Info for changing voltage

Discussion in 'General Electronics Chat' started by spike1947, Mar 4, 2016.

  1. spike1947

    Thread Starter Member

    Feb 4, 2016
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    Hi

    I am attaching a drawing of what seems a simple circuit ! , what calculations would I have to do to have this same circuit run on a 2.4v supply .

    it may seem simple to others ,but not me ! .

    cheers

    spike
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Consider:
    What LED will you use? A white LED needs about 3V to give a reasonable light output. A red LED needs about 2V.
    A 2.4V supply can't charge a 9V battery unless a DC-DC boost converter is used.
    A 1N4001 diode drops about 0.65V; that's a big chunk of the 2.4V you're starting with. Schottky diodes drop less voltage.
    Reistor values would need to be reduced.
     
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  3. #12

    Expert

    Nov 30, 2010
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    You have chosen a random circuit which has nothing to do with 2.4 volts, and it never will.
    Are you trying to charge a 2.4 volt battery?
    Better to say what you need. Somebody here might know how to do it.
     
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  4. spike1947

    Thread Starter Member

    Feb 4, 2016
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    Hi

    Thanks for all your answers , and I am sorry for being to blunt in my enquiry , what I am wanting to do is use 2 x 1.2v AAA rechargable battery's and a solar panel that produces 4.36v , it could be a little higher than that , I am at work and taking this from memory , the resister to the LED I off course can work out , my LED is 1.8 to 2.2 v , I really wanted to know regarding the resister size from the solar panel to the transistor base connector , I do not know how to work that one out .

    cheers

    spike
     
    Last edited: Mar 4, 2016
  5. #12

    Expert

    Nov 30, 2010
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  6. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Start with the desired LED current. Divide that by 20 or more. An ancient rule of thumb says 10, but, like many other things from the Eisenhower administration, it does not apply to contemporary devices and applications. Remember, every uA than goes through the base does not go through the LED. The tradeoff is that too low a base current prevents transistor saturation, which decreases LED current and brightness.

    Rbase = Vbase / Ibase = (Vbat-0.6-Vpanel(dark)) / (Iled/20+)

    ak
     
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  7. spike1947

    Thread Starter Member

    Feb 4, 2016
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    Hi
    I have mentioned the battery voltage as 2x1.2v rechargeable , I don't have battery's as yet , prob NiMH .

    Cheers

    Spike
     
    Last edited: Mar 4, 2016
  8. wayneh

    Expert

    Sep 9, 2010
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    I'd be tempted to use that fact to eliminate the 100Ω resistor. It would require a little experimentation to get the right base resistance to dial in the brightness you want. But when the supply voltage is only 2.4V max anyway, it seems like it should be OK.
     
  9. WBahn

    Moderator

    Mar 31, 2012
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    I'm still unsure of what the goal of this circuit is. Under what conditions is the LED supposed to be on?

    Notice that your base current is a reverse current in the solar cell. What is the effective resistance of the solar cell when it is being overdriven? Does this damage the solar cell?

    But let's start with a statement of the goal -- you have a black box to which you connect an LED, a solar cell, and a rechargeable battery. What is the purpose of the box? What do you want it to do?
     
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  10. HitEmTrue

    Member

    Jan 25, 2016
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    I was puzzled, too, when I first read the thread.

    But my money's on this: use solar power to charge the battery.
     
  11. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Solar power charges battery and inhibits LED. Darkness turns LED on with reverse current through the solar cell.

    ak
     
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  12. spike1947

    Thread Starter Member

    Feb 4, 2016
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    Thanks
    That makes me understand the circuit a lot better now .

    cheers

    spike


    Cheers thanks for that info , again now I understand a bit more in this amazing world of electronics .


    spike


    As mentioned it says what it is in the drawing ! .
     
    Last edited: Mar 6, 2016
  13. spike1947

    Thread Starter Member

    Feb 4, 2016
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    It says in the schematic what it is ! , I want the flickering LED 1.8v to 2.2v to turn on when it gets dark and turn off and charge the batteries 2 x 1.2v NiMH , when it gets light .

    spike
     
    Last edited: Mar 6, 2016
  14. ian field

    Distinguished Member

    Oct 27, 2012
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    Most commercial solar garden lights use a single 1.2V cell and usually a single transistor blocking oscillator (Joule thief) to boost the voltage for the LED. Occasionally I've seen 2 cell types and other varieties of oscillator. Most recent types use a custom chip that integrates the sunlight sensing with the oscillator, usually the only external component is a small inductor.
     
  15. wayneh

    Expert

    Sep 9, 2010
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    Yes, if I was doing this project, I'd just hack a commercial light - which are often free at Menards - to replace the LED with a flickering one.
     
  16. spike1947

    Thread Starter Member

    Feb 4, 2016
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    Thanks for all your replies , have tried with a cheap garden solar light , but the flickering led does not work with a joules thief , I think it has something to do with a chip built into the flicker led ! .

    Cheers

    Spike
     
  17. ian field

    Distinguished Member

    Oct 27, 2012
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    Put a small electrolytic across the LED - maybe about 10 - 47uF.
     
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