I am new to Solar and but I am a High School Science teacher. My question is in regards to a small scale hydroponic project that runs off solar.

I need to get the 6v 2w 350mA solar panel to power up a Koge air pump that draws 4.8v at 100-250mA. When I used alligator clip leads to connect the terminals and had a 100w incandescent bulb 5 inches from the solar panel...nothing happened.

What am I doing wrong? What do I need to add? Your help is GREATLY appreciated!

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    \$\begingroup\$ Measure (a) the open circuit voltage and (b) the short circuit current from the cell under the light source you are using, and compare that to the values you get in full sunlight. That'll give you an idea how much more light you need. \$\endgroup\$ – Brian Drummond Feb 26 at 20:18
  • \$\begingroup\$ Thanks, so you happen to have this equations on hand? I don’t wanna search the internet to find the wrong solution and mess the whole system up! \$\endgroup\$ – DTK421 Feb 27 at 2:04
  • \$\begingroup\$ No, this is something you will have to measure (with a DMM). At a guess you may be getting 10% of the rated current from a mere 100W bulb. \$\endgroup\$ – Brian Drummond Feb 27 at 3:15
  • \$\begingroup\$ Also note that you need just a silly bright light source to make a panel perform as if it was in sunlight. Sunlight is very broad spectrum and almost all lightbulbs are narrow spectrum. If you find out what part of the spectrum your panel responds to, you could find a bulb in that spectrum or try some much brighter light sources for testing, like a 500W halogen work light at point blank. If you test the panel with bright sunlight hitting it perpendicularly, you can get an idea of the expected response and then test a few light sources. In bright enough light, as Brian says, it may work. \$\endgroup\$ – K H Feb 27 at 23:17
  • \$\begingroup\$ Most solar panels are rated for their maximum performance in very bright light. The farther you are from the equator you are, the fewer hours of sunlight you get and the worse of an angle it will hit at. Even on a bright day, if there is haze, smoke, clouds or fog cover, the panel will only put out a portion of it's rated current. At least one brand(powerfilm) makes panels that perform better in indirect or dim light. If you live in a desert and/or close to the equator, you can expect panels to provide roughly their rated power. Anywhere else and you have extra factors to consider. \$\endgroup\$ – K H Feb 27 at 23:24

Almost certainly your air pump needs way more than 350 mA to get going from a dead stop. Since your panel can't supply the required current surge, the motor simply shorts it out and nothing happens.

You might be able to deal with this by connecting a largish (several thousand µF) capacitor across the solar panel to supply the startup surge. But here's the key — you need to let the panel charge the capacitor up before you try to start the motor.

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  • \$\begingroup\$ Thanks! So as to not destroy my circuit perhaps you can explain or point me in the direction of how to measure what size capacitor I would need? I am having a hard time locating capacitors over 1000mF \$\endgroup\$ – DTK421 Feb 27 at 17:18
  • \$\begingroup\$ Look at the current surge for several startups using an oscilloscope, and try to estimate the total charge that it represents (area under the current-vs.-time curve). In this case, a "supercap" or "ultracap" might be the right thing to use. Just make sure the voltage rating is high enough. \$\endgroup\$ – Dave Tweed Feb 27 at 17:21
  • \$\begingroup\$ Also note that you're looking for a few thousand uF(microfarad) not a few thousand mF(millifarad). Dave says several, so I'm guessing you'd want 3000-7000uF, which is 3-7 mF. \$\endgroup\$ – K H Feb 27 at 23:16

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