direct drive Q

hey, if i direct drive straight off a cell a Q5,
1: how many lumens start up ?
2: will it constantly dwindle untill the cells dead
3: and how long will it take.
4: If i add a resistor, will it limit current to the led, increasing runtime ?

havnt played around before but planning too, just wondered on some very general info

thanks
:twothumbs

1) This is dependent upon many factors. Heatsinking, the voltage of the supply, the forward voltage of the LED, what type of cell, chemistry the cell is, etc. So this is technically un-answerable...

2) Yes

3) Depends upon many factors listed above... Again, technically un-answerable...

4) Yes, a resistor will decrease current, increasing runtime. As to what type of resistor to use... I have no idea.

A resistor will decrease current but I would think it would not increase runtime. I would think it is just converting current to heat.

Correct me if this isn't the way it works.

ahh this is what i was wondering about the resistors, do they convert all the energy to heat or not, thanks gcbryan

i would have thought asuming good heatsinking, using firstly cr123x1 then again using rcr123x1 . forward voltage of led i guess i could choose to suit ? what would be best for runtime ? if it makes a diference ? initial lumens is not important on this. but as allways could be fun !
thanks for your time john_galt.

gcbryan said:

A resistor will decrease current but I would think it would not increase runtime. I would think it is just converting current to heat.

Click to expand...

zs&tas said:

ahh this is what i was wondering about the resistors, do they convert all the energy to heat or not, thanks gcbryan

Click to expand...

A resistor converts current to a voltage drop and heat, but because the total resistance of the circuit will increase, the total current will drop, reducing brightness and increasing runtime.

zs&tas said:

hey, if i direct drive straight off a cell a Q5,
1: how many lumens start up ?
2: will it constantly dwindle untill the cells dead
3: and how long will it take.
4: If i add a resistor, will it limit current to the led, increasing runtime ?

havnt played around before but planning too, just wondered on some very general info

thanks
:twothumbs

Click to expand...

I've done it, but I sorted through a bunch of emitters to find a few that would work without using a resistor.

A bench power supply comes in real handy doing this.

HKJ said:

A resistor converts current to a voltage drop and heat, but because the total resistance of the circuit it will increase, the total current will drop, reducing brightness and increasing runtime.

Click to expand...

thats brilliant and makes sense, thanks.

i was asking as i have no room, but may be able to get a small resistor in there if it helps runtime.
im guna learn ( alot ) as i go along but atleast i have someidea where to start

You need an emitter with a low enough Vf to work. Out of all the XR-E's I've tested, many were too high to ever work in a single 3.7 volt Li-Ion battery. Not enough current. A precious few had low Vf's and were set aside.

One of my DD XR-E lights is a Raidfire Spear that the driver failed on after a hard drop. I took out the old driver, but kept the original idea by using a 60 ohm resistor for low, and when the head is screwed all the down it goes DD, which is ~1.6 amps on a fresh battery, IIRC. Somewhere around there.

Very bright (and dim when I want it), and dependable.

sounds like a neat set up.

what sort of low are you getting with 60 ohms greg ?
if you were to choose an led for direct drive single cell looking for low vf, which ones generally have lower vf's to pick from ? or is it the same for all, just gota search and find one ?
thanks