Solar to LM7805 to Powerbank

Discussion in 'General Electronics Chat' started by alecksha, Jan 27, 2015.

  1. alecksha

    Thread Starter New Member

    Jan 27, 2015
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    Hi everyone i got this school project which will be needed on march for my final project, im just new to electronics. here is the picture my concern is if sunlight is stable will it charge my powerbank continuously? please do give some advise or do and don't on the picture thank you
     
  2. pwdixon

    Member

    Oct 11, 2012
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    Rather depends on the voltage from the solar panel and what the current is going into the power bank, you might find that the regulator gets hot if the solar panel outputs much more than 7V or 8V.
     
  3. alecksha

    Thread Starter New Member

    Jan 27, 2015
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    here is the solar panels specs.

    10W Polycrystalline Solar Panel
    Cells per module: 36pcs
    Maximum System Voltage: 800V
    Test Condition: 1000W/M² AM 1.5 Global, 25 °C Cell Temperature
    Maximum Power (Pmax): 10W
    Tolerance: 5%
    Voltage at Pmax (Vmp): 18.0V
    Current at Pmax (Imp): 0.555A
    Open-circuit voltage (Voc): 21.6V
    Short circuit current (Isc): 0.621A

    some of my classmate says i have to put heat sink for regulator since the panel says its 18v i hope my solar panel wont burn my regulator. thank you for the fast reply
     
  4. MikeML

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  5. nsaspook

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    alecksha likes this.
  6. MrChips

    Moderator

    Oct 2, 2009
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    Why do you want to use an LM7805 regulator?
     
  7. wayneh

    Expert

    Sep 9, 2010
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    You want your system to operate well at full (max) power conditions, and the 7805 would not be able to handle that much. It would be dropping 13V at 0.5A, requiring it to dissipate 6.5W. It will go poof.

    Perhaps worse than that, your panel is cranking out 9W and you are burning off and wasting most of that.

    Forget the 7805 and look into a DC-DC converter. These wonderful things can convert your power to the lower voltage while losing only ~15% of the power your panel is making. They are cheap and ready-made from many sellers on e-bay. Get one rated for well over your max conditions of open-circuit voltage and short-circuit current, so that nothing your panel puts out can blow it.
     
  8. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    You have given only half of the system-- need specs on power bank. Nice advantage of buck converter is that output V is regulated & adjustable. When all ic connected, adjust buck converter, BC,output so as to load SP V to about 15.75 V ( 75% of OC V ) which will give near max. power.
     
    Last edited: Jan 27, 2015
  9. wayneh

    Expert

    Sep 9, 2010
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    Bernard, see the PV panel specs the OP offered in #3, giving the max power point settings.
     
  10. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    The TO-220 package without a heatsink will overheat with more than about 1 W, so you *will* need a heatsink. If there is no fan and you are relying on radiation and still-air convection, find one about the size of a golf ball.

    ak
     
  11. Bernard

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    Should have used 83 %, seems a little high, but my panels are a little old.
     
  12. Evil Lurker

    Member

    Aug 25, 2011
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    I believe the OP is inferring to use a 7805 to regulate the charging voltage of the solar panel. This is incorrect on so many levels.

    Solar panels charge batteries in two states based upon the luminous intensity at which they are being exposed and ohms law. With a battery that is depleted but otherwise in good condition it will have very little resistance to accepting a charge and in this state the solar panel is said to be operating in constant current mode. As long as the current output from the solar panel is less than the maximum permissible charging rate of the power bank then no sort of regulation is needed during this phase. When the battery voltage is approaching the level where it is almost at full charge the internal resistance of the cell will increase, charging current will drop, and the voltage provided by the solar cell will rise accordingly. It is around this point where charging shifts from a constant current mode to a constant voltage mode to ensure that the batteries do not get overcharged. The way to go about this is not with a linear voltage regulator that controls the charging voltage but rather to use a shunt regulator that dumps excess power to ground. The difference between the two is that with a linear regulator it would operating all the time and wasting massive amounts of power coming from the panel, but with a shunt regulator it only works when the battery voltage reaches a certain level. The device is known as a TL-431 (many variants exist with similar names) adjustable shunt regulator. And since your panel is likely to exceed the current of the shunt regulator you will need to rig up a pass transistor to the TL-431 to handle the extra power.
     
  13. alecksha

    Thread Starter New Member

    Jan 27, 2015
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    good day my teacher says i will try to use LM7805 i hope there is another possible regulator to use beside it
     
  14. MrChips

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    Oct 2, 2009
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    As Evil says, you do not necessarily need a voltage regulator to charge a battery.
    What you need is a voltage monitor to check when the battery is fully charge and then switch to a float charge.
     
  15. alecksha

    Thread Starter New Member

    Jan 27, 2015
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    thank you i already got a a heatsink its more than a size of a golf ball i got it from an old computer processor with add on fan with 5v power
     
  16. alecksha

    Thread Starter New Member

    Jan 27, 2015
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    @ Evil Lurker thank you sorry im not good at electronics but ive been reading your post it does mean i need some switching device that shuts the regulator feeding my powerbank as soon it is full charge btw sir im using Powerbank 8800mAh it has a charge protection and led indicator on full charge and it automatically trip off power after full charge i hope that alright thank you sir
     
  17. alecksha

    Thread Starter New Member

    Jan 27, 2015
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    Powerbank specs
    Type: 8800mAh Mobile Power Bank
    Available Colors: Gold/Silver/Green/Black/ Blue/Yellow
    Material: ABS Plastic
    Dimensions: 110mmm x 28mm x 80mm (L x W x H)
    Packaging Dimensions: 112mm x 31mm x 215mm (L x W x H)
    Charging cable: 300mm(L) Micro USB
    Net Weight: 215g
    Gross Weight: 251g
    Capacity: 8800mAh
    Charging indicator: 4 Blue light (running)
    Full charge Indicator: 4 Blue Light (Stable/Auto trip off on full charge)
    Input Voltage: DC 5V / 1000mAh
    Output Voltage: “ USB1: DC 5V / 1000mAh “ USB2: DC 5V / 2100mAh
    Battery: Lithium-ion
    Charging time: 8-10 hours
    Battery usage time: 5 Hours

    thank you so much sorry for the late reply
     
  18. alecksha

    Thread Starter New Member

    Jan 27, 2015
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    thanks sir but its not possible for us to use ready made converter since its a school project also i need to put them piece by piece in a place so that i can tell them what this part is use for.
     
  19. wayneh

    Expert

    Sep 9, 2010
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    The 7805 will work. It just has two major shortfalls: 1) Poor efficiency and 2) Limited capacity due to heat.

    But if you just want 100mA at 5V from your panel and don't care about the efficiency, the 7805 will be just fine. You can add a power transistor to help handle the current and heat, so that your load can draw more total current. This cannot solve the efficiency issue.
     
  20. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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    I think it is time to find a new teacher.
     
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