Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Results make no sense regarding Kirchoffs laws?

  1. May 11, 2017 #1
    For an experiment we configured three 6V solar panels in parallel connected to a 10 ohm resistor. Each panels outputs roughly 0.5A each (measured) so thats 1.5A through the resistor. But the voltage across the resistor measure 15V. Does anyone know why this happened?
  2. jcsd
  3. May 11, 2017 #2
    Someone made a mistake. Either a bad measurement or the information on your equipment is incorrect.
  4. May 11, 2017 #3
    Thats what I was thinking, just didnt know how to explain it for a discussion besides human or instrumental error.
  5. May 11, 2017 #4


    User Avatar
    Science Advisor
    Gold Member

    Solar panels behave like current sources, not voltage sources. So your results make sense. I'm not sure what the 6volt spec means - perhaps open circuit voltage? Did you measure the voltage out of the panel when it is hooked up to no load?
  6. May 11, 2017 #5
    We measured voltage across the load originally with one panel and had about 6V across. Thats why Im confused, shouldnt the voltages be the same? But when consider V=IR on the resistor it works.
  7. May 11, 2017 #6


    User Avatar
    Science Advisor
    Gold Member

  8. May 11, 2017 #7


    Staff: Mentor


    Look at those curves. A panel (many cells) can look like constant current or constant voltage. The 6V nameplate is trying to characterize those curves with a single number.

    I think your results are fine.
  9. May 11, 2017 #8

    Solar cells are often specified by max power, open circuit voltage, i.e.. no load and short circuit current V=0. Sometimes they may specify the current and voltage at max power. From that you can determine the load resistance that produces max power If your resistance is less the voltage will drop and the current will begin to increase slightly. If you use a greater resistance the current will drop and the voltage will rise until you reach a maximum voltage at which the current will drop to zero. typically the max voltage is 25% than the voltage at max power.

    Assuming 0.5A and 6V are at max power 3W for a 10 ohm resistor at 6V you should have drawn about 0.6A close to the spec of the panel. So one panel works fine. If connected in series your numbers suggest that the current is 1.5A which is not possible. If connected in parallel your results suggest the power delivered is 22.5W which is not possible. So something is amiss.

    below is a typical characteristic curve for a 70 W solar panel If you draw a line on this diagram for V-=IR where R is the load, where the line and curve intersect will be the actual current and voltage for that resistance.. if you had a similar curve for your panel you could predict the current and voltage for your 10 ohms.


    Attached Files:

  10. May 11, 2017 #9

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    I'd say anorlunda and jason see the same thing i see. You wired the solar panels in SERIES not PARALLEL.
    If each panel put 'roughly 0.5A' into ten ohms then it made roughly 5 volts to do that.
    Three of them in series made 15 volts which Ohm tells us will push 1.5 amps through ten ohms..

    Have you a picture of your setup ?

    Are they 2 amp panels ?
  11. May 12, 2017 #10
    But the panels cannot supply 1.5 A (OP spec'd them at 0.5A at 6V).

    Since each panel can only supply 3W of power you only have 9W which for 15 volts across the resistor would limit the current to 0.6A which would be consistent with panels in series. But a 10 ohm resistance with a 15 volt drop uses 22.5W of power.

    As I see it with the info provided the only resolution for this discrepancy in the results of the OP is that the panel are in fact in series and the resistance is closer to 25ohms.
  12. May 12, 2017 #11

    jim hardy

    User Avatar
    Science Advisor
    Gold Member

    Hmmm i took this imprecisely worded sentence fragment :
    to mean
    'We measured roughly 0.5A (but not stated :when we connected each panel in turn across a 10 ohm resistor)'

    other interpretations are certainly possible

    but i didn't see 'specification' mentioned,
    and a 6 volt panel would push roughly half an amp through a ten ohm resistor, even if it were capable of more...

    Perhaps he'll clarify.

    old jim
  13. May 12, 2017 #12
    Sorry guys for a late response. For a singular panel we measured the voltage and current across the 10 ohm resistor - which ended up being about 6V and 0.5A.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted