# Lumen arithmetic

#### zgurl

##### Newly Enlightened
[edit: maybe there is a better place to post??]

Hi guys,

I am thinking of a biking light with good autonomy without sacrificing the brightness.

The baterries would be 6x2400NiMh (or 2x18650)

I am thinking of two possibilities:
1. A double cree + Downboy500 + short circuit for one led -> two brightness levels
2. A simple cree + nflex -> nflex (or similar) -> many brightness levels

I am questionning about the brightness I would get with the two leds. I know what gives a single led driven at 750mA (must be close to my Lumapower M1), but I dont know what will give for example 2 Crees at 500mA: similar to one at 1A? Does the lumens add themselves? Is there a more complicate combination rule? :thinking:

Any idea on this or any comment on the advantage of each of the solution?

(I read in another thread that using a switch to short-circuit the led could damage the board? I cannot see any reason for this :huh2: )

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#### Erasmus

##### Flashlight Enthusiast
zgurl said:
I am questionning about the brightness I would get with the two leds. I know what gives a single led driven at 750mA (must be close to my Lumapower M1), but I dont know what will give for example 2 Crees at 500mA: similar to one at 1A? Does the lumens add themselves? Is there a more complicate combination rule? :thinking:
Just total up the output of individual LEDs to get total output Yet 2 XR-E's driven at 500 mA will put out more lumens than a single XR-E since the efficiency is higher when you drive them at lower power. You can see that clearly in this great contribution from Jtr1962 : http://candlepowerforums.com/vb/showthread.php?t=89607&page=2

For example the XR-E P4/WH from my groupbuy that Jtr1962 tested will put out :
- 2x500mA = 2x114,15=228,30 lumen
- 1x1000mA = 184.48 lumen
The 2x500mA option is 43.82 lumen or 24% brighter due to higher efficiency.

Now it's upto you to decide whether this efficiency increase is worth the money of an additional LED/optic/...

Bonne chance!

#### zgurl

##### Newly Enlightened
:thanks: "Merci" for the enlightenment!!

I do understand that it is far more efficient to use two leds (apart from the initial cost and the led extra weight ).

I further can see another advantage of this: there is still the possibility to push the leds at a higher rate for a limited amount of time and get more light!!

But the other question remains: is it safe to temporary short-circuit one of the two leds if unnecessary?

#### Erasmus

##### Flashlight Enthusiast
I don't quite understand what you mean but in any case it is not a problem to cut one LED out of the circuit if the other LED isn't overdriven then.

##### Flashlight Enthusiast
zgurl,
I am stuck waiting for georges80 MaxFlex board that I'm going to use in a 8AA to 2D Quad Cree Mag mod for a bike light. Since I have gone through many designs from MadMad driven Luxeon I R bins to NFlex driven W bin LuxeonV's, I have a few thoughts.
If you are going for two LEDs, use two regulators--one for each LED. Shut off the seperate regulator to turn off the LED and you will be fine.
Another option is to use a boost regulator and run three Crees in series at 350mA. Three P4 bin Crees will put out 255 lumens at 350mA (3.42 watts total) VS 228 lumens for two at 500mA (3.36 watts total) If you are using a Mag 2D Mag body, a 3 (or 4) Cree heat sink called the PXR is available to simplify the build.
The NFlex and BFlex are buck (step-down) regulators which will work for one or two LEDs, for three LEDs you need a boost regulator. Not sure when the MaxFlex multiple output boost regulator will ship though.

Another option is to purchase two Fenix L2D CE lights and put them together :green: I use a Fenix L1D CE on "turbo" as a helmet light and it works very well. My plan is to attach a L2D CE to the frame for my summer time light while I complete the 500+ lumen Mag Cree mod.

What are you planning on mounting the LEDs in? Will it be a Mag, a specially made device or an external battery pack?

#### zgurl

##### Newly Enlightened
zgurl,
I am stuck waiting for georges80 MaxFlex board that I'm going to use in a 8AA to 2D Quad Cree Mag mod for a bike light. Since I have gone through many designs from MadMad driven Luxeon I R bins to NFlex driven W bin LuxeonV's, I have a few thoughts.
If you are going for two LEDs, use two regulators--one for each LED. Shut off the seperate regulator to turn off the LED and you will be fine.
Another option is to use a boost regulator and run three Crees in series at 350mA. Three P4 bin Crees will put out 255 lumens at 350mA (3.42 watts total) VS 228 lumens for two at 500mA (3.36 watts total) If you are using a Mag 2D Mag body, a 3 (or 4) Cree heat sink called the PXR is available to simplify the build.
The NFlex and BFlex are buck (step-down) regulators which will work for one or two LEDs, for three LEDs you need a boost regulator. Not sure when the MaxFlex multiple output boost regulator will ship though.

Another option is to purchase two Fenix L2D CE lights and put them together :green: I use a Fenix L1D CE on "turbo" as a helmet light and it works very well. My plan is to attach a L2D CE to the frame for my summer time light while I complete the 500+ lumen Mag Cree mod.

What are you planning on mounting the LEDs in? Will it be a Mag, a specially made device or an external battery pack?
Thank you for your answer. But can you give me more explanations on why using two regulators instead of one for two leds?
Actually, I wish to be able to save battery and cut off one of the two leds so a step down driver seems better to me (1 or 2 leds are always lower than the input voltage). Maybe my reasoning does not make sense:thinking:.

Indeed I am not sure I need those 250 lumens for biking (except of course for impressing friends :naughty: ). But I am sure I need extra long run time and definitely do not want to carry many cells!

Of course I also considered the Fenix+Lockfish option (currently what I use, with Luma M1), but I also like the DoItYourself way . So my present option is to make a simple mount based on square aluminium bar like the one nighrider built. I think it could be light, cheap, easy to handle, ... nearly perfect

##### Flashaholic
If you use a constant current converter to drive two LEDs in series, you can (theoretically) put in a switch to ground out the input of the second LED so the volatge flows through converter then through led1 then to ground.

The current should remain the same (500ma) whether it flows through one or both leds. I don't know if the sudden change in load will blow the converter.

Daniel

#### 2xTrinity

##### Flashlight Enthusiast
If you use a constant current converter to drive two LEDs in series, you can (theoretically) put in a switch to ground out the input of the second LED so the volatge flows through converter then through led1 then to ground.

The current should remain the same (500ma) whether it flows through one or both leds. I don't know if the sudden change in load will blow the converter.

Daniel
This is true. However, a much better way to do dimming would be to simply change the current limit. As we just talked about earlier, running two LEDs at 250mA would be significantly higher efficiency than running 1 at 500. You're best off simply powering all your LEDs in series and changing brightness by changing the current limit.

Most constant current regulators have what is called a "sense" resistor that has high resistance, and permits a small current to flow through to a meter, this gives feedback to the current regulator. If you swap in a higher resistance instead, this causes the current passed through to the meter to go down, causing the regulator to increase the output to correct it. You can get multi-levels by having a switch connected to sense resistors of different values.

You can use this to regulate the light in other ways as well. On one of my night-lights (regulated 25mA 5mm LED), there is also a photoresistor -- photoresistors conduct when exposed to light, and are insulators in the dark. When it is light out, this component conducts more and bypasses the sense resistor completely, causing the light to go off. Likewise, a thermistor conducts better at higher temperature -- the hotter it gets, the more current gets through to the feedback mechanism, which reduces the current to the light. This is how the maglite LED modules keep from burning themselves up since they have no heatsink.

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#### chris_m

##### Enlightened
I agree - is better to dim than to shut one LED off. Which is why I wonder why you don't have nFlex and 2 LEDs in your options, as that would be ideal?

Some regulators even have a pot input (like the Fatman), where you can either attach a potentiometer for gradual dimming, or switched resistors for dimming steps. Or in my case my own uC board and UI to result in a multi-level boost converter

##### Flashlight Enthusiast
zgurl said:
Thank you for your answer. But can you give me more explanations on why using two regulators instead of one for two leds?
Actually, I wish to be able to save battery and cut off one of the two leds so a step down driver seems better to me (1 or 2 leds are always lower than the input voltage). Maybe my reasoning does not make sense:thinking:.

Indeed I am not sure I need those 250 lumens for biking (except of course for impressing friends :naughty: ). But I am sure I need extra long run time and definitely do not want to carry many cells!

Of course I also considered the Fenix+Lockfish option (currently what I use, with Luma M1), but I also like the DoItYourself way . So my present option is to make a simple mount based on square aluminium bar like the one nighrider built. I think it could be light, cheap, easy to handle, ... nearly perfect

Z,
I am a bit paranoid about shorting LEDs to regulators on bike lights. Nothing worse than wanting to shut off one LED to extend runtime and poof! no more light. Shutting off the regulator gives a nice backup to your light in case one malfunctions.
I can see why you would want two LEDs though, they have the same throw but the beam width will change when using one or two LEDs.
I use a Fenix L1D CE single AA Cree light on a helmet mount myself. A P2D CE is on order as a frame mounted light presently as I will check that configuration out for summer use. The long runtime 8AA frame light is a winter use thing so the shorter 2.5 hour runtime of the 2AA P2D CE is not a concern.
Figure it would be interesting putting two or three P2D CE heads in an aluminum mount and running two 12,000mAH NiMH D cells for 4.5 hours of runtime with all the lights running. Simple to wire up and it takes three holes in an aluminum plate with artic silver thermal epoxy to complete. Add a rubber gasket on the back of the thick plate and screw on a cover plate and you're done. No control over the beam though.
The NFlex or BFlex (with bicycling UI) would be your best bet for simplicity and dimming control. The step-down current regulator can use anything from 7.2V to 20 volts so you have options. Use a laptop battery for extra runtime if you please.
Keep us posted on what you plan to use and how the light construction is proceeding.

#### zgurl

##### Newly Enlightened
:thanks: for all this light on my project.

I conclude that since I have two leds, I better drive them with lower current than drive only one with the same current. I did not get this point before you put the stress on it (but this was actually my question: lumen arithmetic:huh2: )

Actually I was bit afraid by the *flex boards that seem more complicate to use/configure than a simple driver+ switch. But now I undertand it is worth trying to understand how they work.

But a few points keep unclear to me...

Z,
I am a bit paranoid about shorting LEDs to regulators on bike lights. Nothing worse than wanting to shut off one LED to extend runtime and poof! no more light. Shutting off the regulator gives a nice backup to your light in case one malfunctions.
I don't see the problem here: if one led goes break, since they are in series, the whole light will be off :thinking:. Or maybe you are talking about breakdown of the regulator?

I will keep you posted with the result of this...

##### Flashlight Enthusiast
zgurl said:
But a few points keep unclear to me...

I don't see the problem here: if one led goes break, since they are in series, the whole light will be off :thinking:. Or maybe you are talking about breakdown of the regulator?

This is just the paranoid of me speaking :green: If you are running the regulator with two LEDs in series and you short one of the LEDs while the regulator is running, (I am assuming you are using a switch) the switch contacts will arc slightly when the switch closes. This is that "surge" or "spike" that I worry about. You can use a capacitor inline with the switch to filter the spike but it might screw up the regulator with the extra capacitance. There is a simpler way. (sorta)

Use a three-position switch. On (two LEDs) then off then on (one LED) flipping the switch quickly can surge the regulator though

As you point out, in series if one LED fails (or one connection in the string) all the lights go out. The same thing will happen if you use one regulator and a switch between the two LEDs. One connection on the regulator, battery or wiring can fail and lights out. Two regulators and two LEDs add some backup to the system although it adds cost.

My work around is to use two different lights. One frame mounted high output light and a seperate helmet mounted single AA Cree LED light. In my case the Fenix L1D CE is the helmet light. March through October I use the Fenix L2D CE as a frame mount light and Nov-Feb I'll use the Quad Cree XRE 8AA Mag for longer runtimes and much brighter output. Two equals one and one equals none (law of survival)

Yes, I have had a light blow out in complete darkness with a 4 meter fall on either side of a curving road. This is why I like to use two completely different LED lights while riding. Cheaper than medical bills.

#### zgurl

##### Newly Enlightened
This is just the paranoid of me speaking :green: If you are running the regulator with two LEDs in series and you short one of the LEDs while the regulator is running, (I am assuming you are using a switch) the switch contacts will arc slightly when the switch closes. This is that "surge" or "spike" that I worry about. You can use a capacitor inline with the switch to filter the spike but it might screw up the regulator with the extra capacitance. There is a simpler way. (sorta)

Use a three-position switch. On (two LEDs) then off then on (one LED) flipping the switch quickly can surge the regulator though .

As you point out, in series if one LED fails (or one connection in the string) all the lights go out. The same thing will happen if you use one regulator and a switch between the two LEDs. One connection on the regulator, battery or wiring can fail and lights out. Two regulators and two LEDs add some backup to the system although it adds cost.

OK now I see better the possible problem (sort of a side effect, but with dramatic consequence)

My work around is to use two different lights. One frame mounted high output light and a seperate helmet mounted single AA Cree LED light. In my case the Fenix L1D CE is the helmet light. March through October I use the Fenix L2D CE as a frame mount light and Nov-Feb I'll use the Quad Cree XRE 8AA Mag for longer runtimes and much brighter output.
This must give you a bit of light...

Two equals one and one equals none (law of survival)
Yes, I have had a light blow out in complete darkness with a 4 meter fall on either side of a curving road. This is why I like to use two completely different LED lights while riding. Cheaper than medical bills.
I can now understand your paranoya... keep cool, lay down on the sofa and talk to us about your feelings

##### Flashlight Enthusiast
zgurl said:
I can now understand your paranoya... keep cool, lay down on the sofa and talk to us about your feelings

Z,
Now you have to figure out what you want the light to do. The BFlex has a cycling user interface so it should make your light more adapatable for your use. By using the square aluminum channel, the choice of optics are up to you.
The helmet light bulb blowing story happened to me in the spring of 2002. The 4 meter drop off on either side of the road landed in partially frozen rice paddies when the bulb blew. I continued to lean and brake estimating were the road went. Luckily, I stopped about a meter from the edge of the elavated road. My rear red LED flasher showed how close I came to the edge. Hmmm, the LEDs don't blow like bulbs so that started the quest for high output white LEDs. 5 years later, a handful of Crees and some NiMH batteries and my problem is solved.
Plenty of Luxeons have gone through the process to get me to this level though. Time to get off the couch before you start charging money that I can use for more mods.

#### zgurl

##### Newly Enlightened
Bent,

sorry about your story. But it sounds like it finnaly ends well

Time to get off the couch before you start charging money that I can use for more mods.
Too late, you owe me a beer!

To be perfectly honnest I am the kind of man always searching for the very most up-to-date-less-than-10g-hyper-top products... and always have a good heavy light in the bottom of my bag... just in case!
Maybe I owe you a beer too . It reminds me Winnicot, a famous psychoanalyst who used to say something like "my patients paid to teach me" :lolsign:

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