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Basic Electronics » Solar cells in parallell, but no additional amperage

November 06, 2010
by Shanonius
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Hi Nerdkits, I bought some of those night garden solar LED lamps to I could do some experiments with the solar cells, but one thing has me stumped at the moment. Firstly, and in keeping with my current understanding of electronics, the more I put in series, the higher the voltage readout on the multimeter, (amperage not increasing at all). Then, I took 2 sets of 5 cells and put them in parallell (negative merged with negative, positive with positive, expecting to get a noticeable increase in the amperage. But there was no increase in the amperage at all. The readout is almost identical to the amperage of just one of the sets of solar cells. In fact, it was actually a tiny bit less. My head hurts. What is going on here? Many thanks

November 07, 2010
by esoderberg
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Shanonius,

Don't know for sure is this is your issue but, from http://www.solarbotics.net :

Some more subtle properties of solar cells also need to be accounted for in their use. In particular, when connecting solar cells in series, care needs to be taken to give all cells roughly equal access to light -- the weakest solar cell in series (or one that is shaded) will determine the total current. In a pinch, reverse Schottky diodes can be wired across each cell to automatically bypass any cell that may get shaded.

Similar issues can occur when wiring solar cells in parallel. In that case a shaded cell can act as a short circuit to the output of its more active neighbors. Here, a germanium diode in series with each separate cell can be used to mitigate problems.

November 07, 2010
by mongo
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One of the funny things with solar cells is that they are really inefficient. Unless they are all perfectly matched, it would be difficult to get them to produce in a parallel configuration. There will always be weaker cells that are actually loading down the others.

In order to wire them in parallel, you need to add bypass diodes. Since a single cell only delivers about half a volt, you would not see anything come out since it takes .7 volts to get past the diode. You would best be suited to put several in series first. Then take those sets and then wire them in parallel with an isolation diode on each one. Some times, you might also want to add a bypass diode as well, across individual groups. This way, if there is a shadow or a poorly lit set of cells. This would prevent them from limiting the voltage at the output.

By using germanium diodes instead of silicon diodes, you can use fewer cells as they have a voltage drop of only .2 volts. Most however, have a much lower current carrying capacity.

November 07, 2010
by mrobbins
(NerdKits Staff)

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Hi Shanonius,

It sounds like you're making two measurements: one is of "open circuit voltage" (at nearly zero current), and the other is of "short circuit current" (at nearly zero voltage). These are easy to make with a multimeter, but neither actually measures a usable level of power (which would need voltage and current to both be non-zero). And due to the variation in different cells and different illumination, it's likely that that power will not add up when combining cells, as the other contributors have already mentioned.

However, I have to agree that I am surprised that measuring the short circuit current of several solar cells in parallel would be less than that of any individual cell (under the same illumination per unit area). Mathematically, the measured short circuit currents really should add perfectly linearly! (We've taken advantage of this in the past in some projects involving optical sensors and photodiodes, which are basically tiny solar cells...)

Mike

November 07, 2010
by Shanonius
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Hey thanks everyone for your insightful and helpful comments, much appreciated. When I get back home from work I'll do some reading on the topics discussed. Thank you for pointing me in the right direction!

November 07, 2010
by mongo
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There is something I didn't think of. Open circuit and closed circuit measurements. True enough... You need a known load to be able to measure much of anything. Combine the voltage with the current and work it out in wattage. Whatever max wattage you get will likely be at the highest of both worlds.

November 12, 2010
by BobaMosfet
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Shanonius, can you tell us anything about the item you purchased, so we can determine the specifics about the photocells? In order to create a proper expectation (what you believe should happen), we need to make sure we understand the capabilities of the components in question.

BM

November 14, 2010
by Shanonius
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Hi BobaMosfet, Thanks for your reply. These are square shaped, "post-it note" sized solar cells which I pulled out of a pack of 10 solar garden lights I purchased from a variety store. Is there any other measurement (eg. volts in full sunlight or similar) you would like me to make on an individual cell?

November 24, 2010
by BobaMosfet
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Shanonius-

Remember, your solar cells are resistors. Measure with an ohmmeter please, with a bright light on a cell, and then with it really covered. What are the values on each extreme?

BM

November 24, 2010
by mongo
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Are these solar cells or photo cells? Solar cells generate electric currents, photo cells are light sensitive resistors. (though I am told that photo diodes can do both...)

November 24, 2010
by BobaMosfet
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mongo-

Good thought. In my experience with both, I've found they act similar, and the only real difference between the two in terms of generation .v. resistance, is surface area and sensitivity. I realize that my experience is not guaranteed to be universal, and specifications, materials, and design considerations for either component may preclude its ability to behave as I've generally experienced.

Take your garden-variety photo-cell from a standard issue plug in nightlite from wallmart- you know the one, AC outlet, 120VAC 5W bulb. If you plug your voltmeter into that photocell and shine a flashlight on it, you're going to get about 5mV. If you put your ohmmeter across it, you'll see somewhere between 12K Ohm in the dark (or higher), to maybe 200 Ohm with the same flashlight shining on it.

Solar panels are the same. Take a 4VDC, 100mA 3" x 5" panel and hook a voltmeter to it- Shine the same flashlight on it as you did on the photo-cell, and you'll see about 1.77V, at an expected current. Switch to ohmmeter and you'll see 2.2M Ohm (or more) in a shadow, and zero resistance in any reasonable light.

The solar cell is much more sensitive and higher quality, and has a larger surface area, so the potential voltage and current is greater than a photo cell-- but still, they can be used in both circumstances-- handy to know, eh?

BM

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