Measuring Current of a Solar Panel, Confused

Discussion in 'General Electronics Chat' started by Guest3123, Nov 15, 2014.

  1. Guest3123

    Thread Starter Member

    Oct 28, 2014
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    I was always taught to read current like this.

    IMG00000001 2014.11.15 11.40.31 PM.jpg




    But now I'm seeing lots of people measuring current like this..

    IMG00000001 2014.11.15 11.45.17 PM.jpg

    I don't know.. Which one is the correct way to measure current when working with Solar Panels?

    What's the difference? Why are people (even professionals) measuring current like the above image?
     
  2. Lestraveled

    Well-Known Member

    May 19, 2014
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    The first picture is the way to measure current that is being supplied to a load. The second picture is measuring the max current output of a solar panel. They are both valid and measure different things.
     
  3. Guest3123

    Thread Starter Member

    Oct 28, 2014
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    I don't understand.. I always thought that measuring the way that the 2nd image is showing, that it would short the circuit out.. I tried doing it that way before, and blew my meters 10A fuse. Went to Mouser and bought new one, cost me $5 for a BRIT 240v 10A Fuse.
     
  4. Lestraveled

    Well-Known Member

    May 19, 2014
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    It is simple. Your solar panels max current was higher than what your meter was rated for. A better way is to put a low ohm resistor across the output of the panel and measure the voltage across the resistor. Then use ohms law to calculate the current.
     
  5. NorthGuy

    Active Member

    Jun 28, 2014
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    That's why they call it "Short Circuit Current" - Isc - look it up on the label on the back of the panel.
     
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  6. Guest3123

    Thread Starter Member

    Oct 28, 2014
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    No actually it was a circuit involving a LM2940CT-5.0/NOPB IC. Was using it to drop 8VDC to 5VDC @ 500mA. Turns out not good to measure it that way in parallel with the circuit, turns out the proper way was to measure a circuit in series, so you don't blow your digital multimeter, like I did. I guess when a person does it that way, it's safe to say they need to go online "like I did after" and learn how to measure current. So I did, and like I said, the proper way to measure current in a circuit is to measure it in series, not in parallel, like the 2nd image notes.

    So.. Still confused. Unfortunately.. Tried looking online for the answer, and couldn't find any decent sites that could give a good explaination as to why people are measuring current like the 2nd image notes. I've learned the hard way, and the IC got very hot, even when I used a heat sink with my chip, and the meter displayed 10+ Amps.. Then when I did learn how to measure current the right way (SERIES), it was giving incorrect readings on the DMM. So I checked online, and read up on a couple other forums, and turns out that there was a good chance my meters fuses were blown.. Went to mouser, bought brand new BRIT 240v 10A & 1A fuses, and when I put the new ones in, the meter was displaying the proper readings.. So.. that's why I'm just bewildered as to why in all of gods green earth, why would anyone try to measure anything like that if there's a risk of blowing a fuse, or in a worst case senario, ruin the $50 or $300 meter altogether.
     
  7. Guest3123

    Thread Starter Member

    Oct 28, 2014
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    What is "Isc" why would you want to measure a the current in a short circuit? What's the point.. It's not the correct reading for the current. Is it..?
     
  8. Lestraveled

    Well-Known Member

    May 19, 2014
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    Yes it is. Solar panels are constant current devices and Isc (current short circuit) is the current outputted by the panel into a dead short. It is a measurement of the maximum current that the panel can produce at that illumination.
     
  9. Guest3123

    Thread Starter Member

    Oct 28, 2014
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    Currently (12:26AM). So latter on today, when the suns pops it's head from beyond that horizon, instead of measuring my solar panels in series like the 1st image notes, I should measure the solar panels current (A) the way that the 2nd image notes (parallel)? That's the correct way? What's more accurate when measuring the current, when hooked up to a load, 1st image or 2nd image..? 2nd image right?
     
  10. Lestraveled

    Well-Known Member

    May 19, 2014
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    A better answer as to "why" you make these kinds of measurements is, suppose you are building an electronic device that will be driven by the solar panel. You would need to know what the maximum conditions that your device would have to survive under.
     
  11. Guest3123

    Thread Starter Member

    Oct 28, 2014
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    Survive under..? I don't understand the terminology.
     
  12. Lestraveled

    Well-Known Member

    May 19, 2014
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    It depends on what you want to measure. If you want to know the amount of current your load is using, use picture #1. If you want to measure the max current potential of your panel, use picture #2.
     
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  13. Lestraveled

    Well-Known Member

    May 19, 2014
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    OK, suppose you were supplying power from a solar panel, to a regulator that could only handle 3 amps. You would want to know what the max current output of your panel was, so you could "size" or select the right sized panel for your regulator.
     
  14. Guest3123

    Thread Starter Member

    Oct 28, 2014
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    I think I understand now. Thanks.

    I'm a little new to solar panels, and have only used solar panels to charge a cellphone using a step up boost converter so far..
    So what about people that have 2000 watt solar panel systems..? How do they control all that current? They don't go out and buy a solar panel for each device in they're house.. do they?

    A House will only supply voltage to the control panel, and hook up circuit breakers (20 & 15 amp). The electric company doesn't supply current, it only supplies electricity in the form of voltage, typically AC or alternating current.
     
  15. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    "The electric company doesn't supply current"

    erm... I think you'll find that it does.
     
  16. Guest3123

    Thread Starter Member

    Oct 28, 2014
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    How many Amps do they usually give you?
     
  17. blocco a spirale

    AAC Fanatic!

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    It depends on your electricity supplier. Here in the UK, consumer units are usually protected by a 63A breaker but this is preceded by a 100A fuse which is the property of the electricity company. More rural areas may have lower current supplies.
     
  18. NorthGuy

    Active Member

    Jun 28, 2014
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    The mains power is a voltage source - it provides certain voltage regardless of current (to some extent of course). Each load draws as much current as it wish. If you short it with your ammeter, it'll try to draw enormous amount of current, which will kill the ammeter or flip the breaker.

    Solar panel is a current source - it provides certain current regardless of voltage (to some extent). If you short it with your ammater, it still be the same current (but very little voltage), which will not harm the ammeter if the ammeter current rating is above Isc printed on the back of the panel.

    If the panels you use are big (more than 100-200W) you may get some sparks and fire when you connect/disconnect the ammeter, so cover panels with something while connecting/disconnecting.
     
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  19. Guest3123

    Thread Starter Member

    Oct 28, 2014
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    Thanks, very helpful. I knew the electric company doesn't give you current, only voltage. I'm really excited about getting into Solar. So far I only have a tiny three solar panels each rated at 1.5w @ 4.5v. But maybe my next paycheck I'll get one of those 12v @ 100 watt solar panels. Not bad for only $150. Inverters really aren't that expensive either, nore are the MPPT charge controllers. For what you get out of the deal "Free Electricity" it's really impressive. Paycheck after paycheck, could invest in solar and get panel after panel, until I have like 5kW, lol idk.. but $150-$200 per monocrystalline panel, isn't very much. Not if you take your time, and don't blow your whole paycheck on a 5 solar panels every month, maybe 1 or 2.
     
  20. NorthGuy

    Active Member

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    250-300W panels will be much cheaper per W. You also will need batteries unless you want to grid-connect your panels.
     
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