# Solar panel characteristics..

Discussion in 'General Electronics Chat' started by Vorador, Aug 13, 2015.

Oct 5, 2012
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I don't understand voltage-current dependencies of solar panels. I know that current in solar panels is affected by illumination and voltage by temperature, but I don't fully understand the implications of these facts. For example, looking at the characteristic curve (http://www.pddnet.com/sites/pddnet....nd_editorials/editorials/2010-04/lt_fig_1.jpg), it shows that for the same voltage, a solar panel can produce different currents depending on the level of illumination. But I'm not sure what this really means. If I connect a fixed resistive load to a solar panel, is it possible that it can provide different currents through it (caused by different illumination levels) even though the voltage drop across it remains the same? How can this be understood in terms of ohm's law?

Thank you, people!

2. ### dl324 Distinguished Member

Mar 30, 2015
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No; I=V/R. If the voltage across your resistive load stays the same, the current stays the same. Different illumination levels will change the output voltage of the solar cell.

3. ### nsaspook AAC Fanatic!

Aug 27, 2009
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If the load is the same, once the illumination level is above the point where the cells Isc is greater than the load current the cells voltage will remain relatively stable into that fixed current draw.
At any one point in the curve Ohm's law is correct but a solar cell is a current-source with a limited max voltage (non-linear) so over a range of current from zero to a point less than the short circuit current (Isc) where its unable to maintain max potential it acts like a voltage regulator feeding a load.

4. ### ronv AAC Fanatic!

Nov 12, 2008
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The voltage is pretty constant regaurdless of illumination, but it could supply more current in brighter light if the resistor were smaller. I=E/R

5. ### MikeML AAC Fanatic!

Oct 2, 2009
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A better way of thinking about it is that the solar panel that at any given illumination level, has a Current vs Voltage IV curve shaped like this:

The plot shows a nominal 20V 2A panel at four different levels of solar illumination; the more the illumination, the higher the output current. . Green is the lowest illumination level; Lt.Blue is the highest.

Note that for output voltages below ~16V, it acts like a (less-than-ideal) current-source, while for low output currents, it acts as a (less-than-ideal) voltage source. If illuminated at a level to get the Green curve, the short-circuit current (Vout=0V) is ~0.5A, while the open-circuit voltage (Iout=0A) is ~18.5V. But the shape of the curve between these points is anything but linear; hence, you do not apply Ohms Law to the panel; only to the load connected to it.

There is a Maximum Power Point for every panel (at any given illumination) where the product of Iout * Vout reaches maximum.

Last edited: Aug 13, 2015
6. ### dl324 Distinguished Member

Mar 30, 2015
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That isn't what the graph or experience tells me. Without a load, with varying illumination the output voltage will vary between 0V and maximum cell voltage. With a load, it may never reach maximum design voltage.

7. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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It differs very little from the model of a battery when internal resistance is included in loading calculations.

8. ### Lestraveled Well-Known Member

May 19, 2014
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Another way to look at solar cells is like a bench power supple that has voltage and current controls. The number of cells sets the voltage and the amount of light hitting the panel sets the max current. So if the load draws less than the max current the panel will provide a constant voltage. When the load draws more than the max current the voltage will drop to maintain that (max.) current.

Oct 5, 2012
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That's a nice way of looking at it. Thank you and everyone else!

Can anybody please explain how I can obtain the VI characteristics of a solar panel for different illumination levels? I thought of connecting a potentiometer as a load to the panel and then making current and voltage measurements with the illumination fixed for one set of V-I measurements. But for the next set of readings, with illumination set at a different level, and my potentiometer going through the same range of resistances as before, I'm not sure how I would be able to get different readings of current, since my resistance and the voltage drop across will be going through the same range of values as before..

I want to get the characteristic graph that shows current levels increasing with increasing illumination, like that showed by MikeML.

10. ### nsaspook AAC Fanatic!

Aug 27, 2009
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Last edited: Aug 13, 2015
11. ### dl324 Distinguished Member

Mar 30, 2015
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Do you have a way to measure the intensity of the light spectrum solar cells respond to?

If you attempt to measure current with an ammeter, be sure to consider how it's internal resistance will affect your measurements. Measuring the voltage drop across known resistances would be better...

12. ### MikeML AAC Fanatic!

Oct 2, 2009
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Relatively easy to do if you do it at noon, on a sunny day, with the panel orthogonal to the sun. Use an adjustable power resistor rated for more Watts than the panel can deliver; i.e a 25W rheostat for a 20W panel. Use two DMMs, one set to voltage, other set to measure current.

Rheostat like this:

Slowly vary the resistance (by turning the knob), and measure V and I at each new knob position. Plot I vs V on graph paper.

Oct 5, 2012
87
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Would it work if I shine a light bulb on a small panel? Because I need to measure V and I for several different illuminations, which will be easier to control with a bulb.

Thank youuuuu!

Oct 5, 2012
87
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I was thinking of using a lux meter.

15. ### MikeML AAC Fanatic!

Oct 2, 2009
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Light from a light bulb is nothing like sun light. Different spectrum.

It will be extremely difficult to correlate what the panel does with solar illumination to what you measure using a lamp.

What you need are some neutral-density films that block a known percentage of the sunlight.

Oct 5, 2012
87
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I understand that. I'm not particularly interested in sunlight at the moment. Rather trying to do it as an exercise to better understand the behavior of PV panels.

Thanks for all the great help, guys!

17. ### dl324 Distinguished Member

Mar 30, 2015
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A lux meter measures light intensity as perceived by the human eye. The range of frequencies the eye and solar cells are most sensitive to overlaps, but it's unlikely that the response at each frequency will match. It may be okay for relative measurements.