Dim Bulb Tester (light bulb current limiter, North American, 120V)

This is a box for a light bulb current limiter.  A light bulb current limiter uses the properties of an incandescent light bulb to protect a device under test from over-current situations.  It is commonly used in troubleshooting and testing relatively low current line powered electronic devices.

It works by placing an (incandescent) light bulb in-line with the device to be tested.  When turned on the current-pull from the device under test can be inferred from how much the light bulb lights up, whether it flashes and then fades (indicative of caps charging up).  

More importantly, if the device under test experiences a significant short, the light-bulb will fully light, with it's now hot filament going into a high resistance state (~240Ω for a 60w bulb), in turn reducing the voltage experienced by the device under test and limiting the current to 0.5A worst case (dead short). With prompt action to turn the device back off, additional damage, breakers tripping, expensive flashes and noises can be avoided.

This is a box that is designed to take off-the-shelf components to build such a device.  It is designed for North American power standards (120V and NEMA standard components). I sourced the following items from my local Home Depot:

1. Leviton 5241-WS (Switch and 3-0Way Switch, 15A-120/277V AC/CA). This dual switch has both an on/off switch (single throw) and a 3-way switch (double throw), the later of which can be used for a current limited / bypass (unlimited) switch.

2. Leviton T5015-WS (Tamper-Resistent Single Outlet, 15A-125V)

3. Leviton R60-00055-000 (Weatherproof Socket, Black 55, SKU 078477839591)

4. 9 ft. 14/3 Medium Duty Tool Replacement Cord, Black (or similar, SKU 850047764119)

5. WAGO 221 (3 wire, qty. 2)

Additionally, a few general hardware items:

1. Heat Set Insert, M3, 4mm, ø4.6 OD (qty. 10)

2. Heat Set Insert, M3, 10mm, ø5mm OD (qty. 2)

3. M3 Hex Socket Head, 10mm (qty. 6)

4. M3 Hex Socket Head, 20mm (qty. 2)

5. M3 Hex Button Head, 10mm (qty. 4)

The box is broken into four parts which should be printed from the noted materials.

1. Cover Plate (PETG)

2. Main Box (PETG)

3. Cable Clamp (PETG)

4. Strain Relief (TPU 95A)

Assembly:

Insert 10x 4xø4.6 O.D. M3 heat inserts into the six perimeter locations and four interior standoff posts within the main box.

Insert 2x 10xø5 O.D. M3 heat inserts into the two holes in the cable clamp part of the main box.

Insert TPU strain relief into box and glue with cyanoacrylate glue (CA/superglue).

Insert cable through strain relief, and clamp using the cable clamp and 2xM3 Hex 20mm socket head screws.

Pass the wires from the light socket through the faceplate (from top to bottom) and fasten the lamp socket to the faceplate using cyanoacrylate glue.

Wire the lamp socket, plug socket and dual switch, optionally using two WAGO connectors which can be located in the two slots within the box (one for earth, one for live connections to plug socket).  

N.B. I am NOT including a wiring diagram, If you don't know how this should be wired then you don't need this and shouldn't be building it. 

Fasten the dual switch and plug socket into the standoffs within the box, using 2xM3 Hex Button Head 10mm. Don't use socket head screws or the faceplate will not fit flush.

Fasten the faceplate to the box using 6xM3 Hex Socket Head 10mm screws.

Insert 60W (or 40/75/100W depending on your requirement) incandescent light bulb.  If you think you can use an LED, CFL or other non-incandescent light bulb here, then you don't know what you are doing and shouldn't be building this. An old-school incandescent light bulb is absolutely required.

Now, test connectivity thoroughly with a multimeter whilst disconnected from power. At the minimum:

1. No connectivity between earth, live.

2. No connectivity between neutral, live.

3. No connectivity between input live and socket with main switch in off position.

4. No connectivity between input live and socket live with main switch in on position and bypass switch in current limit position (and no bulb inserted). Connectivity should be present only after bulb is inserted

5. Connectivity between input live and socket live with main switch in on position and bypass switch in bypass position.

Additional testing of voltages in on/off and bypass/limit (with and without bulb) should be done after first connecting to power.

Ensure the switches are clearly labeled for function.  In the example picture provided, i've labeled mine using a laser.  I've provided STEP files, so if you want to use multi-color you can modify the faceplate to add your lettering.