Lux III + trustfire 3.7v x2 + 3.9 ohm 5w resistor

[Danz409]

ok. according to the chart found here
http://www.thecustomsabershop.com/resistor.aspx
the resister needed for 3.7x2= ~7.4 volts closist i found on the chart was
3.9 Ohm 5 Watt for 7.2v i hooked this up but not to my lux without testing, i tested the current on the resistor only and it was coming up to about 6.6
and wasn't even about to plug that up to the lux so i garbed a tiny sacrificial LED found on a old toy game. and hooked it up... needless to say it gave a microsecond of a flash and died... im afraid to hook this thing up. i don't know the exact math but according to the chart this should work, what went wrong?

[MoonDragn]

how did you measure the current? Can you describe what leads you put it on?


If you only have the resistor without anything else, the current flow through that resistor is : I = V/R or 7.4/3.9 or 1.89 Amps

But because you have a load condition in the LED, thats not the actual current flowing through the LED.

[Jay-gon Jinn]

Why are you using 7.4volts to light a Luxeon III? That's overkill.....

[swear000]

Very true unless he is powering something else. Would a single 3.7v trustfire work well or would it be underdriven?

[Danz409]

how did you measure the current? Can you describe what leads you put it on?


If you only have the resistor without anything else, the current flow through that resistor is : I = V/R or 7.4/3.9 or 1.89 Amps

But because you have a load condition in the LED, thats not the actual current flowing through the LED.

welp. that would explane it. i compleatly forgot about the load of the LED. being that a lux III acculy has about 50x more of a load then that tiny thing i used

[Danz409]

Why are you using 7.4volts to light a Luxeon III? That's overkill.....

because the batteries was the cheapest rechargeable ones the shop has and they aren't relay half bad. thats why the resister is there too to lower the voltage

and the target amps is 1000 MA or just 1 amp right? and that formula is showing its giving 1.8 which is way to much, what can this LED be over driven to safely?

[Jedi-Diah]

Just use one battery. You can even do a direct drive with one to a green or blue Lux III like Novastar does but it would be better and safer if you got a buckpuck.

If you have to use 7.2V (for some reason) then definitely get a buckpuck or you'll be just burning off the extra power heating up your resistor.

Better to just use one of those batteries. Simpler that way.





ok. according to the chart found here
http://www.thecustomsabershop.com/resistor.aspx
the resister needed for 3.7x2= ~7.4 volts closist i found on the chart was
3.9 Ohm 5 Watt for 7.2v i hooked this up but not to my lux without testing, i tested the current on the resistor only and it was coming up to about 6.6
and wasn't even about to plug that up to the lux so i garbed a tiny sacrificial LED found on a old toy game. and hooked it up... needless to say it gave a microsecond of a flash and died... im afraid to hook this thing up. i don't know the exact math but according to the chart this should work, what went wrong?

[Danz409]

problem with that. im also running a Hasbro board and 3.7 might be too little for that wouldn't it, i tested with the Lux III LED and the resister and voltage across the LED is 3.5 and is CONSIDERABLY brighter. but gets much warmer

[Novastar]

Danz... you have to slow down and understand the way an LED reacts to voltage and current.

The "forward voltage" of an LED pretty much denotes the voltage that it takes to get an LED to begin drawing current at a "nominal" brightness. That is sort of a subjective thing, but... anyhow, forward voltage is the GENERAL jist of where an LED is going to be "best" at.

If an LED's fwd v is rated at "2.95v"... essentially 3v... it means it will look pretty darn good around there.

HOWEVER... you also have the current. The current is most important on an LED, and each have a "maximum continuous current" rating, denoting the recommended FINAL current you can CONSTANTLY give it. This does not consider pulse-width-modulation, nor "blinking" the LED or whatever. It means 100% "on" at that current.

If an LED's max. cont. current is "1000mA"... about 1 amp... it means that's about the most you ever want to feed it, although you might be able to give it more within REASON.

You cannot give an LED that is rated at "3v forward"... 7+ volts. That will destroy it. You MAY be able to give it 4v (MAYBE)... but even that would be pushing it. However, 2.5v would likely light it (dimly)... and 3.5v might be really pushing it.

Examples of "below normal", "nominal" and "over-driving":
(remember, these are just as a guide, not 100% accurate "specs")

Luxeon V, Royal Blue (700mA rating for max. cont. curr, and fwd v of 6.85v)

Below normal: 6v, 500mA (barely lit)
Nominal: ~7v, 700 - 800mA (lit very well, longest lifespan, coolest run)
Over-D: ~8v, 1000 - 1200mA (lit VERY VERY BRIGHTLY, reduced lifespan, warm to hot run--depending on heatsink)

9v would destroy it, and even when I say "8v"... this is only for a short time (so far in my experiments).

This is from a direct-drive saber I've created, here:
http://www.youtube.com/watch?v=Os7TxBQfEZs

This is NOT to say that what I'm doing is "good" or "bad" or correct or otherwise... it just is what it is. You have to be careful when running an LED without a driver. Basically *YOU* have to set things up manually to ensure the LED will not be destroyed.

A current-regulating driver solves a BASTION of problems... example drivers of which would be: Crystal Focus, the upcoming Ultrasound board, buckpucks, the Corbin v2 board and... that's about it for now.

[Danz409]

i used the resister chart on TCSS and i used the one for the 7.2 volts. im running 7.4 so it IS overdriven but not by much so this setup seems to work fine and to my surprise even the Hasbro board without a resister works fine at 7.4 volts. this thing is like a tank :D only having 1 problem, the LED is brighter but comes at a price of one toasty resister. should i consider getting a higher wattage resister?

[Jedi-Diah]

You should really consider using less voltage. The wasted energy is turning into a hot resistor.


i used the resister chart on TCSS and i used the one for the 7.2 volts. im running 7.4 so it IS overdriven but not by much so this setup seems to work fine and to my surprise even the Hasbro board without a resister works fine at 7.4 volts. this thing is like a tank :D only having 1 problem, the LED is brighter but comes at a price of one toasty resister. should i consider getting a higher wattage resister?

[MoonDragn]

Well the reason I asked which leads you measured the current on is because you measure current with your meter in SERIES with the resistor. Not across the two leads in parallel

Also since the lux III forward voltage is around 3V, the voltage drop across the resistor has to be around 4.4 volts. Depending on which Lux III you use, to get 1 amp flowing to the lux III you'll need a resistor of 4.4/1 or 4.4 ohms, for the 1400 mA lux IIIs you need about a 3.3 ohm resistor.

Its much better if you just use the two trustfires in parallel with a resistor. Then it is only 3.7 volts and the led will last longer.

In that 2nd setup you can use a rectifier diode instead of a resistor. The typical voltage drop across a rectifier diode at that voltage is .7 volts which will bring you to about 3v.