Tech Forum

My suggestion would be to put an LM7812ACT linear three-pin regulator (about $0.69 each, plus shipping, from Digi-Key.com) between the solar panel and each fan. Be SURE to read the spec sheet carefully and use bypass capacitors on BOTH the input and output. (Guess how I know to warn about that!) You'll also need some sort of heat sink on each, though some scrap of aluminum should suffice -- my guess would be four square inches or more should do the trick. (If necessary, cut up an empty aluminum can, though fold the edges over to avoid sharp edge hazards.)

Using a "low dropout" regulator would mean more efficiency, but at a noticeably higher cost.

P.S.  I notice that you're in San Diego.  You can probably get the parts at Fry's Electronics, though for a little bit higher cost. The spec sheet can still be had at Digi-Key.com. (Wish Fry's would provide scooters for their mobility-limited customers!)

It’s not likely that the voltage from the solar collector, under load, will be enough to damage the fans. However, to be safe, you could put a 13 volt, 15 watt zener across the collector output. The fans will be drawing 1.53 amps while the collector is trying (best case) to put out 2.5 amps, so the zener will have to sink 0.97 amps. I could not find such a zener in the Mouser catalog, so the alternative is to use a power transistor amplifier, see diagram (the part numbers are Mouser).

The zener current is just the base current, about 33 mA. It is DO-41 (1 watt) and costs 22 cents. The transistor sinks 0.97 amps and dissipates 13 watts.  It costs 99 cents. A heat sink will be needed; Mouser part number 588-FA-T220-51E is suitable and costs $1.56. The heat sink is mounted vertical. It has two 0.091 solderable posts, diameter spaced one inch.

A good solution to this issue is to use a switching regulator. It has very high efficiency, close to 90%, and will take the 20+ VDC from your panel and even increase the current output some!! Go to "sul-tech" on YouTube, and you will find the solar optimizer schematic....hope it helps.

The 30W rating on your solar panel is very optimistic and requires a LOT of direct sunlight to reach that rating. You might need more than one panel to do what you want to do. I think you will find that you have the opposite problem of what you are concerned about. Solar panels have a very large output impedance, which means that the voltage will drop to a low value when you connect a load like an electric motor that has a low input impedance.

Also note that electric motors are rated at their RUNNING power requirements and they need much more current to START. You will need a buck-boost voltage regulator to do what you need to do. It raises the voltage when the light is dim and lowers the voltage when the light is bright.

Since your motors will require more than one amp you will need to find a regulator that is capable of supplying at least 2 amps and I recommend one rated at 3 amps or more.

Your solar panels only show that high of voltage when they are unloaded. Once you put the fans on them, the voltage will adjust to their rated voltage, providing you have enough sun light to drive them properly.

The other good news is that the fans will not burn up when more voltage is applied to them. They will run faster and wear out sooner, but to fry them, you would need much more than a single solar panel can produce.

The solar panel you have says 30 watts, but what they actually mean, is that under ideal conditions, it can produce 30 watts. Two panels would assure that the fan would run in less than ideal sun light.

I would use a simple LM7812 voltage regulator for each fan. The 7812 can take up to 18V in and as low as 5V and has a current rating of 1 amp. Based on your specs, you will only be drawing a little over half an amp on the 7.6 W fan and a little under a half amp on the smaller fans.  If you find that they get hot, you can easily add a heat sink.

I will also mention that I did something similar, running a boom box radio at the beach, and found that unless the panel is pointed at the sun constantly, the output will drift when fair weather clouds cross overhead. So, I connected 3 1000000 20V caps in series, which slowed down the drifting effect caused by clouds.

Another option is to use a solar charger controller like the one I picked up at harbor freight tools. You would hook up the solar panel to the controller, then the controller to the battery (car/marine), and run the fans off the battery. I would still use the LM7812 voltage regulator just to protect the fans.