Intro:This circuit is a 1.2V to 35V DC Regulated Power Supply, which is an essential bench tool for electronics hobbyists and engineers alike.
Its primary job is to take an unstable or higher alternating current (AC) voltage from a transformer, smooth it out, and convert it into a steady, adjustable direct current (DC) voltage that won't fluctuate—even if your load changes or the main power grid dips.
Core Highlights
The Brains (LM317 IC): At the heart of this circuit is the incredibly popular LM317 adjustable linear voltage regulator. It handles the heavy lifting of adjusting and maintaining the target voltage.
Highly Adjustable: You can dial in any custom voltage you need between 1.5V and 35V simply by turning a built-in potentiometer (a variable resistor).
Built-in Safety: The circuit is remarkably resilient because the LM317 IC features internal short-circuit protection and thermal overload protection. If you accidentally short the wires or pull too much current, the IC will safely limit the current or shut down before destroying itself.
Smooth Output: Thanks to a combination of filtering capacitors, it boasts an impressive 80dB ripple rejection, meaning the final DC output is clean and free of unwanted AC electrical "noise."
What Can You Use It For?
Because it is fully adjustable and can deliver up to 1.5 Amps of current (with proper heatsinking), this circuit is incredibly versatile:
Powering prototype projects on a breadboard without relying on batteries.
Testing small DC motors or controlling their speeds.
Powering low-voltage bulbs, LED strips, or testing sensor modules.
Essentially, it acts as a reliable, protective gatekeeper of power for your electronics workspace!
Components: To build this
Core Electronics
Integrated Circuit (IC): *
LM317Adjustable Voltage Regulator (1 qty)Diodes: *
1N4007Rectifier Diodes (4 qty) — Used to form the bridge rectifier that converts AC to DC.Resistors:
120 ΩResistor (1 qty)4.7K ΩPotentiometer / Variable Resistor (1 qty) — Used to manually dial in and adjust the output voltage.
Capacitors (For Filtering & Smoothing)
C1:
100nF(63V) Polar Capacitor (1 qty)C2:
1µF(35V) Polar Capacitor (1 qty)C3:
10µF(35V) Polar Capacitor (1 qty)C4:
2200µF(35V) Polar Capacitor (1 qty) — The main reservoir capacitor to smooth out AC ripple.
Hardware & Power Selection
Transformer: You will need a step-down AC transformer depending on your desired maximum voltage output. Refer to this quick selection guide:
| Desired Max DC Output | Required Transformer Rating |
| 3V – 5V | 9VAC / 15VA |
| 5V – 8V | 12VAC / 30VA |
| 8V – 13V | 15VAC / 30VA |
| 13V – 15V | 18VAC / 30VA |
| 15V – 18V | 22VAC / 30VA |
| 18V – 22V | 24VAC / 50VA |
| 22V – 35V | 28VAC / 50VA |
⚠️ Note: If you plan on drawing close to the maximum 1.5 Amps of current or dissipating higher power (up to 15W), it is highly recommended to mount a heatsink onto the LM317 IC to prevent it from overheating and shutting down. For low-power applications, passive cooling isn't strictly necessary.
How It Worked: Here is a detailed breakdown of how the
How the Circuit Works
The circuit processes power in four distinct stages to turn unstable alternating current (AC) from your wall outlet into a clean, adjustable direct current (DC).
1. Step-Down Stage (The Transformer)
High-voltage AC power from the mains (usually 110V or 220V) enters a step-down transformer. The transformer safely drops this dangerous voltage down to a lower AC voltage (up to 28VAC), depending on the rating of the transformer you chose.
2. Rectification Stage (The Diode Bridge)
The low-voltage AC power then flows into a bridge rectifier made of four 1N4007 diodes.
AC power constantly flips polarity (positive to negative).
The diode bridge forces the current to flow in only one direction, converting the AC wave into a bumpy, pulsating DC wave.
3. Filtering Stage (The Capacitors)
The pulsating DC wave is still too rough to power electronics.
C4 (2200µF): This large reservoir capacitor acts like a water tank. It stores electrical charge when the voltage peaks and releases it when the voltage drops, smoothing out the bumps into a steady DC voltage with minimal "ripple."
C1 (100nF): Placed close to the LM317 input, this small capacitor filters out high-frequency electrical noise from the line.
4. Regulation & Adjustment Stage (The LM317 IC)
The smoothed DC voltage enters the IN pin of the LM317 regulator.
The LM317 maintains a constant reference voltage of $1.25\text{V}$ between its OUT and ADJUST (ADJ) pins.
By turning the 4.7KΩ Potentiometer (P1), you change the resistance at the ADJ pin. This forces the LM317 to change its output voltage to maintain that reference balance.
C2 and C3 provide final stabilization at the output, preventing the circuit from oscillating and keeping the voltage rock-steady even if the load suddenly changes.
Practical Applications
Because this circuit can deliver a fully adjustable output from 1.2V up to 35V (at up to 1.5 Amps), it is incredibly versatile:
Variable Bench Power Supply: It is the ultimate tool for an electronics lab or maker space. You can dial in exactly 3.3V for microcontrollers, 5V for Arduino projects, 9V for guitar pedals, or 12V for automotive accessories.
Component Testing: It allows you to safely test individual components like LEDs, relays, and sensors at their exact operating voltages without risking damage.
DC Motor Speed Control: By manually turning the potentiometer, you vary the voltage fed to small DC motors or fans, smoothly controlling their rotational speed.
Battery Charging: With proper monitoring, you can set the exact float voltage required to charge small lead-acid, NiMH, or lithium-based rechargeable batteries.
Educational Training: Because it covers all fundamental power electronics concepts (rectification, filtering, and linear regulation), it is widely built by students to learn circuit design.
1. Electronics Bench & Prototyping
Variable Test Bench Power Supply: It serves as an excellent, low-cost entry-level bench power supply. You can dial in exact, non-standard voltages required by different prototype boards.
Microcontroller Power Source: Easily dialed down to provide a clean 3.3V or 5V line required for sensitive development boards like Arduinos, ESP32s, or Raspberry Pi Pico modules without needing separate wall adapters.
Sensor & Relay Testing: It allows you to safely test individual components—such as 12V relays, 24V industrial sensors, or 9V audio sub-circuits—to verify they function properly before integrating them into a larger project.
2. Hobbyist & Consumer Electronics Projects
DC Motor Speed Control: By turning the potentiometer to change the voltage output, you can directly control the rotational speed of small DC motors, miniature drill presses, or 12V cooling fans.
LED Strip & Lighting Driver: Perfect for powering and testing various lengths of 12V or 24V LED strips, low-voltage incandescent bulbs, and custom architectural lighting arrays.
Battery Charging Station: With proper manual voltage monitoring, it can be set to the exact float or trickle-charge voltage required to recharge small lead-acid batteries, NiMH packs, or legacy rechargeable cells.
3. Education & Lab Learning
Academic Lab Work: Because this circuit cleanly demonstrates the four pillars of power design—step-down conversion, diode rectification, capacitor filtering, and linear regulation—it is heavily used as a foundational DIY project in high school and university physics/electrical engineering classes.
Safe Circuit Troubleshooting: Thanks to the built-in short-circuit and thermal overload protection of the LM317 IC, it acts as a safe power source for students; if a student accidentally wires a prototype incorrectly and creates a short, the IC safely throttles down rather than damaging the components or causing a fire hazard.
