Project Log

November 11th, 2003: Lab Day 3

Where: Jobst 248

What got done:

We got the motors and solar cells in on Thursday, so today was spent testing them. The motors look acceptable, but the solar cells are a bit of a disappointment.

The motors are Mabuchi FA-130RA-18100 models, which have a peak RPM of around 12300 at 3V / 0.15A. That is a lot of current, but motors are current-mode devices so they'll run with less, just slower. We need to test that theory with a real variable current source, but these little guys look very nice. If they're not the motors we need, they're at least close.

The solar cells, on the other hand, are a problem. Though the "data sheet" promises 55mA @ 3V, we were utterly unable to get that much power out of them. Our best result was with a 200W incandescent bulb held 8 inches from the panel, which gave a whopping 4mA @ 3V. That's, er, not a lot. It forces some rather serious redesign decisions on us.

What needs doing:

We need to find out why the solar panels are so weak. There are at least two possibilities here. One, the electrodes have a thin plastic coating. It doesn't peel off, you have to either melt it off or solder through it. If we didn't get all the way through the plastic, it would present a series resistance, which would severely limit the current.

The other, more plausible idea is that the frequency response of the solar cells doesn't match the light spectra we tested it with. Fluorescent lights don't put out the same kind of light as the sun, and if they're sensitive to sunlight - as you'd expect from a solar panel - then they're just not going to be as effective indoors. The problem with this idea is that we've been unable to find any datasheet for any solar cell that shows a frequency response curve, or even a graph of power per lumen. This makes it a bit hard to design with.