Lab 2L.2.3 and 2L.2.4 (as well as some exercises in Lab 3) called for using a transformer to step mains voltage (115V) down to 6.3Vrms and then do various stuff with that voltage. The parts list simply called out a transformer part number (https://www.mouser.com/ProductDetail/546-266JB6) , which arrived with bare wires:
I wasn’t about to stick the bare wires into the socket, and even just soldering on a power cord seemed unsafe. I wanted a proper enclosure and a fuse. So I decided to build one. This proved a bit more of a saga then I expected.
The first thing I wanted was an entry module. Following advice in the AoE, I didn’t want to roll my own, but rather purchase one that took a standard IEC 320-C14 cable (the standard “computer” power cable), had a fuse holder and a switch, as well as safety features like making it impossible to open the fuse compartment while the cable was plugged in.
After poking around on Digi-Key, I settled on the TE Connectivity PS0S0S000 unit. The basic line is available with all sorts of features like noise filters, but I didn’t want that, since the noise was part of the lab. The module required more work than I expected, The three basic parts of the module – the receptacle, switch, and fuse holder – weren’t connected, I had to make up a bunch of jumpers.
The module uses automotive-style quick connect tabs in a less-common .187″ x .032″ size (.25″ is much more common). I found some at a local AutoZone, but they were meant for 16-14 gauge wires, whereas I was working with 20 and 22 gauge (the transformer leads were 22 gauge, so I saw no reason to go any thicker than 20). Fortunately, there’s a trick where you can use a larger crimp terminal to hold a thinner wire plus a stub to make up the thickness:
Using a good crimper is a must. Forget the cheap sheet metal ones that come in kits with the crimp connectors. Get a good ratcheting one. I don’t recall where I got mine, but something like this one from Amazon will work great. Or you can get a kit with a bunch of jaws.
The next challenge presented by this entry module was figuring out the fuse. The datasheet says “1 or 2 fuses 1/4″ x 1-1/4″ or 5 x 20mm”. As far as I can tell, the “two fuses” are only meant for 220V setups, so I opted for one. I had trouble finding a 1/4″ x 1-1/4″ fuse that I liked (I wanted ceramic, not glass), so I bought a 5x20mm kit from Amazon. What I didn’t realize is that the metric fuse was much smaller, and it took a bit of experimentation to get it into the fuse holder just right.
The datasheet didn’t help much, but Digi-Key had a video on how to replace the fuse in these modules, and between that and some probing with a continuity tester I was able to figure it out. The fuse holder can go in two ways, depending on whether you use it for 110V or 220V. Unlike the one in the video, mine didn’t labels. The important aspects were that the smaller fuse has to go into the bottom (towards the “feet”) part of the holder, and the clip (the bit with the blue pen on it) goes on the other side. Note the orientation of the power rails in the photos. I ended up labeling the top to make it clear in the future:
Jumpers had to be run from the plug terminals, to the switch (which is DPST, disconnecting both sides of the line, important in case of an improperly wired outlet), to the fuse holder. Be careful of the switch orientation – it’s rotated 90 degrees to the other parts – use a continuity checker. (Note: I added heat shrink to the jumpers later on, for extra safety)
Wiring the transformer came next. The primary coil had two ways to be wired, one for 115V the other for 220V, and that was fine. For the secondary coil, the included instructions offered “parallel” and “series”, with no further info. A google search found their website which showed that I wanted the series connection.
To house the whole thing, I chose an aluminum project box I found at Digi-Key – at 6″x4″ it was the perfect size, and it came with perforated side and top panels, which meant I didn’t have to worry about the transformer overheating.
To make this work, I needed to make a rectangular opening for the entry module, plus some holes for the binding posts, transformer hold-down screws, and a grounding lug.
Holes are easy – best way to make them in something like this is a step drill, available at most hardware or building supplies stores. This one goes up to 1/2″, which is perfect.:
To make the rectangular opening for the entry module, I scribed some lines based on the datasheet, drilled a starter hole, and then used a nibbler to cut to the lines. The nibbler was a cheap one from Amazon. There are definitely better ones out there, but this one worked ok. Jammed on a bit of waste occasionally, but was easy enough to clear.
To get the clearance to use the nibbler from the “inside”, I had to unbend the box. The aluminum looks like it can (and did) take a single incident of being unbent and rebent at each bend, but I wouldn’t do it twice. Hint: Unbend the side you want to nibble, not the opposite side (the way I did initially). Much easier that way.
The transformer flange had two mounting holes. Two holes in the bottom of the box, plus some generic bolts and nuts, secured it next to the output box:
I should probably add some stick-on vinyl feet or something, since right now the box rests on the two screws.
For the output, I set up binding posts. I wanted paired ones, so that a standard double-banana plug could go in, and didn’t want to rely on getting the distance just right, so I got a dual set made by Pomona. Unfortunately, I could only find double binding posts in red and black – not really right for AC.
To provide isolation from the chassis, the holes had to be 1/2″ in diameter, so that the plastic bushings engaged. The step drill worked great for this.
The secondary coil of the transformer were crimped to some ring terminals (correct gauge this time, 22-16, found at a local Ace Hardware store) and then attached to the binding posts with the provided nuts. The two halves of the coil were connected together with a crimp-on butt splice.
Finally, a ground lug and jumper were made, to tie mains safety ground to the chassis. This is an important safety measure – if the hot wire should come loose on the inside, it could electrify the case, presenting a serious shock hazard. By grounding the case, such a failure would short the hot wire to ground, blowing the fuse or popping the circuit breaker rather than shocking the user.
The grounding bolt is also brought to the outside, in case one wants to tie the rest of the project to mains ground, though this is rarely a good idea – one of the advantages of using a transformer like this is to provide galvanic isolation from the mains.
That’s about it. I added heat-shrink tubing over the crimp connectors to everything not protected by the fuse, printed on some labels, and closed up the box:
The project works flawlessly, in that I was able to get the correct AC voltage across the binding posts and complete the rest of the lab. More on that in Part 3.
(While I have a pretty good metal shop with all sorts of measuring and marking tools, it’s currently in storage. So this project was done without them, and it shows – nothing is square to the box. Really should have gotten a machinist’s square or something before doing this. Oh well).