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Discussion Starter · #1 ·
I'm converting an old metal ViewPoint 12w/20w MR11 lamp to LED. My plan is to have one side be a single Seoul P4 bin U driven at 700ma, with the other beam being a triple P4 driven at 700ma. I will be using a 14.8v Li-ion battery and a 700ma BuckPuck for a driver.

My question is, if I want to somehow retain the pushbutton switch set up or improve upon it, is there a way that I can switch from lowbeam (single LED) to highbeam (triple LED) with the push of one button? I don't just want to have all four LEDs going, since my Vf is likely to exceed my battery output with the driver (3x3.6v=10.8v+2v for the driver = 12.8v which is ok, but 4x3.6v=14.4v+2v for the driver = 16.4v). I also don't want to have to turn one light off and another one on since it risks a moment of darkness if I hit the wrong switch.

I've also considered breaking these two lights apart and just running them both with triples at 250ma for a low beam, 700ma high beam. Any thoughts?
 

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I can't think of any way to do it using the existing push button (you'd have to build another board and program it). Also, if you're switching the power (like with a double pole toggle switch) you may run into issues with the single LED not having enough Vf for the driver (I don't know if the buckpuck driver has a minimum Vf). You'll probably be better off using two triples with the low and high settings.
 

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Discussion Starter · #3 ·
Thanks. I know the BuckPuck can work on just one LED, so I was hoping for an easy solution.

Thinking out loud as I type, perhaps I'll just break the pair apart (they use a common mount, but could be mounted separately with minimal modifications) and go with a 250ma/700ma (using the on position with a ~680 ohm resistor on the control pin and off giving me an open circuit for the full 700ma from the puck). I guess that'll be smaller and lighter and give me the other head to use on another project. I just don't see the need for two triples on the bar when I have a triple on my head already. Alternatively, I could have 700ma constant and have the switch short out two LEDs, so I'm just running one LED or three LEDs at the click of a switch. Hmmm, seems much simpler. Why didn't I think of that in the first place? Thanks for the sounding board.
 

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Since the driver is constant current (or should be) you could use the switch to short out the LED's you don't want "on" when in low beam mode.

This means that all LED's will be "on" in high beam though.

If it was a double pole double throw switch, you could use it to short out alternate LED's, one position shorts out three LED's (leaving your low beam on), one shorts out one LED (leaving your high beam on)...

Although having a high/low current setting would make more sense, unless you are actually doing a high/low beam, ie two different beam positions and for some reason you don't want them all "on" in high beam...
 

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Discussion Starter · #5 ·
The existing switches are single pole, single throw. I'm not sure I'd be able to find a DPDT pushbutton and I really like the idea of keeping a pushbutton on this project.

The problem with having them all on is that it would demand two drivers. I haven't ruled that out entirely, but it's not ideal.

I've also thought about an overvolted 15w as my high beam, but the idea of burning 18w+ for the same amount of light I could get from 7-8w of LEDs is almost repugnant anymore.

It seems the simplist solution is to run all three LEDs at 700ma and have the switch in the on position short out two LEDs. The most desirable would be to use a dimmable puck and have the switch utilize a resistor across the control pin or provide an open circuit.
 

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maker of trail
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Oh wait.... there is a way to do it... :)

Put the switch in series with the single LED, then put that switch/LED combination in parallel with your triple cree, when you open the switch the three cree's will run, close the switch the single cree will run, the much higher forward voltage of the three cree's in series will ensure that only the single LED will run when the switch is closed...
 

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I struggled with this problem with my bflex driven light. I wanted a narrow beam for the road and a wide beam for the trails.
Talking with George at taskled he said shorting out LEDs wasn't a good idea as there would be a power spike to the other LEDs. ... if you have 4 leds driven at 700mA and 3.5V each, when you short out three the other will briefly see 14V. If you want to do this use a pot you can turn so the change is gradual.

What I did was run two strings of two LEDs in parallel, with a 3ohm resistor in the wide string that would limit the current to 300mA (giving 700mA to the narrow beam). Shorting out the resistor evens out the current to each set of LEDs, giving equal power(but shouldn't overly stress the others). Works well, but I should probably up the resistor so its more like 800/200.
http://www.candlepowerforums.com/vb/showthread.php?t=174539
 

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Discussion Starter · #9 ·
@troutie: I currently have a 700ma dimmable Buckpuck driver on hand. They're cheaper and a bit more idiot proof (good for me) than the B-flex. Buckpucks don't care about open circuits and deal better with shorts and such, where there's been a number of threads complaining about the fragility of the B-flex (wiring came loose and the driver stopped working). I plan on trying a B-flex one of these days, but I don't think this is necessarily the application for it. I'll keep in in mind though if I get into size/space challenges, as that feature of the B-flex is very appealing.

@essenmeinstuff: I'm having a hard time visualizing what you are saying. I'll try breadboarding this with what I think is your idea and see if I can get it to work. This is when I with there was a drawing feature in the forum. One appeal of keeping this dual is that when I'm climbing, I want the minimal light close to my front wheel, but when I'm descending, I'd like that light 20-30 meters out there. There's no way to make these adjustments while racing, so having it set up that way is ideal.
 

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znomit said:
I struggled with this problem with my bflex driven light. I wanted a narrow beam for the road and a wide beam for the trails.
Talking with George at taskled he said shorting out LEDs wasn't a good idea as there would be a power spike to the other LEDs. ... if you have 4 leds driven at 700mA and 3.5V each, when you short out three the other will briefly see 14V. If you want to do this use a pot you can turn so the change is gradual.

What I did was run two strings of two LEDs in parallel, with a 3ohm resistor in the wide string that would limit the current to 300mA (giving 700mA to the narrow beam). Shorting out the resistor evens out the current to each set of LEDs, giving equal power(but shouldn't overly stress the others). Works well, but I should probably up the resistor so its more like 800/200.
http://www.candlepowerforums.com/vb/showthread.php?t=174539
Yeah that "spike" may or may not be an issue depending on how much output capacitance is in the driver. Like you say, adding a current limiting resistor of an ohm or three with the single LED would reduce it (and not affect the current the LED gets).

Plus its a one off low energy transient, even if the cap is huge, say 1000uF, your talking less than 70mJ of energy, which the LED should be able to handle easily (no liability accepted lol).

These controllers must have a current shunt somewhere, which would also limit this current depending on how big it is...

Also your thing with simply doing a 2X2 parallel series string is not a good idea without something that ensures current sharing, the Vf of diodes is not constant, and changes with temperature...
 
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