Old-World Light Bulb Load

When working with power electronics, quite often the right piece of test equipment is an old-fashioned incandescent lamp. They make good ballast resistors and dummy loads for a variety of devices. A 100W bulb starts at about 10Ω when cold and works its way up to about 150Ω, so they even adapt well to a wide range of voltages. Plus, they are cheap, available in a several values, and generally do not explode on failure.

As I have been designing a little power supply myself of late, I decided to make myself a little box to hold a pair of light bulbs wired to standard 5-way binding posts. For some reason, I decided to make something nice, and to try to improve my woodworking skills at the same time.

I am a fan of all things steampunk, and so I decided to try to make something with a similar old-time feel.

I started by finding the essential electrical parts. I found the small porcelain light bulb sockets at Home Depot, and I picked up the standard 5-way binding posts from Radio Shack. I have to admit that I was very tempted to use knurled brass thumb nuts instead of modern binding posts, but since this device is also meant to be useful, it has to accept banana jacks, and so I resigned myself to having a little bit of plastic.

My original plan was to simply mount all these parts to a board and get back to work on my power supply. The problem was that the binding posts expect rear connections. The light sockets are more flexible, but they do allow rear wiring.

I quickly came to the conclusion that I would need a box.

With the idea in mind that I would need a box to mount these parts on, I started looking around my workshop for something suitable. I had plenty of plastic and metal project enclosures, but nothing had the right look. Finally my eyes came to rest on a sheet of oak craft board that I had sitting next to my scroll saw.

The board was 5.5" wide, just big enough to mount the two fixtures side by side. I measured off a piece 4" long and cut it. Then I measured a set of pieces a nudge more than 1" long, and cut those off. Two of them I left 5.5" long, two of them I cut down to 4" long.

I measured and marked the location of fingers for box joints, marking on the inside surfaces the order in which they assemble. This is important: while they are all measured the same way, they only go together in one order, as slight errors occur from freehand cutting.

I cut the slots out with my scroll saw. To make these square cuts, start by cutting two slots straight in, keeping the blade on the waste side of the line. After cutting to the final depth, back the blade straight out. Once both slots have been made, put the piece back into the blade at an angle, cutting through the waste piece while turning, reaching the bottom about half-way between the two slots. At this point, the blade should be facing in the right direction to finish half the cut, resulting in a nice sharp corner. Then just make another cut, starting from where the last cut began, but in the opposite direction, making another sharp corner.

The mating pieces will usually not fit the first time you try. I used the scroll saw to take off paper-thin curls of wood until the parts fit. You could do the same, or if you are not as comfortable with scroll saw operation, you could file them down with a wood rasp.

After the box was ready to assemble, I marked and drilled all the mounting holes.

No old-time woodworking project is complete without a touch of brass. The shiny yellow metal complements stained hardwood nicely. This was one of the most complicated parts of the project, and not even strictly necessary, but it looks good.

I started with some craft store brass strip, which I cut to length. Brass is remarkably difficult to cut with a scroll saw. My first attempts caused a blade break, so I searched the internet for advise before continuing. It turns out that the trick is to use a backer board, so I picked a lump of pine from my scrap bin and put it under the brass. After that, the scrolling went smoothly, although slowly and noisily. I imagine that the correct tool for the job is an abrasive band saw, but I do not own such a tool.

After cutting, I rounded the corners on a stationary disk sander. I then tilted the sander table to a 45 degree angle and beveled the edge. The resulting edges were rather sharp, so I cleaned them up on the sander without using the table. When using a disk sander without a table, hold the piece at an angle away from the direction of rotation. Otherwise, the grit will catch the workpiece, and even assuming you can keep a grip on it, this will cause the piece to vibrate. Holding the piece at an angle causes the grit instead to push the piece away from the wheel, allowing fine control and reducing the amount of material it removes.

I then took the plates over to my drill press and drilled out the mounting holes. I have to admit that I made a mistake here. Brass is a slippery material. Make absolute sure that you have a good center punch before trying to drill it, and drill a pilot hole. My own center punches were not well enough defined, and the pilot holes drifted. Thankfully, I managed to mostly correct their locations before drilling the mounting holes. Mostly.

After that, one thing led to another, and I had managed to scratch the brass. Brass is, after all, a relatively soft metal. I sanded it down with 400 grit sandpaper. If I had any 200, I would have started with that, and it would have given me better results. After sanding with the 400 grit, I ran over it with a Dremel wire wheel, giving it a nice shiny brushed appearance.

Some time after I took this picture, I decided to refinish the brass parts, as I had picked up some 220 grit sandpaper. I did it the same way, but I decided to forgo the wire wheeling.

Because friction fit is never enough, the time arrived for me to glue the box together. I used Elmer's brand "Stainable Wood Glue", because I figured that I would probably end up with glue in a few random places and a stainable glue would probably help to hide it when the time came to stain the wood.

I started by taking the workpiece to the scroll saw and enlarging a couple corners to improve the fit under pressure. If anything, I may not have been aggressive enough in doing this, but it helped a lot.

I rolled out a sheet of aluminum foil, poured a little glue out onto a corner, and used a Q-tip to apply the glue to the mating surfaces of the box joint. As the process is rather time sensitive, I wasn't able to actually get pictures of this step in progress.

After the glue was applied and the box was assembled loose, I clamped it every which way and left it to sit for a few hours.

The next day, I removed the clamps. The box was, indeed, a single solid piece. I applied some wood filler to the surface, and scraped the excess off with a putty knife, and then left that to dry overnight.

The next day, I sanded the surface smooth with a bit of 220 grit hand sandpaper. Then, using some 80 grit sandpaper on my stationary disk sander, I cleaned up any overhangs on the box finger joints. I then rounded down all the edges and corners.

After this, I applied a second coat of wood filler and then sanded it down in preparation for surface finish.

After filling the grain and the box joints with wood filler and sanding, I applied a coat of Minwax water-based rosewood color wood stain with a cotton cloth. I was pleasantly surprised by how well the wood filler soaked up the stain, leaving a very even finish. I then left it to dry overnight.

I waited another day for the weather to clear up, and on a nice sunny day with little wind, I applied three coats of Minwax Polycrylic matte topcoat, allowing an hour between coats, and sanding lightly with 220 grit before applying the next coat.

The results are pretty good for a first try.

The last step remaining is to assemble all the parts. I had to clear the holes with a hand-drill, as some wood filler had adhered to the insides, but otherwise, this was completely straightforward. I mounted the fixtures down with some #10 screws.

I fed some 18AWG wire through the holes in the back of the bulb fixtures, and soldered them to the binding posts. I fastened the other ends in the screw terminals. After that, I put the retaining rings on the fixtures, and added some bulbs.

I made a bottom out of a sheet of mat board from a craft store, which I cut to size using a rotary cutter. I rounded the corners of the mat board on my disk sander, which worked far better than I expected. I then glued this to the box with some rubber cement.

I was out of rubber feet, so I made some with a bit of craft store self-adhesive foam sheet. I made a punch by sharpening some 3/4" brass pipe on my disk sander, and while pressing the punch into the sheet against some scrap wood, turned it until the material was cut. This seems to be a pretty easy way of making non-marring feet, although the foam is a bit soft. I applied four of these improvised foot pads to the bottom.

For the purposes of demonstration, I connected the bulbs in series and then to 120VAC.