A stable power supply is one of the most important tools to have in an electronics lab. Whether you’re testing microcontrollers, experimenting with sensors, or running small DC motors, you need a reliable voltage source. While you can buy lab power supplies, building one yourself is more rewarding and, honestly, teaches you a lot about regulation and voltage control.
The circuit we’re looking at here is a regulatable 1.5V to 12V power supply built around the LM317 IC. This regulator is a workhorse — it can output a range of voltages depending on how you configure the resistors around it. In this particular design, the circuit provides multiple fixed steps like 1.5V, 3V, 4.5V, 6V, 7.5V, 9V, 10V, and 12V with the help of preset resistors and a rotary switch.
Why Use LM317?
The LM317 is a three-terminal adjustable regulator IC. Unlike the 7805, which only gives 5V, the LM317 can be set anywhere between 1.25V and 37V (as long as your input voltage is higher than what you need).
Some advantages:
- Adjustable output: Easy to set precise voltages.
- Current up to 1.5A: With proper heatsinking, you can run small motors, LEDs, and circuits.
- Built-in protection: It has short circuit protection and thermal overload safety.
- Widely available and inexpensive: Almost every electronics hobby shop stocks it.
That flexibility is why it’s being used here to make a mini regulated supply with predetermined voltage steps.
Looking at the Circuit
At first glance, the design might look busy with all those resistors, but let’s break it into parts.
Power Input
- The supply takes a 12V DC input from an adapter, battery, or rectified transformer.
- A basic on/off switch controls the input power.
Filtering
- Capacitors C1 (0.33µF) and C2 (33µF) help stabilize the regulator and reduce oscillations.
- Capacitor C3 (100µF) smooths the output to reduce ripple and provide a clean DC voltage.
The LM317 Regulator
- The LM317 has three pins: Input (pin 3), Output (pin 2), and Adjust (pin 1).
- You set the output voltage by creating a divider with resistors between the output and the adjust pin.
- In this circuit, a set of resistors (R2 to R9) is carefully chosen to create specific voltages.
Rotary Switch (SW2)
- Instead of using a single potentiometer to vary voltage, this design uses a rotary switch with resistors tied into each position.
- Each position corresponds to a standard DC voltage: 1.5V, 3V, 4.5V, 6V, 7.5V, 9V, 10V, and 12V.
- This way, you can quickly select the voltage you need without fiddling with dials.
Load Indicator
- A small digital voltmeter module is connected at the output to show the regulated output voltage.
How the Voltage Regulation Works
The LM317 formula for output voltage is:
[ V_{out} = V_{ref} \times \left(1 + \frac{R2}{R1}\right) + I_{adj} \times R2 ]
- Vref is 1.25V (internal reference voltage).
- R1 is a fixed resistor (here, 150Ω).
- R2 is variable in this circuit, depending on which resistor is selected by the rotary switch.
For example:
- If R2 = 30Ω → Output ~1.5V
- If R2 = 60Ω → Output ~6V
- If R2 = 180Ω → Output ~9V, etc.
By carefully picking resistor values, you get precise voltage outputs without the need to constantly measure and tweak.
Applications
This regulated supply is useful in a ton of places:
- Testing small circuits: Perfect for powering prototypes, breadboard circuits, or Arduino projects.
- Replacing battery power: Many toys and devices use 1.5V, 3V, 6V, or 9V batteries. This supply can substitute for them.
- Learning tool: Great DIY project to understand how adjustable regulators work.
- Bench power supply: With 1.5A capacity, you can run LEDs, relays, sensors, and small DC motors.
Advantages of This Design
- Multiple Fixed Steps: Instead of randomly turning a potentiometer, you get clean voltages instantly.
- Simplicity: The LM317 circuit is simple, reliable, and robust.
- Flexibility: Covers the most commonly required voltages for electronics hobbyists.
- Cost-Effective: Uses only one IC and standard resistors.
Limitations
Like any design, it has some downsides:
- Current limited to 1.5A: If you need more, you’d use external pass transistors.
- Heat Dissipation: The LM317 will heat up at higher currents or when dropping from 12V to low voltages (say 1.5V). A heatsink is essential.
- Requires 12V input: You can’t get more than 12V out if your input is 12V. To push higher (say 15V), you’d need a higher input source.
Practical Tips
- Always use a heatsink: When set to low voltages while drawing current, the IC drops a lot of power as heat.
- Use quality capacitors: This keeps the output stable and avoids oscillations.
- Tighten resistor values: If you want more accurate voltages, use 1% tolerance resistors.
- Expand if needed: If you want a continuously variable version, replace the rotary switch and fixed resistors with a potentiometer in the R2 position.
- Fuse the input: Always a good idea for safety when experimenting.
Variations and Improvements
Once you understand the basics, you can expand this design. Some ideas:
- Add a current-limiting feature with another transistor.
- Add a digital volt/amp meter on the output for both voltage and current.
- Replace resistors with a precision multi-turn potentiometer for smoother adjustment.
- Build it into a metal box with banana jack outputs for a mini bench PSU.
Conclusion
The 1.5V to 12V regulated supply with LM317 is a classic DIY project and one of the most practical things any electronics hobbyist can build. It uses simple parts but gives you a powerful tool for powering circuits and experimenting with different voltages.
By using a rotary switch with fixed resistors instead of a continuous potentiometer, this design makes it easy to quickly select standard voltages like 1.5V, 3V, 4.5V, 6V, 7.5V, 9V, 10V, and 12V. That makes it especially handy when replacing batteries or powering hobby circuits directly.
It’s efficient, reliable, and a great way to learn how voltage regulators work in real-world projects. If you’re putting together your first workbench, this supply is a must-have.
