LM317 12V to 1.5V

Discussion in 'The Projects Forum' started by pedecamera, Jun 16, 2009.

  1. pedecamera

    Thread Starter New Member

    Jun 16, 2009
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    I have these LED lights in the garden with the little solar panels.
    well they worked for one year, I checked them out.. switches are rusted batteries are a mess. ( I did keep them indoor in the winter )
    But anyway I opened them up, clipped the solar panels, short wired the switch, and used just a battery 1.5 V AA and all works fine.

    I do have a small solar system I use in the house for some small things.

    So what I am going to do is run a wire to the outside 12V and now is my problem I need to drop this to my 1.5 v ( no idea what these LEDs draw. )
    I was thinking to use a LM317
    but when I look at other posts and web stuff I am getting more confused.. so what would the best way be.
    Could I just use a resitor ????
    :cool:
     
  2. kurf

    Member

    Jun 15, 2009
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    0

    Yes I would use a LM317. Find the data sheet online for it. You need to regulate the voltage because if its coming out of a solar panel the voltage and current are going to change. A resistor or voltage divider network will not regulate the voltage for you. Most likely your LEDs need 1.7Vdc and 30mA. Adjust the LM317 circuit to output this voltage and you should be able to supply your LEDs outside.
     
    Last edited: Jun 16, 2009
  3. Audioguru

    New Member

    Dec 20, 2007
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    Solar garden lights usually use a 1.25V rechargeble battery and a voltage stepup circuit similar to a Joule Thief. Then it feeds 3.5V to a white or colours-changing LED.

    You can make an LM317 power supply with a +12VDC input and a +1.25V output with just an output capacitor. Connect its ADJ pin to 0V.
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    The LM317 will throw away a lot of the energy in the form of waste heat. If you are using a power supply then the power is relatively minuscule, but if it is from something like solar cells then every watt counts, and there are much more power efficient ways to do this.
     
  5. pedecamera

    Thread Starter New Member

    Jun 16, 2009
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    Thanks so far..

    If there is a better tool to do this what would it be. Yes I am using Solar panels to Batteries

    Peter
     
  6. kurf

    Member

    Jun 15, 2009
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    Would using a simple common-collector amp with a zener diode work better? Vout = Vzener - 0.7V.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    Yes and no. The transistor will absorb less insertion voltage, but it will still get hot. In my article LEDs, 555s, Flashers, and Light Chasers I show a schematic of something like that in Figure 6,

    [​IMG]

    Is that where you got the idea?

    Wattage of the regulator in this case is dropping voltae X current. ½A is hot, more than that will burn flesh.
     
  8. kurf

    Member

    Jun 15, 2009
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    0
    Nope haven't read that yet, will look through it though.
     
  9. pedecamera

    Thread Starter New Member

    Jun 16, 2009
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    0
    so bottom line.. I am on thin ice here I do not know the LED V and A ( it is an LED changing colours )

    the only thing I know I need to replace a AA battery with something that comes from a 12 V battery ..
    I am not this electronic whiz, I can build something if I know the facts, but this figuring out what I need is beyond me... I am just a little green frog.
     
  10. Audioguru

    New Member

    Dec 20, 2007
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    The colours-changing LED uses 3.5VDC. A simple circuit inside the garden light steps-up the 1.25V from the rechargeable battery cell to +3.5V.

    An LM317 is fine to have a 12V input and a 1.25V output. The current for garden lights is low (22mA measured a minute ago from a 1.25V battery cell) so the LM317 can power many of them without getting too hot. Then nothing in the solar garden lights need to be changed.
     
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