Resistor with a Li-Ion?

[RevengeoftheSeth]

Hey all,
Normally I don't have a problem with resistor calculations, but this is more of a question of theory. Since a Li-Ion puts out 3.7V and a green Lux III has a Vf of 3.9V, would the current from a protected 18650 blow the LED or will it be fine without a resistor? I do know it will get hot, and I'm not using an MHS setup for this build. I don't have room for an alternate battery solution and I'd hate to stick a fat 1Ohm 1W resistor in there (boo for resistor bricks).

I know some people go resistorless when the voltages match, so I am looking for voices of experience to chime in. I'd use a 14500, but I'm looking for run-time and I haven't found one that actually puts out a steady 1000mAh, yet.

[Rhyen Skytracker]

You should be fine without a resistor. I just did a set up with a P4 blue and a 3.7V Li-Ion and it works fine. Keep in mind the 3.7V Li-Ion runs normally around 4Volts on full charge. I overdrove my LED by not using a resistor and anytime you do that you take a chance. So, do so at your own risk.

[RevengeoftheSeth]

Hopefully the econo board will draw the extra voltage off of the full charge and I won't encounter any 'imperial entanglements'.

[Novastar]

Hey all,
Normally I don't have a problem with resistor calculations, but this is more of a question of theory. Since a Li-Ion puts out 3.7V and a green Lux III has a Vf of 3.9V, would the current from a protected 18650 blow the LED or will it be fine without a resistor? I do know it will get hot, and I'm not using an MHS setup for this build. I don't have room for an alternate battery solution and I'd hate to stick a fat 1Ohm 1W resistor in there (boo for resistor bricks).

I know some people go resistorless when the voltages match, so I am looking for voices of experience to chime in. I'd use a 14500, but I'm looking for run-time and I haven't found one that actually puts out a steady 1000mAh, yet.

Basically, if the rated voltage of the cell is equal to or a bit less than the forward voltage of the LED... you should be good without a resistor. I know this for a fact because I've been doing it with no problems for the last 5 years with multiple sabers, all with differing setups... but with the same basic principle: batt v = (or ~=) LED fwd v

This, however, is all about driving an LED with no special board. That is to say... if you're not using an FX board, a "Corbin" board or an "Ultrasound", CF, etc. etc. In the case of most of the common boards we all use... some kind of controlled and/or fixed current is being supplied, and a resistor is not needed--because it's already present on the driver board in a certain fashion.

Still... what Rhyen is saying is also true--you take a risk if you have more voltage from the battery source over what the LED "wants" (fwd v). But even with THAT said... you have to remember:

* The fwd voltage of an LED is basically the voltage at which it lights up "well" or "properly". You may be able to have it light up with less voltage... and more voltage too! Within reason (say + or - ~0.5v)
* As an LED attempts to draw more current from a cell... it requires more voltage to do so. Because voltage and current are relational. If the cell cannot provide more voltage... the LED will be unable to keep drawing more and more current... and you won't run into "current over-run".

Hope it helps... I know it can be "hard" to understand. But one thing is a good bet--if you "match" voltage & current to the best of your ability... a resistor may not be needed. And having less resistance means less waste, which provides more runtime in the long run. Good luck!

[RevengeoftheSeth]

Thanks, guys. That is the same conclusion I had come to, and it is always nice to have others back you up. I'm pretty confident, now, that this will work just fine.

I did decode the numbers on the back of my lux III green, and when matching up the Vf code, it said 3.27-3.51V. Is that right, when it is listed on the spec sheet as 3.9V?

[Novastar]

Right, this is where it can get a little confusing...

So... from my understanding, "forward voltage" is technically the most minimal voltage that can be used to light any given LED. Otherwise--the diode simply doesn't work and no current flows. Unfortunately, due to the way LEDs are made... you never have a "definite" number you could use that would be the perfect one for ALL LEDs of that type. Which is why you have the range that you see.

If this is getting confusing (not just for "revengeofthesith", but for all new readers, etc.)... do the learning by experience: TEST IT!

* Try feeding that Lux III green 3.2v. Like, use 2AA Alkalines. That ought to suffice... and you should see light--but... it will kinda suck, lol. Won't be bright. It's probably @ around 200mA to 500mA if you are lucky. So, not optimal current (obviously).
* Then try ~4v... which should be safe, given that the Vf code is 3.2 to 3.5... AND the fact that in order to feed the maximum current to an LED... you must also increase the voltage a bit
* When trying 4v (say with alkaline again for reference/testing)... it should be QUITE a bit brighter. At this point, it's likely to be drawing closer to 700mA or 1A or something... vs. the maybe 200mA to 500mA or whatever from before.

This goes back to:

VOLTAGE AND CURRENT ARE RELATIONAL

and

LEDs are DIODES

and

Spec sheets are good guidelines--but they are not "perfection", nor are they always accurate in every instance. For example, if you use a driver with PWM (pulse width modulation), it's a sure bet that you at least SLIGHTLY exceed the spec sheet "maximum" current (or voltage!)... because you are now feeding the LED "waves" of current vs. a steady, unmoving stream of energy.

ANYhow... There ya go. LEDs. Sabers. Meant for testing, experimentation, creativity... in a world with "rules"... ....

...which are meant to be bent!

[cannibal869]

Thanks Nova - question though about accent LEDs - would you likewise be more conservative with the smaller LED sizes given that they are probably not quite as "sturdy" or resilient? i.e. would you power a string or even a single 5 or 3mm LED with a single trustfire without a resistor?

-C

[equinox13]

i just tried powering a rainblow LED rated at 3.4V with a fully charged Li-Ion. it melted after a few seconds. on the flip side, 3 of them are perfectly happy with a single 82 ohm resistor and wired in parallel.

[RevengeoftheSeth]

Good read - thanks, Nova. I've collected some drained AA batteries (so I can get 3 of them to add to 4V), so I'll tinker around with it tonight. I'll also monitor the heat and see if my heatsink is going to be enough for it.

[Novastar]

i just tried powering a rainblow LED rated at 3.4V with a fully charged Li-Ion. it melted after a few seconds. on the flip side, 3 of them are perfectly happy with a single 82 ohm resistor and wired in parallel.

Right, this somewhat makes sense, since even a Lux III red, amber, red-O would likely give the same result (if powered directly with no resistor), because the fwd voltage is now TOO far off! I'm not familiar with a "Rainblow" LED either, so... I can't vouch for that. A question would also be... did you have the LED(s) on a heatsink during testing? You should...

A fully "topped off" Li-Ion cell can have as much as 4.25v (!!), so... be careful.

In general, I actually look for LEDs that are more around 3.85 Vf or 3.9 Vf (so, ok "~4v")... such as Lux III Green or certain Lux III Blues, etc. when giving a single Li-Ion cell. And (as stated above), I'd never give a single Li-Ion cell to a Lux III red (~3 Vf), since it's almost 1v less. At that point, a 1 ohm resistor with 1watt capability or > should suffice.

@equinox... hmm... I'm surprised about the whole "82 ohm" (!!!!!) resistor thing. Uh, I guess I'd have to see the wiring/photo... but otherwise, from the way *I* understand things... 82 ohms would be FAR FAR FAR too much for them to even light. So... whatever, I'm not sure what the heck is going on. I mean, if you're going 4.2v to the rated LED @ 3.4v... you need a 1ohm resistor. 82 ohms would be ASTOUNDING and I can't imagine how they would light up. But what do I know. I don't have any idea on how you wired it up. And even if the 82 ohms was being split in thirds (?) over the 3 LEDs... that could possibly yield only a lowest value of around ~20 ohms...

Hmm. Come to think of it... I'm beginning to think you are talking about LOW-CURRENT "little indicator" types of LEDs!!! Is that true? As in... they are looking more for ~20mA (milli-amps!), as opposed to ~1A (aka ~1000mA)? If so... then we have a different conversation going...

AND IN ~THAT~ CASE... even there, you'd want something around 40 ohms of resistance with 1/4 (ish) watt capability, again depending on wiring.