Retro Circuit & Power Tester

Introduction

The idea for this Retro Circuit & Power Tester came from my need for a simple, safe, and stylish tool to test and control line-powered devices on the workbench. Commercial solutions were either too bulky, too expensive, or lacked the tactile, analog charm I enjoy. So I decided to build my own.

At the heart of this tester is an old-school filament bulb, serving not just as a nod to vintage tech, but as a clever and effective current limiter. If a connected device has a fault or short, the bulb limits the current and lights up—instantly alerting me without tripping breakers or damaging components. It’s a time-tested trick, now neatly built into a permanent tool.

I wanted full visibility and control, so I added analog voltmeter and ammeter gauges, giving real-time feedback on the test load. A rotary regulator allows me to gradually bring up the voltage, and several switches let me toggle modes and outputs cleanly.

The project box was custom-designed and manual-cut with hand U-type saw bow, but I also prepared DXF files, and I took care in arranging the front and back panels for both functionality and a classic aesthetic. Inside, all wiring is securely connected and neatly routed, with isolation in mind for safe operation.

This tool quickly became a go-to on my bench, and I’m excited to share it with the community. Whether you’re repairing vintage gear or just want a safer way to test projects on mains power, this Retro Circuit & Power Tester is a fun and useful addition to your workspace.

Safety First

Before working on this project, remember you are dealing with mains voltage (230V). Always take proper precautions:

• Double-check wiring before powering up.
• Never touch internal components while connected to power.
• Use insulated tools and wear safety glasses.
• Test your connections with a multimeter first—don’t assume anything is safe.
• If you’re unsure about mains wiring, consult someone experienced.

This project is designed to be as safe as possible, especially with the current-limiting bulb, but safety still starts with you!

Building Instructions

1. Front & Back Panel Preparation

I built my prototype by manually cutting the openings using a U-type sawbow, caliper, and compass—a bit old school, but rewarding. If you prefer precision, I’ve also included DXF files that can be used with laser cutters or CNC machines.

• Mark all hole positions carefully on the front and back plates.

• Use a drill and saw to cut out apertures for meters, switches, sockets, and connectors.

• Clean up the edges with a file for a proper fit.

• Apply labels using self-adhesive lettering (such as nail art letters) or a similar method.

• Also on back panel.

2. Mounting Components

Start by installing all panel-mounted components:

Front Panel:

• Analog Voltmeter and Ammeter
• Rotary voltage regulator knob
• Function switches (D / PL / RG)
• Output terminals (GND, N, PL)
• Neon Indicator lamps

• Note the use of a plastic potentiometer extension shaft between the regulator and the chicken-head knob. Two extensions should be connected to achieve the required length.

Back Panel:

• 220V AC inlet with fuse and power switch
• Power sockets (with protective covers)
• Power indicator lamp

Secure everything firmly with screws and washers.

Cover and Base:

• Lamp Holder (for current limit)

• Triac Regulator - see the attached DXF files for dimensions.

3. Wiring the Internals

Refer to the schematic (also attached in the project files) to wire up the system:

• Use color-coded wires and crimp terminals where possible to keep it neat and safe.

• Mains input connects to the switch and fuse, then branches to the voltage regulator and bulb circuit.

• The bulb is wired in series with the load, acting as a current limiter.
• Connect the meters to measure voltage and current going to the load.

• The rotary regulator controls output power (wired before the load sockets).

• Optionally, secure all wires using clamps or zip ties to prevent movement.

Tip: Always test your regulator circuit with a dummy load (like a lightbulb or resistor) before connecting real equipment.

4. Final Assembly

• Double-check all wiring and tighten screws.
• Mount the internal frame and attach front and rear panels.
• Power up with no load first and check all meters, switches, and the bulb limiter.

• Test each function using known loads (like a desk lamp or small fan).

Testing and Using the Retro Circuit & Power Tester

Once your Retro Power Tester is assembled and powered on, follow these steps to ensure proper operation and safe use:

1. Initial Power-On Check

• With no load connected, switch the device on.
• Verify the power indicator light (on the back panel) turns on.
• Ensure that the analog voltmeter shows around 230V when set to D + Ddirect mode (depending on your mains).
• The ammeter should read zero (no current flow).

2. Check Polarity Indicators

The front panel includes two neon indicator lamps, connected across the Line (L) and Neutral (N) at the output terminals.

When powered:

• The lamp connected to Line should light up.
• The lamp on Neutral should remain off.

If both lamps are reversed, your AC plug is connected with wrong polarity. Simply flip the plug in the wall socket to ensure the phase (Line) is on the correct side.

This is important because the incandescent bulb current limiter is placed in the Line path. Ensuring proper polarity guarantees that all loads are protected through the bulb in current-limited modes.

3. Mode Selection Tips

Use the mode selectorswitches to choose how the connected load is powered:

• PL (Probe Lamp): The load is powered through the series-connected light bulb. The 100W incandescent bulb serves as a current limiter. If a connected device has a fault or short circuit, the bulb limits the current and lights up brightly—instantly alerting you without tripping breakers or damaging components.
• RG (Regulator): The triac regulator controls voltage output. Best used for testing motors or voltage-sensitive loads.
• PL + RG: Combines both current limiting and regulation—useful for cautious ramp-up testing of unknown loads.
• Direct (D): Full 230V bypasses the bulb and regulator—only use this mode after confirming the load is safe.

4. Testing a Device Under Load

• Plug your device into the main test socket (the one labeled for selectable modes).
• Start in PL mode to check for shorts. If the bulb glows dimly or not at all, the device is likely safe.
• If needed, switch to PL + RG and slowly turn up the regulator.
• Observe the ammeter and voltmeter to monitor the current draw and voltage delivered to the device.
• Switch to Direct only once you’re sure the device behaves normally.

That’s all.

Feel free to leave your comments, questions, or suggestions below—I’d love to hear your thoughts and ideas for improving or customizing this build!