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Thread: How do I test amp draw from a flashlight?

  1. #1
    Unenlightened
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    Default How do I test amp draw from a flashlight?

    So I just recieved a DX flashlight that has two modes 1200mAh and 350mAh. The difference doesn't seem that great and I was wondering how do people test these things with a DMM (digital multimeter).

    I figured out how to test the voltage output of a battery, but thats about it.

    This would also be nice to know to test a couple other flashlights I have around the house.

  2. #2
    Thread Killer Illum's Avatar
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    Default Re: How do I test amp draw from a flashlight?

    nevermind....
    I'm sure someone with higher expertise will chime in, identify the errors I have made and make me eat my words
    Last edited by Illum; 08-30-2007 at 10:52 PM.

  3. #3

    Default Re: How do I test amp draw from a flashlight?

    Remove the tailcap. One meter lead goes to the exposed negative end of the cell, the other meter lead must contact the bare metal at the end of the battery tube or threads. If your leads are reversed the reading will have a - in front of the numbers, just ignore it. Obviously the meter must be set to the AMP scale, and the test leads connected to the proper jacks.

    On multi level lights it depends on where the level selection circuit is and how it operates. With for example a Streamlight 2L3W just select each mode with the side switch, and measure the current for each level at the tail end. On a Surefire L2 the tailcap itself does the selection, and I'm not sure the current draw of each level can be checked, probably just the high level drain. If the head end does the level selecting simple measurement of tailcap current as described above should give you the results desired. The Surefire U2 would be an example which can have all levels tested from the tailcap end.

    Do NOT mess around with cutting any wires!

    Paladin

  4. #4
    Unenlightened
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    Default Re: How do I test amp draw from a flashlight?

    Cool thanks. I'll try it out once my lithiums are fully recharged.

    So I just checked the tailcap itself testing the resistance and it seems like the switch is in the tailcap as the resistance goes up once set to low mode.

    It was about
    .5 ohms on high
    3.0 ohms on low

    So if high is 1200mAh drain then low should be 200mAh? (1200/6 = 6x the resistance on low).
    Last edited by guy121; 08-31-2007 at 01:37 AM.

  5. #5
    Flashaholic* Long John's Avatar
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    Default Re: How do I test amp draw from a flashlight?

    guy, if your light is regulated and you want to know the driving level (current draw) at the Led, there is no other way to measure at the Led directly, shown at this picture:



    In case of a direct driven light, you can measure at the tailcap like described.

    Best regards

    ____
    Tom
    It's cheaper to get a high quality light than spending money for many cheap lights.

  6. #6
    *Flashaholic*
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    Default Re: How do I test amp draw from a flashlight?

    longjohn, nice pics!

    two different types of current draw measurements.

    one measures current from cell, other measure actual current to emitter. as shown in diagram above. nice explanation of where to measure VF (forward voltage) too.

    use higher settings on your meter, 10amps. at low current settings, internal resistance of meter will throw off measurements, unless you compensate. much easier to use high settings. besides using low current settings risks blowing internal fuse. Fluke meters fuses are $10+ each.. ouch.....

  7. #7

    Default Re: How do I test amp draw from a flashlight?

    measuring voltage at the led is straightforward enough - the toughest part is trying to avoid getting blinded while touching the legs with the probes! measuring current seems much more complicated - i am not sure how to do it other than by cutting off one of the legs of the led (!) as depicted by longjohn. perhaps there is a roundabout way by measuring voltage and resistance and using I = V/R?

    guy121, it sounds like you have one of those lights where the switch has a resistor soldered in (used in the U2-clone, WF-series etc) so you will only be able to measure the high level current at the tail-cap.
    [SIGPIC][/SIGPIC]

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