I'm new and have questions on 5W LS


Flashlight Enthusiast
Sep 29, 2002
A very strange dark place
I\'m new and have questions on 5W LS

<looking around>
I am new to this forum (been lurking for a few weeks) and have a couple of questions. First an explanation of what I am doing would seem in order. I commute to work on a recumbent bicycle and don't enjoy getting run over by cars. On the back of my bent (short for recumbent) I have a retina burning 4 AA 16-LED red flasher and it works great! Cars avoid following me like the plague.
On my helmet is a 5w 7.2V pencil beam, halogen bulb with an external NiCad battery pack. It works well for nuking the retinas of the sleepy drivers when it is dark (point and shoot) I would love to remove the 5w 7.2V incandescent light and put in a 5W white Luxeon Star with collimator. This project will have to wait a few months until the 5W LS LEDs become available.

I hear that NiCads will burn up LS LEDs from too much current
Is there plans in the mix to have a current/voltage regulator for the 5W LS? Anyone out there messing around with the 5W (or 4 1.2W LS) that can run on NiCads? What about a resistor inline to tame the NiCad? I know that I had better be getting a ceramic one to handle the current of the battery, and the voltage regulator will handle the 7.2V of the NiCad.
On a related subject, I will add a flashlight to my rig for dual lights on the front. Luckily, I will be leaving in a month and be back in mid-December and will purchase the LS driven lights. The 5W LS is SUPPOSED to be out by then.
The 5W LS flashlight is the main thing at this point, something to give light and I can evaluate the thing for mods to my helmet light.
So, the specific questions!
1. Will the 5W white LS be available as a flashlight by mid-December?
2. Will high power resistors tame the current output of a 7.2V NiCad in conjunction with a voltage regulator?
3. Are there any rumblings about a strongly collimated 5W LS "pencil" beam?
4. How much cash is this going to burn?

As a reformed lurker
This is a great forum, I am not the only geek in the world that gets off on LEDs
Any info would be appreciated, and everybody, have a great weekend. Craig, good to hear you are feeling better


Newly Enlightened
Aug 4, 2002
Re: I\'m new and have questions on 5W LS

HI BentHead - you might want to ask these questions on the mod board. Best - Michel


Flashlight Enthusiast
Aug 22, 2002
Re: I\'m new and have questions on 5W LS

Due to their construction, Alkaline batteries have a higher ESR (Eq Series Resistance) than NiCads

Any electronic device can be destroyed by running more current through it than the device was intended for, my suggestion is to hook up the batteries/LED thru slightly higher resistance than you compute - measure the current with a current meter and adjust the resistance accordingly

Putting 10 ohm resisters in par combinations is a very good way to get the current close to the 0.750A design by LS, just make sure you have enough heatsink - you can always bolt the 5w module to your bicycle



Dec 14, 2001
Portland, OR
Re: I\'m new and have questions on 5W LS

Hey there Bent',

I'm into bicycle electronics myself, and specifically have an interest in the 5W luxeons. With regard to your questions, I'd strongly suggest that you spend some time looking back through the archives of the electronics forum, to see how people drive LEDs.

The basic issue with LEDs is that they have a very non-linear voltage versus current curve, meaning that the current flow is very low until you reach a threshold voltage, and then the current climbs rapidly as voltage is increased past the threshold. The second point is that too much current will fry them. The third point is that the threshold voltage is different for each LED, and even for the exact same type of LED (same part number, same production run) there will be slight differences in threshold voltage.

What this all boils down to is you need either a regulator which will properly adjust its output voltage to get the desired current (a current regulating controller of some sort) _or_ you need to have sufficient supply voltage combined with a resistor in order to make the system relatively immune to the various voltage changes.

For a very simple resistor current limiting device, the higher the supply voltage and the higher the resistance, the better the current regulation over supply voltage and threshold voltage changes. The limit of this is an infinite voltage connected through an infinite resistance, producing a constant current. However the higher the voltage and the higher the resistance, the lower the efficiency.

For best efficiency you want battery voltage quite near to the LED voltage, and a very small resistor. The problem here is that you are again in the realm of small voltage changes leading to large current changes. When you say that the LS5W needs 7 volts, you are ignoring the fact that any particular LS5W might need anything from 6.4 to 8V...if you connect a 7.2V supply to an 8V LED, you won't get much light out of it. If you connect a 7.2V NiCd supply to a 6.4V LED, then you will eventually get smoke out of it.

The ideal device to use is a current regulating switching converter, and several are discussed in the archives. Another possible device is a current regulating _linear_ regulator, which could be a voltage regulator modified for current regulation. Linear regulators are less efficient, but quite simple.

The final trick that some people use is to take advantage of the internal resistance of alkaline batteries to provide 'good enough' current regulation. Wayne of electrolumens has been running the LS5W devices with 5AA alkaline batteries, and no additional current limiting. While this is nominally 7.5V, under load it is probably more like 6.5V or less...the same LED, supplied with a 7.2V NiCd would fry.