Re: Solar Cells Won't Turn Motors

[Lee Hart <leeahart@earthlink.net>]

David Dymaxion wrote:
> We decided to do a small toy electric car powered by a solar cell.
> I've tried various solar panels and motors, but it boils down to
> this: I can't get the motors to turn at all, not even in free air!

I've tried a few of the solar car kits, and found them to be expensive
junk. The solar cells and motors are low quality, and design is so poor
that they just b-a-r-e-l-y work if everything is absolutely perfect and
you have intensely bright sunlight available.

> Anyway, the little PC fan motor is spec'd at 0.080 amps and 12 volts.
> Solar cell shows 12 Volts and 0.25 A on the multimeter, right on spec

I'd guess that the solar panel delivers 12v at 0a no-load, and 0.25a at
0v into a short circuit (a multimeter set on its current range). Both of
these data points are zero power; volts x amps = watts. Into a real load
(like a motor or resistor, it may deliver half voltage at half current;
like 6v at 0.125a = 0.75 watts.

0.75 watts is very little power. It won't run a typical made-in-China
toy motor; they need at least a couple watts. It will barely run some of
the smallest toy motors that have above-average bearings and brushes. It
will run an instrument-grade precision miniature motor (like a Portescap
motor), but you'll pay $10-$20 for them.

There are two things I've done to make these solar cars actually work. 
1. Put a large capacitor across the solar cell, let it charge, and then
run the motor on the stored charge in the capacitor. For example, let it
charge for 10 minutes, then get a 10 second burst of power.

2. Use a piece of thread or audio cassette tape on a spool as an
"automatic transmission" (thanks to Paul Compton for this great idea).
Start with all the thread wound on the axle spool, and an empty spool on
the motor. When the motor starts with a full capacitor, it has full
speed and a huge mechanical advantage. As the capacitor discharges and
the car accellerates, the thread winds onto the motor spool which grows,
and unwinds from the axle spool, which shrinks. Thus the gear ratio
changes, and the car "upshifts" as it goes!

> But it can't droop too bad, or these things would never meet their
> claim to charge car batteries.

I have news for you... they DON'T charge car batteries! Their output is
simply too low to do more than counteract self-discharge and very small
parasitic loads like a car radio's clock and presets.
-- 
Lee A. Hart                Ring the bells that still can ring
814 8th Ave. N.            Forget your perfect offering
Sartell, MN 56377 USA      There is a crack in everything
leeahart_at_earthlink.net  That's how the light gets in - Leonard Cohen