First, here is my solar panel: http://www.amazon.com/Instapark%C2%AE-Black-Mono-crystalline-Charge-Controller/dp/B005LR9IOG

And the regulator: http://users.ece.utexas.edu/~valvano/Datasheets/L7805.pdf

And the Schematic: http://freedatasheets.com/blog/uploaded_images/7805datasheet-730098.gif

So here is my predicament, whenever I connect the solar panel to the input, the output is not 5V (or 12V for the L7812 regulator) UNLESS the solar panel is outputting its max voltage. So for instance, when the solar panel reads about 14V, the regulator outputs about 3.5V. These values are tested under no load.

Now I know that it is not the regulator because when I connect a 9V battery in place of the solar panel, the regulator outputs 5V like a charm. I also read that there is a 30V upper limit on the 5V regulator so that can't be it either.

My only thought is that the regulator somehow recognizes the max output of the panel and regulates linearly with that voltage so if the panel outputs less than its 22V the regulator will as well. That is just a shot in the dark and does not make sense to me.

If anyone can let me know how to fix this problem, or why it is happening that would be great. This is for my senior project in college which consists of a solar tracker and battery/usb charger. I got the tracking working and the chargers working, but I am trying to self power the system using the solar panel.

So again, any insight would be a great help because I am kind of stumped and when I asked my teacher about it he just said check to see if the regulator is over-heating which it isn't.

UPDATE: I just checked it now with the panel outputting 18.5V and it did output 5V, so I'm guessing there is a cutoff voltage or something. Another thing I just tried was measuring the resistance of the panel versus the resistance of the battery. This was actually pretty interesting. When the panel read ~10V, the resistance of the panel jumped around in the kilo-ohms (I have an auto-ranging meter). When I went to test the battery ohms, a little battery symbol came up and it would not give me the ohms. Here is the key part, when I read the ohms of the panel at 18.5V, it came up with the battery symbol on my meter as well. This probably means that the panel only acts as a battery above a certain voltage. I am still kind of curious as to why though, so if anyone has any thoughts that would be great.

  • \$\begingroup\$ Sounds like the panel isn't putting out enough current. \$\endgroup\$ – Ignacio Vazquez-Abrams Mar 25 '14 at 22:55
  • \$\begingroup\$ Yea, that was my thought as well, but when I compared it to the battery it was only 10-20 mA less. \$\endgroup\$ – Josh Mar 25 '14 at 22:57
  • \$\begingroup\$ what voltage is on the input of the 7805 when it misbehaves. Also why are you not using switching regulators. They are at least 90% efficient in converting power to a regulated output. \$\endgroup\$ – Andy aka Mar 25 '14 at 23:20
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    \$\begingroup\$ Do not connect your meter, set to measure resistance, to a voltage source! The meter is designed to apply its own voltage across the test resistance and measure the current flow. At best, you will get incorrect readings and at worst, you will destroy your meter! \$\endgroup\$ – DoxyLover Mar 25 '14 at 23:32
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    \$\begingroup\$ I agree, it is likely that the panel is just too weak for your project. In my experience, solar panels can output 90% of their voltage in relatively low light situations. However, their current output seems to drop rapidly as light starts to drop below ideal or maximum. So if you are in a situation where the voltage is significantly below maximum, then I would bet that the current (which is only 0.57 amps max) is very low and that is also a big part of your problem. A second solar panel, a larger single panel, or only using it in very bright sunlight would likely solve your problem. \$\endgroup\$ – Filek Mar 26 '14 at 3:59

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