understanding garden solar light circuit

Discussion in 'General Electronics Chat' started by tab a, Jan 15, 2015.

  1. tab a

    Thread Starter New Member

    Jan 14, 2015
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    I'm playing around with some old garden solar lights, and 'think' I understand most of this circuit, but do not understand the purpose of D2. Could someone en'light'en me? :rolleyes:

    Thanks.

    solar garden pcb232.jpg
     
  2. wayneh

    Expert

    Sep 9, 2010
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    It prevents the small current flow from Bat+, thru the bias resistors, and thru D3, when Q1 is OFF.
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    And, D2/R3 provide positive feedback to latch the regulator ON once it starts. This prevents the LED light output from being detected by the CDS and shutting off the circuit.

    BTW, your diagram would be much more understandable if all of your ground symbols pointed down, and were a correct ground symbol.

    ak
     
  4. ian field

    Distinguished Member

    Oct 27, 2012
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    All the solar garden lights I've seen to date used a single nickel cell, which mandates some form of oscillator/inverter which uses a flyback inductor to generate the 3.4V for a white LED.

    It tends to be the older ones that use an LDR - the cadmium sulphide is toxic and not RoHS compliant, the modern ones sense charging current from the solar panel to shut it down in daylight.

    The discrete component variety are getting rare too - these days its usually a black-blob chip or sometimes a 4 lead chip about the same size as TO92.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    +1 I haven't seen one in a long time, a decade at least.
     
  6. takao21203

    Distinguished Member

    Apr 28, 2012
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    None of these "solar garden lights" is any good

    The output of the tiny solar cell is so little they would only be effective in Africa
    The small battery gives a few hours when new but that becomes less and less pretty quick
    The light output isnt much useful for anything
    Typically the whole thing after a year shows signs of corrosion
    None of these junk properties warrant to use massive metal components for its construction, which indeed could last centuries
     
  7. ian field

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    Oct 27, 2012
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    I found the wreckage of one built with discretes, on the footpath behind the flats.

    It was also a rarity in having 2 Ni-Cd cells - which were still in the holder.

    That was before I was brave enough to make my own lithium chargers for the e-cigarette - Ni-Cd are the only other sealed rechargeable that can handle the current draw.
     
  8. profbuxton

    Member

    Feb 21, 2014
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    I totally agree with takao21203, These things are total waste of precious resources for no good purpose.
     
  9. wayneh

    Expert

    Sep 9, 2010
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    The same could be said of any of us. :cool:
     
  10. ian field

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    Oct 27, 2012
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    I-robot.
     
  11. tab a

    Thread Starter New Member

    Jan 14, 2015
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    Thanks. As usual, makes sense once someone explains it! :)
     
  12. ian field

    Distinguished Member

    Oct 27, 2012
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    Actually; in I-robot, the machines decided humans needed 'looking after' - it was Terminator machines decided the human race was surplus to requirements.
     
  13. wayneh

    Expert

    Sep 9, 2010
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    Well, I was in a hurry and stated the obvious. But if that is ALL it is doing, then removing it would make little difference, just a small current leak. I suspect it does more than that, something about biasing Q2 when Q1 is conducting, but I couldn't quite figure that out. Take it out and see what happens!
     
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