A couple of questions before attempting my first mod.

[The Jinx]

Hi,

I've been lurking around cpf for a while and now I wish to dwell into the modding scene. My goal is to create a simple flashlight/tent lantern. I've been looking all over the internet and decided that the Life+Gear 400 with a little modification would probably best fit my needs. The main idea is to replace the factory handle led of a life+gear glow 400 (http://www.youtube.com/watch?v=bIdFA6GjDtk), with a Cree XR-E. My main questions revolves around how do I go about direct driving this led http://www.dealextreme.com/details.dx/sku.2395?

1. Based upon the configuration of the flashlight I assume the Vbat = 4.5. Vf = 3.7, but how do I figure out the drive current (I)?
2. How big of a heatsink would I need to disperse the heat from a XR-E? I was thinking about sanding down a US quarter and using that as a heat sink
3. Does anyone have one of these Glow 400 and know if it is even possible to solder on a wire where the handle led is located?

TIA
- Jinx

 

[circa]

I don't know anything about the switch in the LG400 but at direct drive you could resistor it for 350ma, the star hardly warms up at that current and would be 100~ emitter lumens

Not sure how the switch/board works so it could be difficult.

 

[The Jinx]

So if I run it at 350mA that would mean i need a 2.7ohm 1w resister correct? I'm basing it on this site

Furthermore, at 350mA would a quarter be big enough as a heatsink? would i even need a heatsink?

 

[circa]

You could probably get away with no heatsink other then the star it's mounted to, if it has the room obviously a little more material wouldnt hurt.

I never have DD with a resistor, but that calculator should be within the ballpark.

 

[The Jinx]

Um I got a couple of more questions..

1) How does one measure the driving current on a flashlight? (How does one figure out how much mA the led is running at?)
2) How does one measure the battery voltage under load?
3) After purchasing the flashlight and using a multimeter check the voltage going toward the led I discovered that the Vbat = 3.4. In addition, I was thinking about using the XR-C from my lowes 2d taskforce for this project and the resistors that I have access to are 10 ohm 1/8w, 10 ohm 1/4w, or 10 ohm 1w what would be the greatest mA I can drive the led without damaging it? Furthermore, would it be all that bad to direct drive the XR-C w/o a resistor (to be honest I am not sure if the pcb already has a resistor on it somewhere because the flashlight takes 3AAA and the voltage going toward the led is only 3.4v)

Im sorry for all these questions, I am an absolute noob with no idea of where to start. I was using the LED calculator as mentioned In my previous post and after a whole lot of trial and error plugging in the values I was hoping someone can just help me or at least teach me how to calculate this.

Thanks

Jinx

 

[The Jinx]

Well I went ahead and and soldered on the XR-C without a resistor and when I turned it on I noticed that it produced a yellowish tint from a side view and a neutral white tint from a top down view. I though that I might be damaging the LED so I replaced it with the old LED. So my question is, from my description does it sound like direct driving the XR-C without any sort of resistor was damaging the LED?

 

[AnAppleSnail]

Well I went ahead and and soldered on the XR-C without a resistor and when I turned it on I noticed that it produced a yellowish tint from a side view and a neutral white tint from a top down view. I though that I might be damaging the LED so I replaced it with the old LED. So my question is, from my description does it sound like direct driving the XR-C without any sort of resistor was damaging the LED?

That's normal.

Current on a flashlight: In series with the LED, in ammeter mode. A cheap ammeter will reduce the current flow, so watch to see if the LED gets dimmer when being measured rather than being wired normally.

Battery voltage: across the battery while it's on.

It's really hard to get DD to the maximum power without damaging the LED. If you design for a fresh battery you'll be dim later; if you design for partly-dead batteries then you'll toast the LED on a new battery, and so on.

 

[HarryN]

It sounds like you are well on your way with LED projects. It is in fact trial and error, just like any hobby.

It might be time to buy a volt meter / multi meter, as there is not a good way to figure out what is going on otherwise.

If you see a white LED turning bluish, that is the time to disconnect it.

 

[The Jinx]

I actually got a multimeter but I have no idea on how to use it other than checking the voltage left on a battery. Does anyone have any good documentations or know of any online videos explaining how to use a multimeter that they would recommend?

 

[HarryN]

Hi,

I don't know of any other than the usual manuals, but here are some ways I use a meter. (I am not an electrical engineer, so this is basic)

Measuring current flow

Method A - Running the current through the meter

Some meters have 2 setups for measuring current
- low current , a few 100 ma
- higher current - perhaps up to 10 amps

On mine, there is one central plug for "ground", and then 2 options for input
- higher currents
- everything else

If you are measuring higher currents, and run it through the low current side, it can ruin the meter.

Method B
- Very useful for measuring currents in a circuit

It is based on the formula Voltage drop = current x resistance

If you know or measure the resistance of a resistor, then you can use it in the circuit. 1 ohm is a handy number just because of the easy math, but not necessary. I keep some 1, 10, 30 and 100 ohm resistors around for this use.

- Setup your wiring to that the power is running through the resistor, then through your led in series
- measure the voltage drop across the resistor
- Plug it into the formua.

If there is a 2 volt drop across the resistor, then 2 = Current x 1 ohm, so the current is 2 amps.

Some meters also have measurements for checking the polarity of a diode and double checking a capacitor to make sure it is the right rating. When you are building suff, it helps to double check the parts sometimes. (or if you spill some together )