Dim-bulb series limiter box

Series limiter-tester

A dim-bulb series limiter provides inrush current limiting for testing electronics by inserting an incandescent lamp in series with the mains supply. It protects the device you are testing and helps prevent fuses from tripping.

I printed this in PETG with rectilinear grid infill at 15%. When I put the frame together I was surprised how sturdy it felt. Parts are designed so that supports will be mostly unnecessary. Maybe just in the screw holes for the countersunk recesses of the power inlet plate and plain plate.

Print parts one by one, decide which ones you will need as you will decide how you want to build your limiter box. I see 3 main options:

1. Full build - with power meter and mains on/off switch

2. Pragmatic build - without power meter

3. Plainest build - without power meter and without mains on/off switch

I would not build the plainest variant, even though that is possible and simple. I value the added safety that comes with the ability to completely switch off power to the circuit, without needing to unplug the box.

If you not only print the box but also do the wiring, make sure you know what you are doing. You can read more about the electrical side of things in the attached PDF (under “other files”). Pay attention to disclaimers therein; they are reasonable warnings to stress that you are to be very cautious with anything that is to do with mains power, and it is your responsibility to ensure compliance with local electrical codes.

More than a Dim-Bulb-Box

Educational/lab value: In terms of testing what you can do with a full build is rather more than just protecting your device under test (DUT). You can measure and compare the parameters both at mains and at DUT inlet, thus you can see how much the bulb is restricting/protecting the device. And using a breakout cable (also known as a line splitter) you can measure full operational parameters of any mains device(s) without dim-bulb  current limiting effect.

If you decided in the future that you needed data logging - say you are developing your electronics lab or smart home - replace the test module with arduino style PZEM-004T board which allows you to continuously output data. You can transfer data either with a straightforward USB cable attachment from the board to your computer, or you could include an ESP32 chip to do data transmission over Wifi or Bluetooth. To see what awesome things can be done with this kind of setup, have a look at the Michael's tinkerings project for smart home.  And designing a different “meter-plate” in FreeCAD would be easy (if you got stuck, I could help) and then we could just add the new meter plate to the design.

Meter I used

AC 80-260V 100A Current Voltage Amperage Power Energy Panel Meter LCD Digital Display Ammeter Voltmeter Multimeter with open CT (Brand: Fasizi) on Amazon UK. I saw quite a few meters with same size specs. Seller says: ABS, small size 8×5.5×6.5 cm, about 150g, palm size.

Printing & assembly

• Print Top and Bottom and sideWall x2. 

• Using M3x25 hex socket bolts and DIN 562 square nuts, tightened finger-tight plus 1/4 turn, assemble the four parts together

  • now you have a sturdy frame

  • you can attach side walls (e.g. socket, mains switch, plates) and do the wiring

• Side wall thickness will vary a bit depending what components you have. You will attach them using M3x20 bolts with normal hex nuts. For some components it is possible you may need to use 25 mm bolts instead of 20mm ones. 

The DUT (device under test) socket, the mains power switch become side walls. Depending on your design you substitute the item you are not using (e.g. Meter Plate) with a Plain Plate. The Power Inlet plate has a small cutout which accommodates standard 7mm power cord (usually referred to as 3-core flex). The incoming power cable is secured against the bottom-plate using a cable clamp (cable grip printed from cord-fix .stl) held by two M3.5 self-tapping screws that thread into plastic bosses on the enclosure. This is the same type of strain relief commonly found in UK plugs and trailing sockets. I am pretty sure those screws will be included either with the DUT socket you will buy or perhaps the DP mains switch, so you will not need to buy the screws, and most likely you will be able to use the cable grip from there (I did, so I did not print the cord fix). Power cord needs to be fixed once on the inlet, after that the internal connections are made with the spliced wires that maintain their insulation intact.

Note for designers about the FreeCAD file:

The parts are made with Part Design Workbench - each is included in a fully independent "body" object. If you modify a part, then you save the file, select the part (a body) and export it as STL (you can use Ctrl+E shortcut). If you visualize the parts all at once, you will notice that some are orientated upside down (like the meter plate). This does not matter for printing; the parts could have been designed even overlapping each other. You can add an assembly to this file, if you wish, but I prefer to keep things simple and working.