string of 4.5V flashlights on 24V transformer

Discussion in 'The Projects Forum' started by RJ2, Aug 4, 2010.

  1. RJ2

    Thread Starter New Member

    Aug 4, 2010
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    I would like to know the resistance needed to run (9 led) flashlights pulling about 45MA on the 4.5v batteries, to run on a 24V lighting transformer. I would like to run about 26 of them in parallel on a string. Thanks
     
  2. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Have you thought about the circuit you need to design?

    With 24 * 2^.5 - 0.7 as the input voltage to a capacitor, you could have more than one of those batteries in series, lowering the overall current in the circuit.

    What is the specification on the 24 V transformer? Do you have a model number?
     
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  3. RJ2

    Thread Starter New Member

    Aug 4, 2010
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    I am made 12 wrought iron plant holders and was going to light them up with leds . I found that the 4.5v flashlight at HD would be a great choice because they are already waterproofed. My problem is the 9 led flashlights already have premade resisters on them to run at 4.5 volt, and am confused on what I need for dropping the other 19.5 volt if I want to run them in parallel
    I have 24 that I would like to hook up. That would be 12 pairs of planters so I could pair off each planter in series if that would help. Thanks
     
  4. RJ2

    Thread Starter New Member

    Aug 4, 2010
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    I made 12 wrought iron plant holders and was going to light them up with leds . I found that the 4.5v flashlight at HD would be a great choice because they are already waterproofed. My problem is the 9 led flashlights already have premade resisters on them to run at 4.5 volt, and am confused on what I need for dropping the other 19.5 volt if I want to run them in parallel
    I have 24 that I would like to hook up. That would be 12 pairs of planters so I could pair off each planter in series if that would help. Thanks
     
  5. Melchior

    New Member

    Feb 2, 2008
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    Go here:

    http://led.linear1.org/led.wiz

    Check how these (PCB?) LEDs are wired; series or parallel with a voltage booster.


    4.5 Volts Seems odd, Usually arrays run from 12 Volts, 9.6, 6 V or some other battery friendly number.


    Basically you can wire up sets of parallel arrays in series - if the voltage can be evenly divided by the number of arrays present - and the voltage will never rise above its usual output.


    Also, 5mm LEDs are usually junk, even more so if they are found in flashlights - they may not last very long (poor quality phosphor - and the LED dice usually fail faster than normal on bottom barrel epoxy LEDs).
     
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  6. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Is that a 24 volt DC lighting system and if so, what is the current rating?

    If it is a 24 volt AC transformer, you'll have to build a power supply. In that case, the DC voltage will be closer to 34V.
     
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  7. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Measure how much current one of the flashlights draws then use ohms law to deterimine what additional resistance you'll need to drop the extra voltage.

    If you want to run all the flashlights in parallel just multiply the amount of current one draws by the number of flashlights to get a total current and use ohm's law again to find what amount of resistance and at what wattage you'll need.

    Just remember that these flashlights are probably not going to draw exactly the same amount of current each so you might want to test a few to get an average value per light. You can also expect that due to non-identical current draws there may be slight variations in how bright each light ends up being, but LEDs are somewhat forgiving about that and you probably won't even notice any variations.

    As mentioned above you will want at least a single rectifier on the transformer, and if you want accuracy stick an LM7824 VReg IC in the circuit as my guess is a dozen of them isn't going to draw more than the 1.5A most 7824 ICs are capable of when heatsinked properly.
     
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  8. RJ2

    Thread Starter New Member

    Aug 4, 2010
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    I do have a 12v 88 watt , 1A, lighting transformer I could use if that would be better than the 24 volt 600 watt ( think it was around 4A). what is confusing me is I am trying to hardwire 25 of the 9 led flashlights in parallel that have already been configured to run on the 4.5 v . Another monkey wrench would be the gauge needed and resistance of the 150 ft of wire .
    i think maybe if i multiply the 25 lights times the 45 milliamps = 1125...... the 12 volt 1A would not work?
     
  9. RJ2

    Thread Starter New Member

    Aug 4, 2010
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    The transformer is 24 volt 600 watts 4.2 amps. the fact that these led flashlights have already been configured to run with 4.5 v batteries and I measured the draw in the flashlight at 45 MA. Now i want to hardwire 25 of them with 150 ft of wire . what are your thoughts on that. Thanks
     
  10. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    RJ2,

    Are those just AC power transformers or are they DC supplies?

    You could run the whole thing with a 9 VDC 1Amp walwart. You would connect two LEDs in series and then have 12 parallel paths. Then the lone LED would require a resistor to drop the 4.5V @ 45 mA, or 100 ohms half-watt.

    Are the bulbs going to be distributed over the 150 ft or are they going to be 150 ft away from the supply? At 150 ft from the supply, there will be a small voltage drop across the wires (from one side to the other). This may decrease the 45 mA drawn per leg. You would have 13 legs drawing 0.045 A or a total of .585 Amps.

    You've just described the transformers as lighting transformers so I suspect they are AC. If you wish to still use the transformer, you can design a power supply to produce the 9V, a full wave rectifier, filter capacitors, and a 7809 regulator.

    What size wire are you using for that 150 ft run? What is the part number on the LEDs?
     
    Last edited: Aug 12, 2010
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  11. RJ2

    Thread Starter New Member

    Aug 4, 2010
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    JoeJester,

    Thanks , I have 9 columns with 2 planters each and 2 corners with 3, so 24 flashlights , with about 15 ft between them totaling about 150 ft wire. The 12 X 2 in series, will work out great the corners with the extra one , I will drop with the resister. What would you suggest on the wire. Thanks Rj
     
  12. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Choose a wire with a low resistance per foot. Even 18 awg wire has about 39 ohms per 150 feet, and that will affect the voltage delivered to the furtherest point.
     
  13. RJ2

    Thread Starter New Member

    Aug 4, 2010
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    Would there be an advantage to placing the power supply in the middle?
     
  14. JoeJester

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    Apr 26, 2005
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    It would cut the wire run to 75 ft ... so yeah.
     
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  15. RJ2

    Thread Starter New Member

    Aug 4, 2010
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    I do have a 5v 750 ma walwart , think I might try using that.
     
  16. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    25 times 0.45 = 1125 mA. Your wallwart won't like that one.
     
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  17. RJ2

    Thread Starter New Member

    Aug 4, 2010
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    I do have a 6v 3a walwart. do you think I would need to drop the voltage down considering the 150ft of wire?
     
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