In every electronics workspace, whether it’s a full-fledged lab or a corner of a garage, power supplies are the quiet heroes. They power your soldering irons, your microcontroller prototypes, your logic analyzers, and even the lights you work under. And yet, many hobbyists rely on cheap, noisy, or oversized power bricks that clutter the bench and break down far too often.

This project aims to solve that with a focused, compact, and reliable DIY power supply using the NCP1337DR2G, a current-mode controller IC that enables the creation of efficient offline flyback converters.

Rather than building a general-purpose SMPS or a complicated variable power source, this project is about constructing a single-voltage, high-reliability power unit tailored for a specific purpose—powering 12V DC workshop accessories like bench lights, DC fans, small routers, or even an external hard drive hub. With a solid enclosure, clean output, and long-term stability, this supply becomes one of those indispensable tools you’ll wonder how you lived without.

Why the NCP1337DR2G?

At the heart of this project is the NCP1337DR2G, an IC from ON Semiconductor designed for low standby power and high-efficiency offline switch-mode power supplies. It’s not the most glamorous component—no one hangs posters of flyback controllers—but it's incredibly reliable.

The NCP1337DR2G offers internal startup circuitry, frequency jittering to reduce EMI, and an auto-restart mode for overload protection. Its features reduce the need for external components and make it a perfect choice for anyone wanting to build a long-lasting, high-performance power supply without going into overly complex topologies.

By using this chip in a flyback configuration, we can safely convert AC mains (110V or 220V) into a clean, regulated 12V output with decent current capacity—ideal for powering 12V tools in your DIY space.

Project Goals

The power supply will be housed in a durable, ventilated enclosure with a fused AC input, an LED indicator, and a single 12V DC output capable of sourcing around 2A—more than enough for most desk-side tools. Key goals include:

● Electrical safety: Isolation, proper grounding, and physical separation of high/low voltage

● Thermal reliability: Efficient layout and cooling to prevent heat buildup

● Low standby draw: Thanks to the low-power features of the NCP1337

● Compact size: It should fit neatly on a shelf or in a drawer when not in use

Planning the Build

The design begins not with a schematic, but with use cases. What needs power in your workshop? Maybe a 12V solder fume extractor, an LED strip under your desk, or a small logic analyzer. These are low-current, stable load devices—perfect for a regulated supply.

Next, you plan the enclosure. Choose one with internal metal shielding or at least a plastic case with internal shielding tape. The idea is to keep both electrical safety and EMI in check. The back will have an IEC power inlet with a built-in fuse and switch, while the front will have a single barrel jack or terminal block output, along with a power indicator LED.

Keep in mind that you’re dealing with mains voltage. This project isn’t for a complete beginner, and it demands respect for safe wiring practices. All high-voltage sections should be physically separated from the low-voltage output path, and the PCB layout must follow basic creepage and clearance guidelines.

Assembling the Core Circuit

The NCP1337DR2G will be mounted on a small custom PCB or prototyping board with careful spacing between its pins. It controls a switching MOSFET that drives the primary side of a flyback transformer, which then provides a stepped-down and isolated voltage on the secondary side.

On the input side, after the AC power comes in, it passes through a fuse, varistor (for surge protection), and a bridge rectifier, followed by bulk electrolytic capacitors. A startup resistor charges the NCP1337’s VCC capacitor, after which the internal startup circuitry takes over.

The controller chip then begins switching the external MOSFET, creating a pulsed signal that energizes the transformer. The transformer’s secondary side rectifies and filters this into clean 12V DC, which is fed to the output jack. An optocoupler provides feedback to the NCP1337 to regulate voltage, using a secondary-side TL431 or similar voltage reference.

That’s the core loop—simple in concept but demanding in execution.

Thermal and Mechanical Considerations

One common mistake in power supply builds is underestimating heat. Even if the NCP1337DR2G is efficient, the flyback transformer, MOSFET, and output diode all generate heat under continuous load. So, make room for a compact heatsink on the MOSFET and leave airflow space around the transformer.

You can mount the entire board on stand-offs inside the case, with ventilation holes near the hot components. Use silicone thermal pads where needed, and keep wiring away from metal surfaces. Route high-voltage AC traces separately and protect them with insulation sleeves if wires must cross.

The power LED can be mounted directly on the front panel, wired to the 12V output through a small resistor. It will give a quick visual cue that the unit is active.

Testing and First Power-Up

Before connecting the unit to mains voltage, do a full dry fit test. Use a continuity tester to ensure there are no shorts between AC and DC sides, and check all solder joints for bridges or cold joints.

When ready, plug the supply into an isolation transformer or use a current-limiting bulb in series to prevent damage from a miswired circuit. If you see the bulb glow brightly and stay that way, something's wrong.

If all goes well, the power LED will light, and the DC output will measure around 12V. You can now connect a test load—a small fan or a resistor—and monitor the voltage for stability. Listen for any hissing or clicking from the transformer, which might indicate saturation or poor solder joints.

Run the supply under load for 15–30 minutes, watching temperatures. If the heatsinks remain within reasonable warmth and the output holds steady, the design is solid.

Real-World Use

Once complete, this supply becomes a silent but powerful part of your bench setup. You can leave it connected to a permanent set of tools, such as:

● A 12V soldering fan or fume extractor

● LED task lighting strips

● A USB charging board powered via 12V input

● An analog scope with 12V input

● Raspberry Pi or Arduino-based tools with DC buck converters downstream

Because the NCP1337DR2G supports low standby power, the supply won’t waste energy when idle. You can leave it plugged in and switched off with peace of mind.

Its flyback design also means you can use it in international settings—just change the fuse and plug adapter as needed. It handles wide input voltages with minimal change in output behavior.

Customization and Expansion

Once you've built your first unit, you may consider building a few more for other voltage levels—maybe 5V or 24V. Since the core control IC (the NCP1337) doesn’t dictate the output voltage directly, you can reuse the same board design with minor tweaks to the transformer and feedback circuit.

You might also consider:

● Adding over-voltage or short-circuit protection using zener clamps and resettable fuses

● Using screw terminal outputs for easier connections

● Building a dual-output version, e.g., 5V and 12V for different tools

● Installing a panel voltmeter to monitor output live

● Adding soft-start or remote-control capabilities using relays and microcontrollers

These aren't necessary for the first version, but they’re ideas for making the most out of the same solid foundation.

Long-Term Value

What you end up with is not just a power box, but a tool you’ll use in almost every project that follows. It won’t be flashy, and it won’t call attention to itself, but that’s exactly what a good power supply should do. It just works.

Unlike store-bought bricks that get warm and emit high-pitched noise after months of use, this one will sit cool and reliable. And when something goes wrong, you can repair it—you built it. That’s power in more than one sense.

The NCP1337DR2G might not be well-known outside power supply design circles, but once you’ve built with it, you’ll remember the name. It earns its place by offering professional-level performance with just enough flexibility to encourage DIY creativity.

Conclusion

This DIY offline power supply project doesn’t try to be everything—it aims to be one thing done well: a stable, reliable 12V supply for your bench. With the NCP1337DR2G at the core, you build not just a piece of hardware but a dependable companion for countless future builds.

If you’ve ever been frustrated with generic power bricks or wanted more control over your workbench setup, this project offers both challenge and reward. It gives you something real, tangible, and enduring.

And it all starts with one unassuming little IC.