Small DC power supply

I mostly end up using bench supplies for two things:

1.) Seeing which color a random unlabeled clear LED is.

2.) Charging batteries, especially pesky 4.35V lithium ion cells that don't get full capacity from standard 4.2V chargers. 

Seeing that I don't need fancy features like negative rails or lack of switching noise, I decided to build a cheap tiny "bench supply" using some Constant Current-Constant Voltage buck converter boards I found off Ebay and various voltmeters and ammeters, and then laser cut a case for it. 

No, the LED is not drawing 10A; the ammeter is broken.

Here's your typical cheap buck converter board off Ebay. 

The older ones have two trimpots (one to set voltage and one to set a current limit), while newer ones like above also have an additional pot to show the end-of-charge indicator LED (seeing as these boards are very frequently used as cheap CC-CV battery chargers). Some of the cheaper ones don't have current limiting, and only have a single trimpot for voltage. This one, being shipped from the US, is a bit more expensive at around $7 for a board, while those shipped from China can be had for $3-$4. 

I had an older one that was previously used to drive LEDs. 

The idea was to replace the trimpots with actual pots to control voltage and current, and add a voltmeter and ammeter, and then power the whole thing off some old 20V laptop power supply. 

In one night I hashed together this unholy mess onto a random scrap piece of acrylic:

Being obviously extremely derpy, this clearly wasn't acceptable for long term use, so I decided to make something neater.

To power the voltmeter and ammeter, which run off of 5V, I found a 78L05 voltage regulator on the buck converter board (which probably powered the current-limiting circuitry) and a ran wire off it.

Strain relief your wires kids

For the pots, I bought your run-of-the-mill cheap 16mm linear pots off Tayda, in the same values as the trimpots. The banana plugs were crufted.

To make the case, I spent an afternoon making derpy CAD models of each component in Solidworks, and CADed a laser cut acrylic case. The case is held together with tabs and T-nuts.

I then used the Epilog laser cutter over at BU to make the case out of some 1/8" acrylic:

And then I assembled the case and inserted the components, bolting them on with M2 and M3 screws and nuts I had lying around. I unfortunately didn't have standoffs lying around, which I should have used.

To insert the tiny M2 nuts into the T-nut slots, I used a magnetic flathead screwdriver tip to hold the nut in place while I inserted the screw, like such:

A couple zip ties were used to clean up the wiring. Turns out my ammeter died and shows "10.7A" no matter how it's connected, which is unfortunate. Also, looking back, I definitely should have used multi-turn pots, as setting precise voltages is very difficult with pots which only have 270 degrees of travel.

Updates

After not posting for almost a year, here are the unfortunate updates to the last two projects:


-The skeletonDAC project kind of died due to me acquiring another laptop. The Thinkpad T410, in addition to having a far better keyboard and ergonomics than my old Dell Latitude E6410, also has a much better headphone output with very little noise. The T410 was in turn replaced with a Thinkpad T450s, which has even better headphone output.
-The scooter project similarly died due to me discovering bicycles. My initial impetus to build a scooter was for short-distance transportation. Unfortunately for the electric scooter project, someone down the street threw out an old French road bike. It turned out bicycles were fun to ride, faster than most electric scooters, cheap, didn't have range issues, and provided much-needed exercise.

I did start building parts of the scooter. I ordered some five feet 5" U-channel off McMaster, which I cut into two 30" sections, which was split with Aidan Rose (who actually built his scooter a couple of months ago).

Only one half is actually a scooter at this juncture. 

Mike at MITERS showed me how to make a battery pack out of A123 26650 cells lying around at MITERS. I decided to run a 10S2P configuration, which fit well into the U-channel and gave me a reasonable voltage to play around with. The cells were arranged in a 5x4 block consisting of 1S2P groups, making two 5S2P sub pack which were charged with two separate 6S chargers. 6S chargers are pretty cheap (a Turnigy Accucel 6 was about $25), while 10S chargers (such as the iCharger 1010B+) are decidedly expensive, so I decided to charge the cells in series using two 6S chargers using non-grounded wall-warts.

First, I checked the cell voltages to weed out dead cells (the A123s lying around MITERS had failed QC and were subsequently donated, so about a third of them were dead)

Then I glued the cells into 2-cell groups, and solder lathered over the terminals with a high powered soldering iron. These were then paralleled to form the 2P part of the pack configuration.

There were these tabs lying around MITERS which were used to connect the cells. The cells are meant to be tab welded, but I didn't have access to a working tab welder, so I soldered them.

The tabs were also then tinned with blobs and globs of solder:

The tabs were then pressed onto the cells, with the blobby and globby sides facing each other, and then pressed down with a chisel tipped soldering iron.

Not exactly the neatest example.

I did the same thing for the other ends.

To connect the sells in series, I put tabs perpendicular to the parallel tabs, and then folded them (it's hard to explain, but in the picture below, the group of cells in the foreground were "folded" to form the group of cells in the background).

I then added JST-XH balance leads and balanced the cells with various cheap 6S RC hobby chargers off Hobbyking. I bought some JST-XH leads off Ebay. Being paranoid, I checked the pinout, and it turned out that the leads I bought were wired such that the black wire was the most positive one. Never make assumptions about colors, it turns out.

To prevent shorts, I wrapped the cell joints in electrical tape.

To add stiffness to the length of the pack, I cut out bits of some plastic lying around MITERS (probably polyethylene) and glued them to the cell group lengthwise.

Then, using some very large heat shrink purchased off Hobbyking, I covered the pack in heat shrink:

In MITERS tradition, I then started to "hard case" the whole pack using 2L soda bottles. Apparently, using sizable heat guns, soda bottles make pretty good heat shrink.

Watt-hour rating is probably incorrect

Also someone threw out a nice Peugeot mixte frame and various components, which was subsequently turned into a functioning bicycle:

Which also later sprouted a milk crate off the rear rack, and afterwards several hundred miles were happily ridden on it, and which kind of obviated the need for a scooter.

Some point later I also realized the whole scooter was going to be excessively heavy and unnecessary and a pain to carry over the summer, and most parts were subsequently sold to my friend Will Zhang (who may or may not have a blog).

Then over mid-summer, I realized that a scooter would still be useful, as I could freely bring them aboard public transportation (such as the MBTA Green Line and bus system, which ban bikes for being brought aboard). I wanted something much more lightweight than the original U-channel design. Inspired by Charles Guan's lightweight RazEr rEVolution, and reading his excellent primer on DIY scooter hub motors, I decided to start building a hub motor scooter. I found a small 12-tooth stator, and several hours in Solidworks and researching magnets resulted in this:

Then I realized folding bikes were a thing, which could be brought on the train and had most of the advantages of bicycles listed earlier. Also school then started, which wrecked my free time for projects. The hub-motor scooter is still technically a project, though one I haven't really been able to work on, between school, work, and BU's new FSAE team.