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  1. #1
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    Quick LED and battery question

    Hi I am making a LED bike light using an osram ostar led. It is a 27w max LED, nominal power is 750ma at 17-26volts. At 24 volts it pulls 1 amp. Is there any problem with powering the LED directly connected to a 24v 1.7ahr battery? If I can save the trouble and cost of using a driver I would like to.

    Also is 510 lumens enough for night riding or should I run dual lights for 1000 lumens?

    Data sheet
    http://catalog.osram-os.com/catalogu...02726e000200b6

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    Your battery won't be at 24V except maybe when it's brand new. So you'd be getting less than that. I"m at work so I don't have my excel spreadsheet to calculate run times and what not, but how much run time do you need?

    Also, what kind of night riding/ On a well lit street? or a pitch dark single track?

    Do you already have that LED Handle? That is quite a draw...
    www.mtbiker.ca

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    FSR XC -R7 Platinum - SRAM X7 (26.5lbs)
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  3. #3
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    Quote Originally Posted by mhaskell
    Hi I am making a LED bike light using an osram ostar led. It is a 27w max LED, nominal power is 750ma at 17-26volts. At 24 volts it pulls 1 amp. Is there any problem with powering the LED directly connected to a 24v 1.7ahr battery? If I can save the trouble and cost of using a driver I would like to.

    Also is 510 lumens enough for night riding or should I run dual lights for 1000 lumens?

    Data sheet
    http://catalog.osram-os.com/catalogu...02726e000200b6
    You will need a driver for current limiting to your LED. I'd suggect you get one that allows you to select the drive current. There are several out there, but I'm not sure which ones handle the 24V range. Buckpuck and bflex drivers come to mind... The bflex is really flexible and worth the extra $ IMHO.

    510 lumens is a whole lot of light...

    Your biggest challenge with the light will be handling the heat it will generate. I'd suggest you start by running it at 350ma ... You could probably plan to fry eggs on it at 1 amp, but you'd just burn them .

    Good Luck!
    GEVELTERSCHMIDT RACING

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    Quote Originally Posted by airman
    You will need a driver for current limiting to your LED. I'd suggect you get one that allows you to select the drive current. There are several out there, but I'm not sure which ones handle the 24V range. Buckpuck and bflex drivers come to mind... The bflex is really flexible and worth the extra $ IMHO.

    510 lumens is a whole lot of light...

    Your biggest challenge with the light will be handling the heat it will generate. I'd suggest you start by running it at 350ma ... You could probably plan to fry eggs on it at 1 amp, but you'd just burn them .

    Good Luck!
    Thanks for the help, I'm a thermal engineer so the heatsink is one part that I have all set.

    On page 9 of that sheet there is a foward current vs. voltage graph. I was under the assumption that at 24v the max current would be right around 900ma even without a driver. Could you describe what the current would be like without the driver?

    I also have a dragonstick made by osram, it has 12 led's for a total of 300 lumens. It is made to be driven by a constant voltage however at 24v.

    http://dafnwebpd.sylvania.com/os_fil...p?id=003691989


    Could you give me an idea of how much 500 lumens is in comparison to the top lights out there on the market?

  5. #5
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    Quote Originally Posted by bbgobie
    Your battery won't be at 24V except maybe when it's brand new. So you'd be getting less than that. I"m at work so I don't have my excel spreadsheet to calculate run times and what not, but how much run time do you need?

    Also, what kind of night riding/ On a well lit street? or a pitch dark single track?

    Do you already have that LED Handle? That is quite a draw...

    Correct me if I am wrong but at a 900ma draw at 24v nominal I will get ~1.7 hours out of a 1.7ahr 24v battery correct? Accounting for the losses of the higher drain than ahr is tested at, I would say 1.5 hours, which should be fine.

    I intend to ride in dark woods, no street.

    Yes I have the LED here in the lab now, it is quite bright

  6. #6
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    When fully charged the battery will be more than 24V. The actual Vf of the LED may be lower than specced in the data sheet.

    I looked at the ostar LED but I didn't manage to get optics and the cree and seoul came out so I concentrated onthem (not tryingto convince you to go that way lol.....just that they are more efficient even though they aren't as bright per LED).

    I was planning on running using a shark driver from sandwich shoppe. Its a boost driver meaning you can run from a lower voltage batteru (you need to increase the AH accordingly to get the same run time).

    Because LEDs are so sensitive to voltage change I would recommend going this route.

    Also, like has been said....you are going to need a big heatsink to tame 24Watts.Its not the same as runing a 24W halogen, halogens chuck out a fair chunk of their heat as IR out the front. LEDs do not, they get hot and need heatsinking. This is PC micro processor teritory hehe.

    I won't nag and its just a suggestion....but if you aren't too far down the build...I would recommend you consisder a multiple cree or seoul design...they are much more efficient....more lumens per watt. The OSTAR is about 20 lumen per watt at 1 amp the cree/seoul about 50 lumens per watt at the same current.

    Also my spread sheet estimate your runtime for the 24V 1.7Ah will be about 1.33 hours assuming you can get a steady 1 amp to the LED from it (suitable current controller used).

    3 cree/seouls at 1 amp would give 600 lumens and last over 3 hours, and won't get nearly as hot.

    If you stay with the OSRAM...post pics anyway....its is a beasty LED, just not that efficient hehe....bit of a sports car I suppose hehe.

    Stu
    What exactly is a rigid hard tail?

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    Quote Originally Posted by mhaskell
    Thanks for the help, I'm a thermal engineer so the heatsink is one part that I have all set.

    On page 9 of that sheet there is a foward current vs. voltage graph. I was under the assumption that at 24v the max current would be right around 900ma even without a driver. Could you describe what the current would be like without the driver?

    I also have a dragonstick made by osram, it has 12 led's for a total of 300 lumens. It is made to be driven by a constant voltage however at 24v.

    http://dafnwebpd.sylvania.com/os_fil...p?id=003691989


    Could you give me an idea of how much 500 lumens is in comparison to the top lights out there on the market?
    lol I was replying as your wrote this.

    A typical HID light is about 500 lumens. The HID is more efficient than the ostar though. It generates that much light from 10-13 watts. Halogen can be driven up at those brightness too....not sure what wattage. They arent' that common any more though for high lights (HID has ruled the roost...till LEDs got bright and efficient over the last year and are taking over now slowly hehe)

    The brightest LED light I have seen from a manufacturee is lupine's new LED light at 800 lumens. I think theya are using 4 crees or seouls.

    Stu
    What exactly is a rigid hard tail?

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    Quote Originally Posted by Stuart B
    When fully charged the battery will be more than 24V. The actual Vf of the LED may be lower than specced in the data sheet.

    I looked at the ostar LED but I didn't manage to get optics and the cree and seoul came out so I concentrated onthem (not tryingto convince you to go that way lol.....just that they are more efficient even though they aren't as bright per LED).

    I was planning on running using a shark driver from sandwich shoppe. Its a boost driver meaning you can run from a lower voltage batteru (you need to increase the AH accordingly to get the same run time).

    Because LEDs are so sensitive to voltage change I would recommend going this route.

    Also, like has been said....you are going to need a big heatsink to tame 24Watts.Its not the same as runing a 24W halogen, halogens chuck out a fair chunk of their heat as IR out the front. LEDs do not, they get hot and need heatsinking. This is PC micro processor teritory hehe.

    I won't nag and its just a suggestion....but if you aren't too far down the build...I would recommend you consisder a multiple cree or seoul design...they are much more efficient....more lumens per watt. The OSTAR is about 20 lumen per watt at 1 amp the cree/seoul about 50 lumens per watt at the same current.

    Also my spread sheet estimate your runtime for the 24V 1.7Ah will be about 1.33 hours assuming you can get a steady 1 amp to the LED from it (suitable current controller used).

    3 cree/seouls at 1 amp would give 600 lumens and last over 3 hours, and won't get nearly as hot.

    If you stay with the OSRAM...post pics anyway....its is a beasty LED, just not that efficient hehe....bit of a sports car I suppose hehe.

    Stu
    Hi Stu, I thought about using Cree's but when you look at the datasheets, and really dig in I think the osrams come out far ahead..



    If you compare running 2 Ostar's against 3 Cree XR-E's BOTH putting out ~500 lumens

    The ostar will have slightly more lumens, but also 3w more power.
    The ostar will only have a 2.9 hour time versus 3.7 hour for the cree (ignoring losses), but i'm not on the bike for 3 hours at a time.

    However the ostar will have the ability to put out nearly 1200 lumens if I need it (downhill or fast section), whereas the cree is maxed out at only 528.

    Also your first impression of the Ostar being harder to cool isn't quite correct if you look deep enough. If you look at the allowable junction temps and the Rjb values you can see the Ostar needs a much smaller heatsink, almost by half!

    This LED light stuff is exciting!

  9. #9
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    I'm working on a driver board for the ostar, getting the boards fabbed soon.

    You don't want to run the ostar at even close to it's max junction temp if you can help it. The light output and life diminish the hotter it is. The ostar kicks butt with it's junction to slug thermal resistance but that just means you have a chance to get the heat out, not that it is going to generate less heat.

    I believe the ostar is about 500 lumens at 15W input so it is quite comparable to a 10W HID, considering HID ballasts are less efficient than a LED driver. Also LEDs are a lot better on the optical side of things because they shine their light mostly forward. A bulb radiates all the way around which is pretty much impossible to capture all that light with a reflector.

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    Quote Originally Posted by mhaskell
    Hi Stu, I thought about using Cree's but when you look at the datasheets, and really dig in I think the osrams come out far ahead..



    If you compare running 2 Ostar's against 3 Cree XR-E's BOTH putting out ~500 lumens

    The ostar will have slightly more lumens, but also 3w more power.
    The ostar will only have a 2.9 hour time versus 3.7 hour for the cree (ignoring losses), but i'm not on the bike for 3 hours at a time.

    However the ostar will have the ability to put out nearly 1200 lumens if I need it (downhill or fast section), whereas the cree is maxed out at only 528.

    Also your first impression of the Ostar being harder to cool isn't quite correct if you look deep enough. If you look at the allowable junction temps and the Rjb values you can see the Ostar needs a much smaller heatsink, almost by half!

    This LED light stuff is exciting!
    lol yes it is exciting...glad you think so too....my mates get bored of me whittering on about em hehe. They still want me to make em lights....so I keep whittering as they aren't going to pay for my labour hehe

    From your first post it wasn't clear you were going to run them at low brighness aswell....you were talking about direct drive.

    Yes the osram is a lot more efficient when you tickle it .

    The beam of the osram will be quite floody as it a multi die design, but I don't think thats a bad thing for a bar light.

    I have just about got my modded cateye triple (seoul LEDs) upto my HID in both brightness and flood. I try to do enduro races so efficiency is important to me (although it will be a while till my next race....recovering from a broken leg/knee).

    You can ride at near day light speed with 500 lumen bar and 130 helmet...shadows being the limiting factor....but 1200 lumen would be fun hehe. I have my double up to about 400 minus reflector losses now too....not ridden with it yet for afore mentioned reason.

    I need to brush up on my thermal engineering.

    Are you using the rated max temps? Are you considering the affect of temperture on lumen output? At the moment I don't so much, but thats due to my weakness in thermal aspects at the moment (Seouls are more efficient than the lux i replaced for the given heatsink...so I have't reminded my self /brushed up yet ...and seouls are an easy retro fit.)

    Not to go too far off topic...but its interesting geeking out with ya.....personally instead of seeking too many more lumens I am experimenting with improving the colour deficiency of white LEDs. They don't have much red in them. I think that if the colour can be improved it might be less strain on the eyes/brain...browns need red to be seen porperly...but I have a thread dedicated to that topic though . Its not gathering much interest even in the geeky section of MTBR lol....ah well....my light will have it hehe.

    I didn't say welcome before either...welcome to the forum. If you visit candlepowerforum already....my name is Rusty/Knee on there

    Stu
    What exactly is a rigid hard tail?

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    Quote Originally Posted by presslab

    Also LEDs are a lot better on the optical side of things because they shine their light mostly forward. A bulb radiates all the way around which is pretty much impossible to capture all that light with a reflector.
    I dare you to say that in the incadescent or HID sections on candlepower forum hehe. It depends what you are after. For what we are after you are correct. I beleive a point source of omni directional light can be collimated into a tight beam better though. The area behind the bulb where light is lost is pretty small relative ot the area of the remainder of the reflector.

    We aren't after tight beams though....and I mean really tight when I say that before anyone says they don't like too much flood hehe. These beams would be near useless for riding with.

    the ostar is 6 light sources in 6 differnet places...so it will be floody like i said before.

    I keep meaning to go to bed early....but I keep reading interesting stuff like this hehe.

    Night

    Stu
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    Quote Originally Posted by Stuart B
    I dare you to say that in the incadescent or HID sections on candlepower forum hehe. It depends what you are after. For what we are after you are correct. I beleive a point source of omni directional light can be collimated into a tight beam better though. The area behind the bulb where light is lost is pretty small relative ot the area of the remainder of the reflector.

    We aren't after tight beams though....and I mean really tight when I say that before anyone says they don't like too much flood hehe. These beams would be near useless for riding with.

    the ostar is 6 light sources in 6 differnet places...so it will be floody like i said before.

    I keep meaning to go to bed early....but I keep reading interesting stuff like this hehe.

    Night

    Stu
    Yep it would be nice if the ostar had a smaller emitter area. But it really is pretty good - I have tested the ostar with a McR38 reflector and it produces a pretty nice beam! A hot circle in the center with a little bit of usable spill. This is an ostar without the lens. With the lens it is not so tight but still nice.

    It's interesting to note that the ostar has a higher lumen rating with the lens than without. I haven't figured that one out yet. I would think the lensed lumen rating would be slightly less because it is not 100% efficient.

    Oh and if you want a source of cheap ostar LEDs check this out, it's where I bought mine. You need to break the glue but it's not too hard. It has three LE W E3Bs.

    https://store.lsgc.com/R30-WHITE-FLO...SE-P47C11.aspx

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    Quote Originally Posted by mhaskell
    Thanks for the help, I'm a thermal engineer so the heatsink is one part that I have all set.

    On page 9 of that sheet there is a foward current vs. voltage graph. I was under the assumption that at 24v the max current would be right around 900ma even without a driver. Could you describe what the current would be like without the driver?

    clip...
    I'm not sure if you have the answer to your question about the led operating curve so I'll have a go...

    The led does not really limit current... It would be the power source that limits the current.

    If you had a whopping big 24V battery and hooked it up to your LED, the current would surge enough to destroy the led. You really need to know the internal characteristics of the battery to know what kind of current it will be capable of driving.

    I'm rambling on now...

    If you are going to the effort to put one of these nice led lights together, then don't leave the "driver" out

    Cheers
    GEVELTERSCHMIDT RACING

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    Quote Originally Posted by presslab
    Yep it would be nice if the ostar had a smaller emitter area. But it really is pretty good - I have tested the ostar with a McR38 reflector and it produces a pretty nice beam! A hot circle in the center with a little bit of usable spill. This is an ostar without the lens. With the lens it is not so tight but still nice.

    It's interesting to note that the ostar has a higher lumen rating with the lens than without. I haven't figured that one out yet. I would think the lensed lumen rating would be slightly less because it is not 100% efficient.

    Oh and if you want a source of cheap ostar LEDs check this out, it's where I bought mine. You need to break the glue but it's not too hard. It has three LE W E3Bs.

    https://store.lsgc.com/R30-WHITE-FLO...SE-P47C11.aspx
    Thanks for the link, That is the cheapest I have seen them.

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    Quote Originally Posted by presslab
    I'm working on a driver board for the ostar, getting the boards fabbed soon.

    You don't want to run the ostar at even close to it's max junction temp if you can help it. The light output and life diminish the hotter it is. The ostar kicks butt with it's junction to slug thermal resistance but that just means you have a chance to get the heat out, not that it is going to generate less heat.

    I believe the ostar is about 500 lumens at 15W input so it is quite comparable to a 10W HID, considering HID ballasts are less efficient than a LED driver. Also LEDs are a lot better on the optical side of things because they shine their light mostly forward. A bulb radiates all the way around which is pretty much impossible to capture all that light with a reflector.

    PM sent

  16. #16
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    Quote Originally Posted by mhaskell
    Thanks for the link, That is the cheapest I have seen them.
    No prob. The LEDs are glued to the heat sink with some kind of epoxy. I just used a flat blade screwdriver and a hammer and chipped them off. Then I used a file to remove the epoxy on the back of the LED.

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    Quote Originally Posted by airman
    I'm not sure if you have the answer to your question about the led operating curve so I'll have a go...

    The led does not really limit current... It would be the power source that limits the current.

    If you had a whopping big 24V battery and hooked it up to your LED, the current would surge enough to destroy the led. You really need to know the internal characteristics of the battery to know what kind of current it will be capable of driving.

    I'm rambling on now...

    If you are going to the effort to put one of these nice led lights together, then don't leave the "driver" out

    Cheers
    It is the LED that limits current. if the LED VF was as stated in the datasheet and the battery discharged enough to give an actual 24V it wouldn't pop. The problem unregulated is that a 24V battery fresh of charge is going ot be near 26-28 volts. given that LEDs respond to a small voltage change withe a big current change at higher levels means a lot more than 1 amp will fliow. this will be determined by the LED still...till pops. the only time the battery would limit it is if this current was higher than what the battery could supply.

    Stu
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    just reading the datasheet you linked....its more detailed than the one i got last year. I don't like all the log scales...especially for the relative brightness against current one.

    which brightness bin are you using?

    Stu
    What exactly is a rigid hard tail?

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    Got all excited for a minute, thinking you'd got one of the new Ostars http://www.ledsmagazine.com/news/4/2/10 - those will be very exciting, but I'm not really so impressed by the current ones given I do have requirements for longer runtime. Not really sure why you need 1200 lumens though - the 500 I have with my triple Cree is plenty even for downhills.

    Unfortunately you are mistaken with your heat calculations - the twin Ostar will likely need better heatsinking than the triple Cree rather than the other way round. The problem is that when you combine multiple units you are still using the thermal resistance of a single unit and calculating on the basis that all the heat goes through that. In fact you should divide the thermal resistance by the number of units, since the heat has multiple paths - this should be obvious, since the jb temperature difference will obviously be the same for 3 Crees running at a total of 11.1W as a single one running at 3.7W. The correct figure is 1.8K/W for the twin Ostar, 2.7K/W for the triple Cree.

    Therefore given your calculation based upon heating the junction to the max allowable temperature, you need a 7.1K/W heatsink for the twin Ostar, an 8.1K/W heatsink for the triple Cree. More realistically if you want to keep the LEDs decently cool (as this will both improve performance and prevent them degrading), with a 2K/W heatsink you have a total temperature rise junction to air of 54K for the twin Ostar, 52K for the triple Cree.

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    cheers chris...you confirmed the parallel thermal res mistake I made ealrlier today too when comparing seouls on individual and combined heatsinks. really glad this thread was posted...cleared some cob webs for me hehe.

    the seoul emitter has a Rjs of 6.7 too. 8 or the star.

    Stu
    What exactly is a rigid hard tail?

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