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  1. #1
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    Beginner LED questions

    Hi guys,

    I've been looking around the forum for a couple of weeks now toying with the idea of making my own mtb light and trying to soak up as much info and learn everything that it needed.

    I have picked up a lot of information and know an awful lot more now than I did a few weeks ago but still have some questions that I am hoping some of the more experienced guys can answer.

    1. When looking at datasheets for LED's I don't really understand the relative luminous flux vs forward current graph means?
    2. Does forward voltage vs forward current just show the current through the LED for a given current?
    3. How do you calculate the required voltage of the battery? Is it just forward voltage of all LED's added together? Do you have to take into account voltage for a driver or a switch with an LED?
    4. To calculate battery life is it for example one battery with 3000maH and one battery drawing 3000ma, battery would last an hour?


    I think those are all my basic questions for now! Hopefully someone can help me.

    Thanks in advance.

  2. #2
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    1. luminous flux for LEDs is usually determined at 350mA (binning current). However, most LEDs can cope with far higher currents than that, so to estimate the luminous flux at the higher current, figure out the conversion rate from the graph and times the lumens at the binning current by that. So if an LED produces 100lm @350mA and has a conversion rate of 250% @1A, flux @1A is 100lm x 250% = 250lm.

    2. LEDs are constant current devices and the voltage varies depending on the fixed current. So, the higher the current, the higher the voltage.

    3. Required battery voltage depends on the driver type (buck/boost/linear) and no. of LEDs. If using a buck driver, no. li-ion cells = no. LEDs + 1.

    4. Depends. If a single cell driving a single LED using a linear driver, then yes, divide battery capacity by current to get runtime. If using a buck driver, you need to know current draw from the battery (which will be lower than at the LED) to do the same.

    HTH

  3. #3
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    here are my simple answers for how I understand these:

    1) The graph basically is how you can quickly cross reference the rated output at a particular current. If you were planning to drive an LED at say, 1.5A - then you could look up what the manuf says it should give as output at that current. also - if you knew you wanted 400lm out of an emitter, then you could look up what current it would require.

    2) correct. Once you've used the graph in question 1, then you know what vf your LEd should present to your driver. Typically, as current increases, vf increases. So you'd always want to caluclate your driver current and battery requirements off the highest vf, but be mindful of what the vf is at lower currents, if you are making a multi-level light

    3)this depends on several things.
    3a)Primarily, you have to decide what type of driver you are going to be using - boost, buck, liner, etc.... Buck drivers require ~1.5volts more than the total Vf of the LED's to maintain constant current. Boost drivers usually need to see less input voltage than the output. Linear need to be as close input voltage to output as possible, to avoid excessive heat output.

    3b) You also have to determine how you're connecting your LED's - typically they are configured series, although some parallel applications could be used.
    (there are plenty of other considerations here, depending on particular applications)

    4) Depends on your selection of driver and application.

    you can download a runtime calculator here that can help you with the math for some applications. LED Calculator Program

  4. #4
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    Matt always makes things make more sense than I do

  5. #5
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    Both answers have helped thanks guys. Some parts seem relatively simple but I think I will need someone to check my sums and wiring diagrams before I went ahead and made something!

    I have one more quick question for now. When I've looked on ebay, there are some lights which are 5000 lumens (supposedly) however they are using T6 emitters and according to the datasheet their max lumens is 975. 3x975 doesn't equal 5000. Are these chinese sellers lying or am I missing something?

  6. #6
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    Quote Originally Posted by adrenalnjunky View Post
    Matt always makes things make more sense than I do


    Quote Originally Posted by kevinv89 View Post
    Both answers have helped thanks guys. Some parts seem relatively simple but I think I will need someone to check my sums and wiring diagrams before I went ahead and made something!
    you're welcome. Just get a plan together and we'll help critique it

    Quote Originally Posted by kevinv89 View Post
    Are these chinese sellers lying or am I missing something?
    yes and "the truth"

  7. #7
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    Yeah I thought they were missing the truth!

    I've seen a lot of people using Taskled drivers but for a first attempt I am looking for a cheaper option. I saw some people using drivers from deal extreme. Would you recommend these or do you recommend any other cheap drivers?

  8. #8
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    A couple of those DX-style drivers are pretty decent units - and for the ~$5 cost of entry, there's not a lot holding someone back from experimenting. I have a couple laying around too, waiting for me to cook up something to put them in.

    Outline the design/build/component mix you have in your head, at least for battery/LED(s)/driver and we could make suggestions based on what you're thinking.

  9. #9
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    Yeah I think it would be best for me to think about the whole setup and get some advice on the best driver for the job! I'll have a good look into what I'm wanting tomorrow and get something together for a critique.

  10. #10
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    Kevin89,

    You also might find the Cree Product Characterization Tool to be very useful.

    Make up a company name, phone number, etc. If you don't want to give your email use 10 min email and they'll send the link within a minute or so.

  11. #11
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    What you guys have given me so far has been really helpful. Can some explain what bin means?

    Sorry for these basic questions, just trying to get my head around this stuff!

  12. #12
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    Can someone also explain these use of this graph?

    http://swhs.home.xs4all.nl/fiets/tes...ANSI-white.jpg

  13. #13
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    The V2 drivers from Kaidomain are good cheap drivers. You can also buy custom programmed versions off a couple of people on the Budget Light Forum DIY section which have almost all the functionality of the TaskLED drivers but for a lot less money. There's also a guy called PilotPTK who's starting to make his own drivers that look interesting and will be similar in cost to the TaskLED ones. Plenty of options

    Bin = brightness range. Think of it like CPU speed grades, but for light. Some are faster/ brighter than others, so they go in a higher "bin". Cree does it by alphabetical progression (T is brighter than S) then numerical (T6 is brighter than T5). Tint is the "colour" of the output - 1A/1C/1D are cool white so they're very white without being blue, 3C/3D is neutral white so a bit "warmer" with more yellow/red. The warmer tints also have better colour rendition (CRI) so things look more natural than with cool white.

  14. #14
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    I'll have a look around for some cheap drivers. Taskled drivers seem to be the most popular but I don't want to spend that much money and fry it on my first attempt!

    The cree progression makes sense now. The U2/U3 emitters are obviously brightest? I don't think I need to understand the chart at the moment, want to learn the basics first!

  15. #15
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    So on this subject, are buck drivers only really useful for multi LED systems? For single emitters do you just stick with a basic driver?

  16. #16
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    buck drivers are very useful for any time you're going to be driving an LED array with a battery that is higher voltage than what the vf for that array is. That doesn't matter if you're driving one or 5 LED's. buck drivers usually require an input of ~1.5v more than what the vfF of the array is to stay in regulation, so make sure that your pack's discharged voltage is that much higher than the vf. When using lithium batteries, the easy way to figure it is #of leds + 1 cell in the battery. 2 Leds = 3cell lithium battery.

    For single emitters lately, a linear current driver like the TaskLED lflex is very useful for people running a light like a single XM-L (3.3vf) off of a single lithium cell (3.7v), or a multi-cell pack wired in parallel to present a 3.7v (nominal) input.

  17. #17
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    Perfect. That clarifies things a lot. I was wondering why linear drivers would be used, but that clears it up.

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