What Is the LM317?
The LM317 is a popular adjustable voltage regulator used in electronic circuits. It allows you to set a specific output voltage using just a couple of resistors.
Unlike fixed voltage regulators like the 7805, which only output 5V, the LM317 can be set to output any voltage between 1.25V and 37V, depending on the input and resistor values.
It is widely used in DIY electronics, power supplies, and embedded systems for its flexibility and ease of use.
How Does the LM317 Work?
The LM317 works by maintaining a constant voltage difference between its output and adjustment pin. This voltage is typically 1.25V.
By connecting two resistors between the output, adjustment, and ground, you create a voltage divider that sets the output voltage.
This makes the LM317 an ideal component for creating variable power supplies or charging circuits where precise voltage control is needed.
Calculating Output Voltage of the LM317
The output voltage of the LM317 is calculated using the following formula:
Vout = Vref × (1 + R2 / R1) + Iadj × R2
Where Vref is typically 1.25V, R1 and R2 are the resistors used in the voltage divider, and Iadj is a small current (usually around 50 µA) which is often negligible in most designs.
For simplicity, many designers ignore the Iadj term, especially when R2 is small, and use:
Vout ≈ 1.25 × (1 + R2 / R1)
LM317 Example Calculation
Let’s say we want an output voltage of 5V. We'll choose R1 = 240 ohms, which is a common value used with the LM317.
Rearranging the formula, we get:
5 = 1.25 × (1 + R2 / 240)
Dividing both sides by 1.25 gives us:
4 = 1 + R2 / 240
R2 / 240 = 3 → R2 = 720 ohms
So by using a 240-ohm resistor for R1 and a 720-ohm resistor for R2, the LM317 will output approximately 5V.
Why Use the LM317?
The LM317 is extremely versatile. It can be used in a wide range of applications due to its adjustable output, thermal overload protection, and internal current limiting.
It’s also easy to implement in circuits, requiring minimal external components. This makes it ideal for beginners and professionals alike.
It supports input voltages up to 40V and can supply more than 1.5A of current with a proper heat sink.
Interpreting LM317 Datasheets
Understanding the LM317 datasheet is crucial for designing reliable circuits. The datasheet includes details like voltage range, thermal limits, dropout voltage, and pin configuration.
The dropout voltage—typically around 2 to 3 volts—means the input voltage must be at least that much higher than the desired output voltage.
This is important to keep in mind when designing circuits with tight voltage margins.
Practical Applications of LM317
The LM317 is used in battery chargers where you want to control the charging voltage precisely.
It is also found in bench power supplies, where users need to dial in different voltages for testing electronics.
Another common use is in LED drivers, especially when combined with a current-sensing resistor to regulate current flow.
Audio equipment often uses the LM317 to provide clean and stable voltages to sensitive analog circuits.
Tips for Using the LM317
Always use a heat sink if your load draws more than a few hundred milliamps. The LM317 can get hot quickly under load.
Include output capacitors for stability. A 0.1 µF capacitor on the adjustment pin and a 1 µF or higher capacitor on the output is recommended.
Use precision resistors if you need an accurate output voltage. Even small deviations in resistor values can affect your output.
If you need even more precise control, consider using a trimmer potentiometer for R2 so you can fine-tune the output voltage.
Limitations of the LM317
One drawback is its relatively high dropout voltage. If you're designing low-voltage circuits, a low-dropout regulator (LDO) might be a better choice.
It also requires a minimum load current (typically 5 mA) to maintain regulation. If your circuit draws less than this, you may need to add a dummy load.
Finally, it’s not the most efficient choice for large voltage drops with high current, as it dissipates power as heat.
Conclusion
The LM317 is a powerful yet simple tool in the world of voltage regulation. Its adjustable output and built-in protections make it a go-to component for countless projects.
By understanding how to calculate its output and use it properly, you can design reliable power supplies and voltage regulators for a wide range of applications.
Whether you’re a hobbyist or a professional, the LM317 deserves a spot in your component collection.