Power Supply Hurdles

2018-09-01 16:29 - Making

My last post was about a successful step in a long delayed project. I've continued making progress, but mostly progress towards hurdles I haven't quite passed yet. I did cut out my case designs in acrylic plastic:

Laser cutting the bottom case part. The bottom case part, protective layers removed. The top case part, protective layers removed.

My case design involves a bottom part, which is also the back (after a bend). Plus the front part, which is also the sides and top after a few more bends. I didn't quite think about making all these bends, when doing the design. I folded the sides in first, and that went pretty well. Except at the left side, near those big openings in the middle. Acrylic is brittle, and I put some hairline cracks in the very narrow part across the middle and the slightly wider part near the bottom. Mostly cosmetic, so that's okay. Except that left folding the top down, which went even worse.

The case front, shattered after a bad attempt to bend the top down.

In addition to adding weak points, all these holes leave fewer places to hold on. I ended up shattering this larger front piece of the case completely, while making that last bend. Quite the bummer, but at least it leaves me the opportunity to double check a few measurements. I've got one spare piece of plastic to use for a second try. And I know to be careful when I attempt the (simpler) back piece as well.


With that physical part of the project on hold, I returned to the electrical part. There's power in, which goes through a (huge) transformer, salvaged from a bad UPS. This drops line voltage to around fifty volts, which is rectified by a circuit also cobbled together with parts from that bad UPS and some old bad computer power supplies. This goes into a pair of RDTech DPS modules, which handle voltage and current limiting for the adjustable outputs. The transformer has another tap, which produces around 18 volts. I'm going to separately use that to power a fan and a USB supply module (which was visible in the corner of the fresh-cut before-bending case front picture).

An end-to-end test set up of my power supply. Ripple voltage at the rectifier output. Power being drawn during this test.

Here's all the key parts of that set up on the bench as a test. The good news is that it works. The bad news is it doesn't work very well.

The full bridge rectifier has just a pair of 66µF capacitors smoothing it out. At first that seemed okay enough. Since I did some very early tests I've gotten an electronic load, at bottom right of the first picture above, which I can use to do more thorough testing. In that picture, it's drawing twelve volts at two amps, well below the 50V / 5A theoretical upper limits (of the DPS modules). But even at this low power level, it's causing around nine volts of ripple at the rectifier output. Far too much! I know that adding an inductor can create a much more stable rectified output. It drops the maximum voltage level, but this much ripple will too, so I'm going to experiment with inductors I can salvage, for the more reliable of the two lower voltage options. (I don't really need that whole 50 volt output range, anyway.)

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