OK here is the spec:

Solar Panel

  • 4"x6"
  • 200mA
  • 7.2v


  • small : 20x10mm, 12vdc, 60mA, 2.17cfm
  • medium : 40x20mm 12vdc, 140ma, 8.28cfm

What I'd like to do, is to be able to run the fan hook to the solar panel on regular inside incandescent light. Now the fan hooks to the solar panel, and needs direct bright sunlight to run. I think I need a voltage regulator and a capacitor (in micro farad) I'd like to have the fan running only when light on (no need to run at night), running at 1/2 of the 12V speed is fine to me.

So the question is: What capacitor and voltage regulator do I need? I heard that a dc to dc is needed, too.

I know electronics, but I cannot do the maths. I can solder, do breadboard, etc. but know nothing in technical match term.

I know that many capacitors exist, but I like to keep the whole thing under $10, solar panel included. Now the solar panel in retail cost 19.00 but Alibaba can sell it for $5 or maybe you know a US/Canada place to get cheep solar panels. Whole electronic thing excluding fan and solar panel: $3-5.

as told, MAYBE simple incandescent light will NOT be enough, so let think about putting it beside a windows, is it more realistic ?


2 Answers 2


Let me make sure I understand. You want to run a fan from a solar panel, while the panel is only illuminated my in incandescent light? This cannot be done. It isn't a matter of voltage regulators or capacitors. The panel simply will not produce enough power to run a normal fan.

It might be possible to run the fan for say 5 seconds every few minutes by storing the power in a cap, but I doubt this is what you want. (BTW, the times stated here are only guesses. The point is you cannot run the fan anywhere close to continuous.)

  • \$\begingroup\$ ok, beside a windows ? is it possible ?... I need the fan to run INSIDE.... it make no sense to as people to run outside at noon to get it working ! \$\endgroup\$
    – menardmam
    Commented Mar 27, 2014 at 0:33
  • \$\begingroup\$ does it make sense IF i said.... fan need a lot of power to start... but almost noting to keep running ? \$\endgroup\$
    – menardmam
    Commented Mar 27, 2014 at 1:34
  • 1
    \$\begingroup\$ If you have incandescent lighting, why can't you run the fan off of line power? Replace that incandescent with a compact fluorescent or LED, you'll save much more power than the fan will consume. \$\endgroup\$
    – DoxyLover
    Commented Mar 27, 2014 at 2:03
  • 1
    \$\begingroup\$ You will want to research the amount of light that incandescent bulbs output compared to the sun. The specs on the solar panel are for the brightness of the sun, not light bulbs. The power output from the panel under an incandescent light bulb will be significantly lower than under sunlight, in fact that particular panel you linked will likely output a very small current if any under normal indoor lighting conditions. You will need to find a panel that will produce enough power to run your fan under the lighting conditions you describe - I am not sure that one exists that will be small enough \$\endgroup\$
    – Filek
    Commented Mar 27, 2014 at 3:15
  • 1
    \$\begingroup\$ @menardmam I just tested one of my solar panels. It puts out 14.5 volts and 3.5 amps in direct sunlight. I brought it inside and put it on the counter about 4.5 feet below 2 pot lights. I measured only 3.5 volts and 10 milliamps of current! And believe me, it is fairly bright inside where I did the measurement. To give you an idea, the solar panel is just slightly larger than 2 feet wide by 2 feet tall, so you would have to go BIG to get your fan going! (OR maybe a special type of panel, like monocrystilline would do better) \$\endgroup\$
    – Filek
    Commented Mar 28, 2014 at 1:57

Yes, you can run the fan using the solar panel, provided:

1) Fans vary on the design. Some are simple DC motors with brushes (older) while some are driven electronically. In either case, there is a minimum starting voltage & current. I’ll focus on the newer ones.

2) With the electronic design, there is a minimum voltage required to activate the electronics (usually a Hall effect device and a driver IC). Once the minimum voltage is reached (think moving the solar panel closer to the Light), the driver wil turn on

3) Once the driver turns on, the voltage will drop since its now attempting to drive the coils (pulses the coils to kick start the motor). This will cause the voltage to drop due to the load. If the panel is getting enough light to stay above the minimum voltage, the fan may start. This is where current comes in. The panel must be getting enough light to output enough current to keep the voltage above the minimum. (i.e. power: where power = Volts x Amps).

4) Once the fan starts, the current needs will a drop a little until the fan speeds up

5) The speed will increase until it either hits the limit of the fan or the limit of power from the panel.

In your case, your limited by voltage (7.2v panel driving a 12v motor) not by current (200ma vs only 140ma needed). If we assume the solar panel output and motor speed is linear (it’s not), then panel max = 7.2 x 0.200= 1.44 Watts. Motor at full speed = 12 x 0.140 = 1.68 Watts. At best, fan would run at 1.44 / 1.68 or 85%. In reality, your voltage limited, so it’ll probably be 7.2 / 12 = 60% (at best).

The bottom line is getting enough light on the panel to start the fan. No capacitor or regulator needed. You can try putting a cap across the power to help store current for the starting pulses (50 to 400uF as a guess) but this complicate things since now you have to also charge the cap and the first start pulse could lower the voltage below the minimum.

I did a quick test on my bench with a small power supply. The 12v fan started up around 6v, and pulled about 40% of the rated current. At 7.2v the fan was roughly about 50% speed.


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