I posted this in another area, but I think this question belongs here...
I would like to build a simple charging circuit where I can collect light using solar panels (small panels) and store the energy. I would then like to measure the amount of energy collected. I was thinking of hooking up the panels to a capacitor and measuring the voltage at various points in time to see how much energy has been collected. Is it that simple or is there more to it?

Also- I connected the capacitor to the solar panel and it charged up to 3V (the output of the panel). When I covered the panel, the capacitor voltage dropped along with the output of the panel. Why is this? How could I keep the voltage previoulsy stored in the capacitor stable if the input voltage drops? Is there any way for the capacitor (which is a 35V capacitor) to "suck up" the voltage and ever reach 35V?

 

Unless you have a blocking diode, the cap is going to discharge through the panel as soon as the panel is covered.

Regardless of how you do it the cap is only going to have a voltage across it equal to the voltage from the panel.

 

There are a number of problems with this. Not least of which, a very large capacitance indeed would be needed, or it will finish charging up within a matter of seconds or at most a few minutes. For instance, if you have a very big 1 Farad capacitor, and the cell was delivering a maximum current of 10mA, the voltage would rise at the rate of 1V per 100 seconds. Smaller capacitances would charge faster.

When the cell is dark the capacitor will tend to discharge, for several reasons. One of these is leakage through the photocell, which could be much reduced by adding a silicon diode in series with it (but this would waste about 0.7V). There will also be leakage in the capacitor, maybe quite significant if it is an electrolytic. Finally, any load connected, even a DMM, will draw some current.

The maximum voltage attainable will depend on the solar battery, how strongly it is lit, and any leakage current. The capacitor rating MUST be higher than this voltage, or the capacitor may be destroyed. Fitting a very much higher voltage rating capacitor than what is required will not force the voltage up even higher.

 

What you really want to do is to measure the panels wattage. You can do this by measuring the current and voltage with a load attached.

You can do this manually throughout the day or get really fancy and program a micro-controller to measure and log the results.

Once you have all of your measurements you can easily calculate the average power output of your panel.

 

Spinnaker and Adjuster- Thank you for your willingness to help. I understand the need for the diode if I am going to maintain the charge in the capacitor. Do I have to be concerned about the specifications of the diode? I'd like to be able to pick it up at Radio Shack if possible.
By the way- how difficult would it be to build the micro controller? I am handy with a soldering iron- I just don't have the know-how!!

 

You don't build a micro controller you buy one, assemble it into a project and program it.

You want to first learn how to use microcontollers. I chose the PIC. They are low cost but you need to build everything on your own. The most expensive initial item you wll ned is a programmer. I would get the PicKit II or PicKit III, leaning toward the III. I would recommend the kit at the top of this page.

Search around you might be able to find it cheaper elsewhere.

If you decide to go this route post back in the embedded forum. I already designed a solar light controller. Not exactly what you need but it might get you started.


The other way you can go is with Audrino. More expensive but a lot easier to work with. You buy the main mcu then you buy the various modules that you plug into the mcu.

BTW Radio Shack is going to start carrying Audrino so that might be a plus.

As far as the blocking diode, you really aren't going to need one unless you want to charge a battery or go with your capacitor idea (you really won't need it for the mcu if that is all you want to do). You might want to post again on this specific subject in General Electronics for the advice of a more learned forum member but I would think you would want to use a schottky diode. They have the lowest voltage drop. Radio Shack does not seem to carry them. Post of the General Electronics for advice, There are forum members that know the family of diodes so well they will be able to give you a part number based on your specifications.

 

The diode's voltage and current ratings should be greater than the maximum values for the solar panel.

For a large high-current panel it may be better to use a Schottky diode because of its lower volt-drop. If the panel only delivers a little current an ordinary silicon diode may be better because its reverse leakage will be lower.

Since you are only trying this out, providing the maximum current is under 1A, why not use something ordinary, cheap, and easily available like a 1N4001...1N4007.

http://www.fairchildsemi.com/ds/1N/1N4001.pdf

 

Another problem you'll face is that the output current of a solar cell (a panel is just many of them in parallel and/or series) has an output current capability that depends on the output voltage, and vice-versa. So, as your cap charges, the voltage rises, and the output current of the panel falls. Eventually, the cap would fill to the max voltage capability of the solar cells, but by then the current will have slowed to a trickle....

Solar cells usually have to be "conditioned," that is, provided with an electronic circuit to run them at their peak power point.

See: http://www.reuk.co.uk/Measuring-the-Power-of-A-Solar-Panel.htm for an explanation of peak power.

See: http://www.affordable-solar.com/store/inverter-grid-tie-batteries/sma-sunny-island-5048U-inverter for a good example of a typical high-power commercial product.

 

Again- Thanks to everyone for their replies. I added a diode (1N004) to the circuit and the leakage back to the solar cells has diminished greatly. I also reviewed the web site recommended by billbehen- very informative.
Right now, I have the circuit set up so that the capacitor stores the energy from the solar cells and I am able to use a momentary contact switch to release the energy and light up a LED. I plan on using this to see if bioluminescent bacteria can emit enough light to light the LED. I am expecting a very low light level, which is why I want to store whatever I can in the capacitor. Using the switch, I can get a release of voltage from the capacitor which (I'm hoping) will be enough to light the LED (even for a brief moment). That is why I was looking to store the energy.
The first web site on peak power raised a quuestion. Does it matter if I try to find the peak power, or is voltage what I'm looking for here. If I increase the peak power by experimenting with resistors (as in the article) will that make a difference in how much voltage I will be able to collect in the capacitor?