Current sensor needed to monitor solar cell while cell is being used.

[Tedesh]

I'm working on a project for class. The project requires a certain level of difficulty.

The idea:

A system that would theoretically monitor the current through EACH solar cell in a Photovoltaic panel to ensure it wasn't shaded or defective.(For those that don't know, when a solar panel is shaded it essentially turns into a resistor and/or open circuit) If the cell was shaded, my switching relay circuit would bypass the the cell all together so the rest of the cells could add in votlages (minus the shorted SHADED cell) and output some sort votlage.

I'm was hoping to use some-sort of circuit or sensor in parallel so the output of the solar cells could be used.

Now you may be saying, why not just use a phtosensor. Well this is a final project for college and the circuit needs to be a bit more complex than a photosensor, opamp, transistor, and relay.



For example:

----Panel1 (.5v)----Panel2 (.5v)----Panel3 (.5v)---- So total voltage out would be 1v
(Shaded) instead of 1.5v due to the .5v
being shaded and bypassed.
I will try to submit a block diagram soon.

 

[mgb_phys]

Why not just connect them in parallel then you don't need to switch anything?
To measure the current you just put a small resistor in series and measure the voltage across it

 

[Tedesh]

The entire point of the project is to add up as many solar cells function or are not shaded so that essentially a solar panel with many cells would output as much voltage as possile.

The block diagram is below, the sensor measures the current, theoretically if the voltage out from the current to voltage converter is above a certain level, the relay from the next step in the comparator circuit will stay normally closed and the OK cell votlage will be added to the next cell checked. If the next cell is shaded, the voltage output from the current to voltage converter will be lower than th ecomparator reference, the relay will switch to the normally open position essentially bypassing the circuit including the solar cell all together.

http://i8.photobucket.com/albums/a47/elenor911/BlockDiagram.jpg

 

[mgb_phys]

The entire point of the project is to add up as many solar cells function or are not shaded so that essentially a solar panel with many cells would output as much voltage as possile.

So you want as high a voltage as possible at any time?
Normally for a power source you design around a fixed convenient voltage and the power available depends on how much current you can get.

So you just need a simple switch to disconnect each sell and short the the contacts.

------ ------
cell...output
------ ------

------ .\-----
cell...\...output
------ ... \---

The switch can simply sense the voltage from the cell which drops to near zero if shaded.
If the cells are floating there is no problem connecting a bunch of these together in series.

 

[Tedesh]

Understood and voltage in parallel would be very easy to measure and then outputing to some other device like a comparator circuit with transistor and relay to short out circuit or keep it in the circuit. BUT as I just tested to double check, I am working with low amperage (150-200mA) low voltage cells(.5v). I still get a decent amount of voltage out even with the cell slightly shaded but the current is what is cut off almost entirely. The cell will still be receiving some light because it won't be completely darkened out.

This is the other portion of the project. To educate the audience on solar cell properties. So that's why I wanted to do something with a sensor measuring current. I could show shade results with current to voltage converter and just plain readings voltage. Then I could compare voltages to show how drastically current changes as opposed to voltage.

Again I am not looking for simple ^_^ but also only have a week to work on it. (Not cause I waited till the last minute, just a PHd professor at a 2 year school looking at testing us for our full potential. :P

 

[TurtleMeister]

If the cells are all connected in series, wouldn't the current always be the same in every cell? How would measuring the current through an individual cell tell you anything (other than one or more of the cells in the chain have reduced output)? And if the cell becomes a resistor when no light strikes it, wouldn't the voltage measured across it actually be the voltage drop caused by the current produced from the other cells?

 

[mgb_phys]

I still get a decent amount of voltage out even with the cell slightly shaded but the current is what is cut off almost entirely.

Good quality cells give 75% of Vcc with any light at all. The cheap ones used normally used for solar panels (especially battery chargers/toys) drop off very quickly.

 

[Tedesh]

Good quality cells give 75% of Vcc with any light at all. The cheap ones used normally used for solar panels (especially battery chargers/toys) drop off very quickly.

I am aware of that. That's for making it more clear though. This is one of the points I would like to make with my presentation so any extra is great, thanks.

 

[Tedesh]

If the cells are all connected in series, wouldn't the current always be the same in every cell? How would measuring the current through an individual cell tell you anything (other than one or more of the cells in the chain have reduced output)? And if the cell becomes a resistor when no light strikes it, wouldn't the voltage measured across it actually be the voltage drop caused by the current produced from the other cells?

I want to know what cell has reduced output, that is the key to the project. Although not feasible in a real life situation, its the idea that I did the research to find out why.

So yea I want to find out which solar cell is not outputting and short it out with an automated circuit that is some what complex and I can rant on about for about 10 minutes with results, functionality, demonstration and obsticles that were overcome.

The theory of the project is done even though it looks like I'm going to have to change my "sensor" circuit so I can get something going here (Working).

Again, thanks to anyone answering and helping out.

 

[Averagesupernova]

I would say differential amps on each cell. Then feed the output of each into a comparator. Get the drift?

 

[TurtleMeister]

This isn't exactly what you're looking for, but it may help you out or give you some ideas. It's a synchronous rectifier used to prevent a battery from discharging back into a solar cell charger when the cell voltage drops below the battery voltage.
http://www.solarbotics.net/library/circuits/misc_switching.html

 

[shelgenortiz]

Good quality cells give 75% of Vcc with any light at all. The cheap ones used normally used for solar panels (especially battery chargers/toys) drop off very quickly.

you right... Quality http://www.martifersolar.com/gr/pg/Index2 can more expensive but still the life matters and the durability.