# LED voltage tolerance?



## MrNaz (Feb 14, 2009)

How tolerant are LEDs of inaccurate voltage? I know the LED won't light at all if you go much below its Vf, but how about above? Say a Cree R2 with a Vf of 3.7V will that tolerate, say 4.8V if I limited it to 350mA ?


----------



## likeguymontag (Feb 14, 2009)

MrNaz said:


> How tolerant are LEDs of inaccurate voltage? I know the LED won't light at all if you go much below its Vf, but how about above? Say a Cree R2 with a Vf of 3.7V will that tolerate, say 4.8V if I limited it to 350mA ?



That's not possible. If you're regulating the current, then the voltage will be whatever the voltage will be. If you're regulating voltage, than the current will be whatever it will be.







Each individual LED will have its own unique curve, similar to the ones above. You can achieve a point on the curve, but you cannot achieve a point off that curve.


----------



## LukeA (Feb 15, 2009)

MrNaz said:


> How tolerant are LEDs of inaccurate voltage? I know the LED won't light at all if you go much below its Vf, but how about above? Say a Cree R2 with a Vf of 3.7V will that tolerate, say 4.8V if I limited it to 350mA ?



If you control the current, the LED will drop the 3.7V and pass the remaining voltage on to the rest of the circuit.


----------



## old4570 (Feb 16, 2009)

Hmmm ,  I think he may have been wondering about a direct hook up .

IE / 4.7v [ ?? ] from a 3D torch directly fed into the LED [ 3.7v ] , and he may have been wondering [ I know I am ] if the LED will survive this voltage  [ Im wondering in anyway , esp as I have several flashlights id like to mod ] 
lovecpf


----------



## likeguymontag (Feb 16, 2009)

old4570 said:


> Hmmm ,  I think he may have been wondering about a direct hook up .
> 
> IE / 4.7v [ ?? ] from a 3D torch directly fed into the LED [ 3.7v ] , and he may have been wondering [ I know I am ] if the LED will survive this voltage  [ Im wondering in anyway , esp as I have several flashlights id like to mod ]
> lovecpf



Depending on the voltage droop of the D cells and that individual led die's Vf, you might merely drive the led at stupid-high levels. I'd put money on burning it out though. Connecting a led directly to batteries with no resistor or other means of current regulation is just dumb.


----------



## Benson (Feb 18, 2009)

likeguymontag said:


> Depending on the voltage droop of the D cells and that individual led die's Vf, you might merely drive the led at stupid-high levels. I'd put money on burning it out though. Connecting a led directly to batteries with no resistor or other means of current regulation is just dumb.



I'd put that one down as "might work" if properly heatsunk, overdriving the LED some and giving insanely short battery life; the only way to get 4.7V out of 3D is with alkalines, and they (even Ds) drop voltage like mad under load. But if you loaded the same flashlight with NiMH cells, you'd get a direct-drive that would run the LED somewhere around spec on full batteries -- this seems to be a common arrangement for P7s, although I like regulated designs much better. (Dunno if they do this with R2s; I think most people aim a little higher when modding a 3D host... the R2 is gonna need less current, so less voltage sag, so more risk of overdriving with a fresh battery, but probably survives fine.)


----------



## jasonsmaglites (Feb 18, 2009)

if you're wondering if you can wire a 3d direct drive, go for it you'll be fine. even i did one and it was my first mod. with nimh you'll have maybe 3.6 volts (more or less depending on charge state) and will barely drop under the load of your led. if you use 3 alkalines it will want to start at 4.5 ONLY IF the batteries are fresh (and we mean very fresh) out of the box and under a small small load, such as a 5mm led. put a cree or seoul or p7 to those 3 batteries and the voltage will drop down to below what the nimh would be. then see where you are on the curve, and thats how much current its gonna draw (based on what was said above). thats why no one on cpf uses alkalines for their builds. you'll actually have about 1.1 volts apiece under a good load. 

do you guys know what would happen if we tried to drive a p7 by 2 nimh batts (roughly 2.4volts). i read where elektrolumens was gonna try that and the thing was just suppose to run for a long long time, and half as bright. i was thinking of wiring a 2d p7, but according to those charts it wont even fire up.


----------



## BillyNoMates (Feb 18, 2009)

jasonsmaglites said:


> do you guys know what would happen if we tried to drive a p7 by 2 nimh batts (roughly 2.4volts). i read where elektrolumens was gonna try that and the thing was just suppose to run for a long long time, and half as bright. i was thinking of wiring a 2d p7, but according to those charts it wont even fire up.



I've just tested a Rebel on my multimeter in diode test mode - it did light up (but only just) and reported a Vf of 2.59V which is more or less in line with the curves presented above. I reckon you would need a boost driver otherwise you will be left with a torch with a very very very long run time (about the same as the shelf life of the batteries) - cos it doesn't put out any light.


----------



## scientist x (Dec 6, 2021)

I just tried powering a 4.5 volt harbor freight flashlight with a 5 volt dc power supply. What had been a dim battery killing light now is amazingly bright.
I do not expect it to last.


----------

