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  1. #1
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    Driver question for 1 XPE-2 from 7.4v

    Hey all, I have a tiny little light I need to upgrade. It has an older generation LED

    I want a narrow beam, only need about 100-200 lumens, so XP-E2 would be fine. I need to run it from a 2S (7.4v) battery pack, so I'm looking for a driver that will run it at 500-700ma or so, if I have this right.

    I have a 700ma Buckpuck I was using, but I noted that it's output is around 7v. Is this too much for an XPE in a tiny lamp?
    700mA, Non-Dimmable, BuckPuck DC Driver - With Leads

    Here's the page with all the other Buckpucks.
    700mA Low Voltage LED Drivers

    I also checked out drivers on Dx,
    Cheap Led Driver Circuit Boards for Flashlights at Dealextreme - DX

    but I'm getting confused/ overwhelmed with all the different drivers, so I'd really appreciate it if someone can point me to one.


    Thanks!

  2. #2
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    Any buck driver should do, as long as it physically fits. Those 7V output on your buckpuck, did you measure it? If so, did you actually measure voltage drop over an emitter, or did you measure open circuit with no load. Did you also consider using an XB-D. I think its die is even smaller than XP-E so the beam should be a bit narrower, since you mention the light is quiet small.

  3. #3
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    Quote Originally Posted by Toaster79 View Post
    Any buck driver should do, as long as it physically fits. Those 7V output on your buckpuck, did you measure it? If so, did you actually measure voltage drop over an emitter, or did you measure open circuit with no load. Did you also consider using an XB-D. I think its die is even smaller than XP-E so the beam should be a bit narrower, since you mention the light is quiet small.
    The driver is external, so not an issue for size. I measured the output on an open circuit, just across the wires after disconnecting the previous LED.

    Hmm, XP-D, hadn't thought of that. Do they come in warm white? I've only seen cool white so far.

  4. #4
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    I've used BuckPucks for bike lights before, unless you have a defective one the voltage supplied by the driver will equal the voltage drop of the LED.

    Also, I would recommend against using an XB-D. The texture of the emitter causes it to have worse throw than an XP-E2. I had one running with a Carclo plain tight 20mm optic at about 700-800mA, and it had significant issues with color separation--the center was an ugly yellow while the outside of the beam was markedly bluish. This may be less noticable with other tints but the throw definitely seemed worse than a first-gen XP-E in the same setup.

  5. #5
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    OK, I'll stick with the buckpuck, and a warm white XP-E2.

    Thanks all.

    Quote Originally Posted by A10K View Post
    I've used BuckPucks for bike lights before, unless you have a defective one the voltage supplied by the driver will equal the voltage drop of the LED.

    Also, I would recommend against using an XB-D. The texture of the emitter causes it to have worse throw than an XP-E2. I had one running with a Carclo plain tight 20mm optic at about 700-800mA, and it had significant issues with color separation--the center was an ugly yellow while the outside of the beam was markedly bluish. This may be less noticable with other tints but the throw definitely seemed worse than a first-gen XP-E in the same setup.

  6. #6
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    Well, you should consider yourself lucky 'couse you didn't ruin the driver. You should never power a driver without load! Although it might have open circuit protection. What it does is it raises the voltage until preset current is reached meaning without load there is no current running through the circuit so the diver will never be able to set the voltage and eventually will selfdistruct.

    Did you take a look at what cutter is offering? They might stock 4000K or 4500K emmiter which have 80CRI and really beautiful tint.

  7. #7
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    Oh boy, hope I didn't fry the driver.
    Well, if I did, what driver would you suggest? Same one?

  8. #8
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    Buckpucks are good if you only want to feed 700mA so i would run the same unless you want to go for a cheap chinese driver, as long as you have room for the unit. Does yours have the dimming pot on it?

  9. #9
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    No, mine has no dimming pot. Wish I'd gotten one of those- it might have allowed me to dial it back a bit. If I need a new one, do they have them with onboard pots?

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    The buckpuck come with the pot is connected with a couple of leads. I ended up cutting off the pot and adding a 3 way switch with a resistors connected to 2 legs so I got off, low, and high. Just needed to measure the resistance of the pot when the light level was what I wanted and used the same value resistors

  11. #11
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    Which cheap chinese driver would do the trick?

  12. #12
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    Quote Originally Posted by Ofroad'bent View Post
    Which cheap chinese driver would do the trick?
    Don't know, I have never used a cheap Chinese driver

  13. #13
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    Quote Originally Posted by Ofroad'bent View Post
    Oh boy, hope I didn't fry the driver.
    Well, if I did, what driver would you suggest? Same one?
    Buckpucks are open circuit protected. Should be fine.
    DIY LED Bike Lights:
    A few Dynamo builds and some Small battery lights

  14. #14
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    This DX driver seems pretty good so far. I've put a couple to use. The ones I received only drive at ~.7A instead of the 1.25A spec'd on the DX page so might be OK for your application if they are still shipping under-rated units.

    18V 5W Cree Circuit Board for Flashlights (16.8mm*5.5mm) - Free Shipping - DealExtreme

  15. #15
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    A buck driver can only step DOWN. So, with open circuit load, the output voltage can never exceed the input voltage. As long as the output components (caps/switcher element etc) are rated to at least input voltage, then there is no risk running them open circuit. With the OP and only 2 li-ion cells as input, there is no risk at all to a buck puck running open circuit.

    A boost driver steps up, so unless there is over voltage (open circuit) protection in the driver, then it is possible to blow a boost driver if powered up without a load as it attempt to raise output voltage to infinity to try and reach the target output current. I implemented open circuit voltage protection into all my boost drivers a long time ago - to protect my customers from the risk of damage. This means that a boost driver should transition to constant voltage mode once it reaches some designed maximum boost (below the max voltage ratings of the output components) to protect itself.

    cheers,
    george.

  16. #16
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    George, if this wasn't a simple on-off situation I'd have looked at one of your buck drivers, but it's a bit like using a Mercedes to shuttle in the mud on a farm.

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    No worries, picking the appropriate driver for the job at hand should be the first decision you make.

    I was just pointing out that a buck driver is typically safe to operate open circuit since output voltage can't exceed input voltage. It would be rare for a led driver manufacturer to choose the output components to handle less than the max input voltage (for DC drivers).

    cheers,
    george.

  18. #18
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    Quote Originally Posted by Vancbiker View Post
    This DX driver seems pretty good so far. I've put a couple to use. The ones I received only drive at ~.7A instead of the 1.25A spec'd on the DX page so might be OK for your application if they are still shipping under-rated units.

    18V 5W Cree Circuit Board for Flashlights (16.8mm*5.5mm) - Free Shipping - DealExtreme
    Got around to measuring mine, getting 1.24A :-)

  19. #19
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    Quote Originally Posted by brad72 View Post
    The buckpuck come with the pot is connected with a couple of leads. I ended up cutting off the pot and adding a 3 way switch with a resistors connected to 2 legs so I got off, low, and high. Just needed to measure the resistance of the pot when the light level was what I wanted and used the same value resistors
    Any specifics - can I retrofit a pot or resistor to my buckpuck?

  20. #20
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    Quote Originally Posted by Ofroad'bent View Post
    Any specifics - can I retrofit a pot or resistor to my buckpuck?
    If it has six leads/connections you can install a pot or fixed resistors for current control. If it has only four leads/connections then you can not add the pot or resistors.

  21. #21
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    It is reported to be simple to replace the sense resistor (top left in this pic)

    Current output can be vared by changing the loop back resistor. As they come they give 1.2 amps with an R2. Fit an R12 to up the ouitput to about 1.8 amps, or an R27 to get it down to about 900 mA Soldering in a new resistor is not very finicky due to its size.
    Is there any reason you could not remove the resistor and run some short wires to a pot or switched resistors, along the lines of what Brad72 did ?

  22. #22
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    Looks possible F-B on the Chinese driver. Vanc, it's only 4 wires on the buckpuck. Maybe I'll get a Chinese one to play with.

    Thanks all.

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