage.
Constant current sources: The LM317 can be configured as a constant current source for driving LEDs or other devices.
Voltage references: It can serve as a stable voltage reference in circuits requiring a precise voltage source.

Q: Are there any precautions or limitations when using the LM317 IC?

A: Yes, some considerations include:

Heat dissipation: The LM317 can generate significant heat, especially when regulating high voltage differentials or high currents. Adequate heat sinking is important to prevent overheating.
Input voltage range: Ensure that the input voltage does not exceed the maximum specified value to avoid damaging the IC.
Output current limitation: The LM317 has a maximum current rating, and if exceeded, it may affect the stability of the regulator or lead to device failure.
Capacitor selection: Proper capacitor selection and placement are important for stability and noise reduction in the circuit.