Open circuit voltage and MPP of a solar cell

Thread Starter

arhzz1

Joined Oct 21, 2020
22
Hello. I want to figure out an open circuit voltage of a solar cell. Now I've done that like this

20201206_161843.jpg

As far as I know to get the Open Voltage circuit we cant have a resistor. Now when I measure it I get around 0,15V.But the part that is bugging me is even if I shine a light in the cell I still get the 0,15V. I've even tried adding a resistor and still get 0,15V so I'm pretty confident that I'm not doing it right.What am I doing wrong in the measurment of the voltage? And what could I await as open voltage circuit so that I know that my measurment was correct? Thank you!
 

MisterBill2

Joined Jan 23, 2018
8,711
It looks a bit like trying to measure the voltage using an analog input on some sort of data acquisition board. Those are often a bit more challenging than an actual multimeter. So I suggest first trying to read the solar cell output on a normal voltmeter, either analog or digital. And there is no need for a series resistor to do that. Consider that those solar cells in the cheap solar powered lights produce well over a volt to charge the battery during the daytime. That shows up on my analog multimeter quite well. And if it is an arduino device then there is probably some electronic detail that has been left out of the instructions.
 

BobTPH

Joined Jun 5, 2013
3,304
What are you using to measure the voltage? It looks like it is hooked up to some PCB. Are you sure it is reading the voltage correctly
 

Thread Starter

arhzz1

Joined Oct 21, 2020
22
Well I'm using something called eLIT Board.I think i've figured out the problem. My diode is in series with my measerument for the voltage. I need to get rid off that.But how?
 

Bernard

Joined Aug 7, 2008
5,788
It looks like there is something in series with solar cell, resistor or diode? Remove & remeasure. Voltage will
climb rapidly from low light to full sun. OC V will remain almost constant from dim to bright light. There should be about .5V / cell.
 
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Thread Starter

arhzz1

Joined Oct 21, 2020
22
It looks like there is something in series with solar cell, resistor or diode? Remove & remeasure. Voltage will
climb rapidly from low light to full sun. OS V will remain almost constant from dim to bright light. There should be about .5V / cell.
I've done the following. I've tried to avoid the diod being in series with the measerment of the voltage and I get around 0,159V. Would you say that is correct?20201206_175834.jpg
 

MisterBill2

Joined Jan 23, 2018
8,711
Why have a diode in series with the solar cell? For most regular diodes the forward drop is about 0.7 volts, and that matters in many applications.
 

Audioguru again

Joined Oct 21, 2019
3,171
A solar panel is about 0.5V per segment. Solar panels on cheap solar garden lights have 4 segments (2V) reduced by a series Schottky diode to 1.8V open circuit and charge the single Ni-MH battery cell to about 1.3V in full sunlight. The diode prevents the solar panel from draining the battery in darkness.
 

MisterBill2

Joined Jan 23, 2018
8,711
For checking the solar panel device the diode in series is not needed, and it will interfere with detrmining the actual voltage.
I have discovered that some LEDs generate quite a bit of voltage under bright light. And some of them do not produce much at all.
 

Kjeldgaard

Joined Apr 7, 2016
451
When talking about solar cells, one must not forget that there can easily be a factor of 1000 in the difference between light indoors and outdoors, and that the human eye is amazing to adjust for changes in brightness.
 

MisterBill2

Joined Jan 23, 2018
8,711
Do you have another volt meter, or measure a AA or AAA
alkaline battery with your board.
A very good suggestion, since I suspect that there my be an issue with the voltage measuring arrangement. Like a differential input with the negative tied to the wrong point. Also, using a breadboard can add some serious connection resistance. If it is a differential input, there might also be a common-mode issue. and since it is being read by software, there may even be a software problem. Checking a single battery is a good check, even a somewhat used, but not dead, one will be useful, to verify the measuring scheme.
 
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