Debugging the Dead Power Supply

I don't think I mentioned it (and am too lazy to scroll down to check) but a friend of mine stopped by the other day and I was showing her all the stuff I've built so far. I loaded the switching power supply with 3 ohms to demonstrate the noise I get, but later I realized the -5 volt output was no longer working … odd. Tonight (Friday, by the way, and yes, I have had a few drinks — a chilled shot of Ouzo, a Saranac Pale Ale, and a Genesee can at home) I checked the power supply and inexplicably, the LM324 died — the op-amp that controlled the pulse-width modulation stayed at the positive rail for no reason. I pulled it and threw it away, even though it might have just as well been a loose connection. Replacing it worked just dandy. I tried connecting every LED — both strings, all three colors — and the input drew 0.53 amps for 0.76 amps out (in power, that's 6.36 watts in for 3.8 watts out for an efficiency of 59.7% — cool: almost 4 watts of output through LED's!) Anyway, I decided that the switching noise wasn't going to be much of a problem. Even with all three colors on one string lit, the noise is noticeable but ignorable … it adds an element of irritation. If I go to just one color lit (which, technically, is the load under bit modulation) the noise becomes nearly inaudible except very close. I have my fingers crossed.

The Savonius Windmill Generator

I decided that in this last week or so before I leave town, I'd pick up one more task: build a Savonius generator on the Bike With 2 Brains. So on Tuesday morning I got up and started thinking about designing it. I figured out that the bicycle seat posts are the right size to fit the "#608" size skate bearings — I had bought a pack of them because the 8mm hole is just a hair bigger than the 5/16" (7.99mm) threaded rod and bolts I planned to be using.

Wednesday I got up and started designing it in CAD. By 11 I came up with building a basic cube frame with a pair of centered cross-beams from side-to-side to hold the rotor. The key part is the detail below with the arm to hold the generator with a friction-coupling to the baby-stroller wheel (which is ultimately why I went with 5/16": it's the shaft diameter of the stroller wheel.) The diagram of this detail is below:

Windmill tensioner detail showing a bunch of circles and lines.

Basically, the rectangles on the left and right are the frame mounts and the center horizontal rectangle is the support bar. The small central circle is the bearing and the large circle is the baby-stroller wheel with the centered diagonal rectangle representing the 2×4 used for the rotor. The angled dimension lines surround the generator bar — the larger circle below the baby-stroller wheel is the motor housing and the smaller is the motor spindle (a wood dowel.) The line connecting the bolt (it's supposed to be a bolt and looks better in CAD zoomed in) is linked to the center frame by a spring.

I built a rotor that afternoon. The original one I designed had two problems: first, the shaft holes weren't well centered, and second, the top and bottom plates were not exactly parallel. I paid more attention to those two problems and things came out much better. I also used whatever remained of the vent pipe I had bought — it's just like the original prototype except that it's about 14" long. I even got the bearings mounted on the end and thought, "gee, that's it?" expecting it to be harder to make.

Thursday I got up and started working on the frame. I welded together a basic frame and rethought the cube shape: instead I went with a wide bracket for the rotor bearings and put the vertical members on either side of that. By that evening I had finish-welded the frame and tack-welded the brackets to mount the generator. It was almost hard to assemble it in the very light breeze because the rotor would spin so easily.

Friday I built the rest of the motor mount, mounted the preferred motor, and tested it against the fan in the attic — essentially the blower from a furnace. I was able to get about 4.5 volts open-circuit, 2 volts into 5 ohms (0.8 watts) and 1.5 volts into 3 ohms (0.75 watts.) I had to use a weaker spring than I started with because the friction of the stoller wheel against the dowel was too much when it was pulled tight. I don't know the wind speed nor the rotation speed, but it was pretty dangerous looking. I finished up the brackets and threw on some paint in the evening.

The pretty-good 12V-5V DC-DC converter

I put the power supply on a board and wired it all up. I figure I'm getting around 60-65% efficiency — far better than the 42% efficiency of using a linear regulator. In other words, if my lighting stuff (the motivation for the 5-volt supply in the first place) outputs 1 watt, the battery will be hit with 1.5 watts whereas if I had used a linear regulator, it'd see 2.4 watts — the 84 watt-hour battery could run for 56 hours instead of 35 hours. Heck, it's almost half as much time as I put into making the fucking thing!

5 volt success on the breadboard

I got around to the basics of the circuit below. I set up the MOSFET in the configuration below and drove it from the op-amp. I added the PNP in a similar configuration to ensure that maximum current would get to the gate — I need to get that gate to 12 volts, and the 20 mA output on the op-amp wouldn't cut it. I got nice square square-waves but, using a simple pulse-width modulation to the capacitive output, I was getting the same problem of current-in equals current-out. I set up the buck configuration again and finally got some success: 6.6 watts in and 4.2 watts out for an efficiency of 63%.

At this point I realized the output-as-driven would end up as 5-volts from the positive battery rail. I remembered having 7905 negative-rail 5-volt regulators around (and always thinking, "what the hell will I ever use these for?") I switched to the terminology where the positive battery terminal was ground (calling it 0-volts) and the negative rail was -12 volts. I spent some time diagramming the circuit (using CADintosh from Lemke Software, GMBH.) I went back and set things up like I had drawn, made a few changes, and did some final tests on the breadboard. By varying resistors, and using the 51-ohm load I got 97mA out (4.95 volts at 470 mW) with 52 mA at the input (624 mW) for an efficiency of 75%. I tried the 8-ohm load and got 620mA out (4.96 volts at 3.08 watts) with an input current of 419 mA (5.03 watts) which gave me an efficiency of 61%. I think this looks pretty good.

So, after three days of slaving for a total of 20 hours or so, here's the circuit …

Here's the final (so far) circuit diagram for the inverter.

You can look up explanations for a twin-T oscillator and a buck converter for the oscillator and the inductor on the Internet, but one thing that I don't think is too obvious is that the diode after the op-amp is there so the base of the PNP transistor can actually go to the rail — the op-amp may not reach it, but the diode will be shut off so the 470-ohm resistor will shut the transistor down. The other thing is that the inductor has no value. I don't know what it is: it's a yellow torroid with red magnet wire that I pulled out of a dead computer UPS, so I don't know its value, but of the ones I had lying around, it worked best. I guess I should put parallel lines below it because it's iron-core … oh well.