Direct Drive

NO not the car, but my torches.
I have just recieved my first few led's for some modding.
The lights I use are 18650 x 1, or 2 or 1 x RCR123 powered.
So long as I keep the voltage to below what the led maker says it can take, would I be able to just have a switch to switch direct, without a "pill" thingy ?
The led is up to 4.2V, the battery is 3.6Volt, I'm thinking of my Romisen RC-F4, as it just went dimmer a while back for no reason, and I really love it.
Please help as I'm a bit stupid, as I missing something, other than a brain that is, thanks
tabetha

The performance could be poor.
Mfgs don't specify limits by voltage really, the current matters. Current is not well controlled by voltage. One LED could be drawing 250mA @3.7v, another 350mA @3.7v. Actually as a given LED warms up, the current drawn under any specific voltage increases.

The thing I don't like most is that single-cell li-ion direct-drive lights, or linear stepdown-only regs, fade a lot once the battery drops to 50% capacity or so. This sucks, I don't find the light level acceptable so I just pretty much have to say the light has half the useful runtime.

If you have a light that already uses a driver , then swapping in a better LED will improve its performance ..

If you want constant output then a Buck Boost driver might be the way to go , Direct Drive can be a little hit or mis depending on the application .

Both DX and KD sell a 1A buck boost driver that seems to be rather good in 17mm form . Hooked up to Q5 or R2 may very well offer you many battery options and decent power output .

tabetha said:

NO not the car, but my torches.
I have just recieved my first few led's for some modding.
The lights I use are 18650 x 1, or 2 or 1 x RCR123 powered.
So long as I keep the voltage to below what the led maker says it can take, would I be able to just have a switch to switch direct, without a "pill" thingy ?
The led is up to 4.2V, the battery is 3.6Volt, I'm thinking of my Romisen RC-F4, as it just went dimmer a while back for no reason, and I really love it.
Please help as I'm a bit stupid, as I missing something, other than a brain that is, thanks
tabetha

Click to expand...

No, your logic is OK. It really will work OK. The only thing to watch with full direct drive without even a resistor, is the current draw can be very high when the battery is fresh. A Cree XR-E will draw over an amp off a fresh 18650 - the driver may have put out somewhat less. So proper and thorough heatsinking of the LED is required, which may be much more than the original torch provided.

I do it sometimes on a single 18650 light, but I'm careful to handpick an emitter for it. I don't have my notes handy, but my selected Cree R2 LED's will usually start off around ~1600ma on a fresh battery and gradually drop from there. And once you've built the light you can't swap out a cheesy high resistance switch with a McClicky for example because that will raise the current a LOT, so you need to have anything that effects resistance set before you pick out the LED that has the right resistance for the light.

I've got a Tiablo A9 and a Raidfire Spear set up in DD using Cree R2 emitters. No driver to fail, so they are likely the most reliable lights I have, and boy they are bright.

Thanks for all the excellent comments guy, it really is appreciated.
All the mod ones I do are to existing led lights, I understand now about the current being correct.
I have a cycle light to mod as well, it is a CAT EYE front led light, that frankly would have a job being seen in a totally dark room, I'm not really bothered about it as it's rubbish so just going to swap in a LED, a P4, and see what happens.
The original led is on a star, but no heat sink compound on it or anything, guess it's that weedy doesn't need it ?
This is powered by 4 X AA cells, so must use a driver to limit current/voltage.
I'm even eventually wanting to do some day lights like audi's have on them, using led's, to fit on my cossie, would look nice and show up well, otherwise I will copy the US system of main beam on 1/2 brightness as this works brilliantly as well during the day.
Thanks again for the input, anything else would be appreciated.
tabetha