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Led Flashlight Help

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Discussion in 'LEDs and Optoelectronics' started by DiodeDave, Mar 4, 2017.

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  1. Mar 4, 2017 #1
    DiodeDave

    DiodeDave

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    Dec 7, 2011
    I have an LED flashlight that runs on 3 AAA batteries.

    I want to run it with a 12 volt battery.

    I have adjusted a dc to dc converter to 4.2 volts.

    The flashlight gets very hot. (It doesn't get hot when the AAAs are connected)

    I've checked my volt meter to see if the dc to dc converter was adjusted correctly, and it seems to be.


    What am I missing? Why does it get hot on the 12 volt battery power?

    Thanks
     
  2. Mar 4, 2017 #2
    Rayregula

    Rayregula

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    Dec 20, 2016
    Use your multimeter to measure the current through your flashlight using the AAAs and again with the dc/dc converter.
     
    Last edited: Mar 4, 2017
  3. Mar 4, 2017 #3
    DiodeDave

    DiodeDave

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    Dec 7, 2011
    The dc/dc converter is rated at 3A

    What is my solution?

    Do I dial the dc/dc converter down?
     
  4. Mar 4, 2017 #4
    davenn

    davenn Moderator

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    Sep 5, 2009
    no, you weren't asked for it's rating
    you were asked to measure the current being drawn using
    1 --- the battery supply and
    2 --- the dc-dc converter output
    when each is being used
     
  5. Mar 4, 2017 #5
    duke37

    duke37

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    772
    Jan 9, 2011
    The AAA batteries have considerable resistance which limits the current. You can put a resistance in series with the power supply to get the same current as from the battery.
    You could drop the voltage of the psu but it will not be as stable as a resistance when temperature changes.
     
  6. Mar 4, 2017 #6
    Audioguru

    Audioguru

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    Sep 24, 2016
    Measure the voltage of the "4.5V" 3-cells AAA battery when the light is turned on. It is probably only 3V. Then adjust your DC converter to produce the same voltage.

    My cheap Chinese flashlight has 24 white LEDs and it came with three cheap "Super Heavy Duty" AAA battery cells. When the cheap battery soon ran down I replaced it with American AAA alkaline cells then the flashlight became very bright.
     
  7. Mar 4, 2017 #7
    DiodeDave

    DiodeDave

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    Dec 7, 2011
    The LED draws 180ma on my meter. If I use the LED calculators found on the internet, the result is to use a 1 ohm resistor since the source voltage and forward voltage are the same.

    I'm thinking Audioguru has the solution. The voltage drops to about 3.4v when the LED is turned on. There isn't any voltage drop when the LED is connected to the DC DC converter (which has a large 12v battery connected).

    Thanks for the responses

    Dave
     
  8. Mar 4, 2017 #8
    Audioguru

    Audioguru

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    Sep 24, 2016
    A problem with using a current meter in a low voltage circuit is that the meter uses a resistance in series with the load to measure the voltage across the resistor then calculate the current. But the resistor reduces the actual battery voltage and its current.

    You never power an LED with a voltage source because the LED's required voltage changes with its temperature. Then it heats and draws more current which makes it hotter which makes it draw more current which makes it hotter which makes it draw more current which.... (thermal runaway).
     
  9. Mar 4, 2017 #9
    Audioguru

    Audioguru

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    I forgot to say that most LED calculators are VERY wrong when the battery voltage is too low for the resistor to do anything. One ohm with 180mA in it has a voltage across it of only 0.18V. The LED's voltage changes more than that when its temperature changes, then the current more than doubles.
     
  10. Mar 26, 2017 #10
    Herschel Peeler

    Herschel Peeler

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    Feb 21, 2016
    More info on your DC to DC converter, please. 12 V in, 4 V out means you waste 8 V in heat?
     
  11. Mar 26, 2017 #11
    (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    The best solution is to get an SMPS with an adjustable current limit in addition to the output voltage setting.

    Start with the voltage set to something safe (like 4V) and the current set to close to the minimum. Apply power and wind up the current until the torch is the correct brightness.

    The main problem you have is that the torch is designed to use the internal resistance of the cells to limit the current. As @Audioguru has noted, using better batteries you get a brighter torch. A regulated power supply is like really good (almost perfect) batteries. If not limited, it will try to provide far more current than the LEDs usually take. It may be sufficient to cook the LEDs or the power supply.

    Good torches have a constant current driver but in, but good is not cheap, so most cheap torches dispense with it in the hope you only ever use "normal" batteries.
     
  12. Mar 26, 2017 #12
    Audioguru

    Audioguru

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    A cheap Chinese "super heavy duty (carbon-zinc)" battery that is leaking in its package after sitting on a store shelf for months is not what I call a normal battery.

    All 24 very bright LEDs in my cheap torch are still fine but its on-off switch sometimes makes a poor connection.
     
  13. Mar 26, 2017 #13
    (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,500
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    Yeah, by "normal" I mean an airline cell or similar (even that leaky carbon-zinc one) as opposed to something with a very low internal impedance. Most rechargeable cells have significant lower internal resistance.
     
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