Why not use an LED in direct-drive ?

[Changchung]

Hi guys, some friends and I had some arguments about why not DD Leds, I say that is not good for so many reasons, some friends say that if the Led is a 3amp or less, xm-l, P7, xr-e, etc Led you can use any kind of 4.2volt battery because the rest of the current dont go to the Led :S go to someplace unknow, I dont think the same.

Help to understand that and for my friends too...


Sent from my phone with camera with flash and internet on it...

 

[AnAppleSnail]

Re: Why dont use a Led in DD???

You can kind of get away with it. The LED Vf rises with current, and the battery voltage falls with current, and the resistance of the wires and batteries takes more voltage out at higher current.

Alkaleaks are commonly used with direct-drive circuits. Most cheap lights with over 3.4v of battery are direct drive. Their voltage sag makes it safer to use direct drive.

But volts and amps never vanish.

 

[Changchung]

Why dont use a Led in DD???

Thanks... More comments???

MOD please, feel free to move this thread to the LED section were I think this belongs...




Sent from my phone with camera with flash and internet on it...

 

[Kestrel]

Re: Why dont use a Led in DD???

You can kind of get away with it. The LED Vf rises with current, and the battery voltage falls with current, and the resistance of the wires and batteries takes more voltage out at higher current.

I agree with the above, but what about that the resistance of the LED decreasing with increasing temperature (NTC behavior), leading to a substantial decrease in output and possible thermal runaway?

Definitely not a dealbreaker though - I currently have two P60 lights that are DD (the OR/TL V2 XPG copper triple and a Nailbender XML - both of which work exceedingly well).

MOD please, feel free to move this thread to the LED section were I think this belongs...

Moving now ...

 

[IMSabbel]

Re: Why dont use a Led in DD???

Thanks... More comments???

Direct drive used to be a very common way to do things not too long ago. Nowadays, forward voltages have continued to drop while batteries also got lower internal resistances, so it got more and more uncommon despite increased current capabilties of LEDs.

You can easily trip the current protection of a 18650 cell by direct driving a XM-L, for example. Or just annihilate the LED when using an IMR.

 

[mattheww50]

Re: Why dont use a Led in DD???

The basic problem with direct drive is the non-linear V/I curve on the LED. That means that a direct drive LED is subject to enormous variations in output based upon the state of the battery. As a practical matter, it is just about impossible to direct drive a high output LED device such as an XM-L and acheive satisfactory output characteristics over the majority of the life of the battery uless you are willing to throw away much of the battery capacity in a resistor in series with the LED.

The secondary issue is progress in electronics has rendered the cost of the electronics reqiured to provide essentially full regulation over just about the entire life of the battery, a minor expense. Moore's law is in fact alive and well. . . .

 

[sunny_nites]

Re: Why dont use a Led in DD???

In the early days of Pulse Width Modulation one of the arguments was that, when using a PWM to drive an LED, the off times in the pulse cycle would allow the LED to run cooler at higher drive currents.

You don't hear about the above much any more and I don't know if that is because it was proven incorrect or if it is just common knowledge that it is correct and doesn't need to be restated.

If driving a LED with a PWM circuit would allow it to run cooler, that would be a big advantage over a direct drive circuit.

 

[daybringer]

Re: Why dont use a Led in DD???

The secondary issue is progress in electronics has rendered the cost of the electronics reqiured to provide essentially full regulation over just about the entire life of the battery, a minor expense. Moore's law is in fact alive and well. . . .

Oh this is perfect, I was about to start a thread on this topic but I'm glad you brought it up. I want a really cheap setup to power this guy right here: http://www.aliexpress.com/item/wdm-...0mA-clip-size-35-35mil-epistar/574836680.html consistently over the lifetime of 3 AAA batteries in series.

I tried it with just direct drive and it got so hot it melted the solder. The second attempt with a 10ohm resistor yielded this:

So now I'm wondering how to get a more consistent brightness out of these batteries. I'm also looking for a cheap solution. Like, dirt cheap. Anybody have any ideas?

 

[argleargle]

Re: Why dont use a Led in DD???

So now I'm wondering how to get a more consistent brightness out of these batteries. I'm also looking for a cheap solution. Like, dirt cheap. Anybody have any ideas?

Gotta have a driver/regulator circuit. No way out of it that I'm aware of.

 

[AnAppleSnail]

Re: Why dont use a Led in DD???

Bigger capacity-to-current-draw ratio, and a better battery chemistry would help. Right now you're pulling out TONS of power from your batteries. If you increase that ratio, and get a nice flat discharge-curve chemistry (LSD NiMH, NiCd, Li-Ion), then you'll have near-regulated-type output with a long tail at the end of battery life.

 

[argleargle]

Re: Why dont use a Led in DD???

Bigger capacity-to-current-draw ratio, and a better battery chemistry would help. Right now you're pulling out TONS of power from your batteries. If you increase that ratio, and get a nice flat discharge-curve chemistry (LSD NiMH, NiCd, Li-Ion), then you'll have near-regulated-type output with a long tail at the end of battery life.

Agreed, a 5mm led custom flashlight maker I was related to used a larger than normal resistor and under-drove his doubled leds to half effect. Of course, the single 1/4 watt resistor was on the ragged edge of overheating, but he was counting on short occasional activation cycles and his stuff was by no means 100% duty cycle.

On the plus side, the design was wicked cheap and had excellent runtime characteristics for the first 1/3 of the battery life. When you're looking at 10-50 hours of peak runtime capacity with no real damage to the emitters whatsoever EVER, who cares if you throw away a set of alkalines once in a while... right? It was sort of regulated without a real driver circuit

Side note: I really, really liked the guy. There's only one thing I've ever said to him that I wish I could take back and strangely it had to do with flashlights. No editing posts in real life, right? His flashlight design was actually pretty sweet and he even tested them with a lux meter. I said the wrong thing once about an experimental design that I helped build and it was taken the wrong way. I also made him mad by opening the wrong door a couple of times. I had no idea why it made him mad and he wouldn't explain. I'd just screwed up his runtime versus lux test.

He was my first exposure to high UV xenon-filled arc light and didn't even tell me anything or warn me about it, just that "your eyes will hurt if you leave this panel open, here have a margarita." I miss him a surprising amount.

Damn! I'm getting all sentimenal over an LED forum post. Wow! Didn't see that one coming! :mecry:

Life is short. Too short to say the wrong thing IRL.

 

[daybringer]

Gotta have a driver/regulator circuit. No way out of it that I'm aware of.

I searched for drivers on aliexpress.com, but the smallest voltage they have there is 12V and the typical is 85-265V. I'm running 3-4 AAAs which would only come out to 4.5-6V, do they make drivers for this little voltage?

 

[Freeze_XJ]

I would look on the standard forum places, this forum has a nice marketplace where the better drivers are available (or the selling shops have threads telling you about them). For cheaper and less reliable versions see the internet at large, Ebay, DX and more Chinese etailers spring to mind. The Sandwich Shoppe and Der Wichtel might be interesting?

 

[idleprocess]

The short answer on why not to direct-drive LED's is that it typically maximizes component efficiency at great expense in term of system efficiency, performance/operating characteristics, and longevity.

 

[gadget_lover]

I recommend against DD for several reasons.

The first is that you can often get away with it for several hours, then POOF. In 2003 a CPF member made a great 1D cell sized Mag using a 5 watt luxeon and direct drive. One of the first in the area. It was wicked bright... until it burned out at a CPF get-together. Turns out he was driving it at 150% of rated current on fresh batteries. It had lasted through several sets of batteries.


The second is that the light output will vary a lot, even with nicad or NiMH. The fresh voltage after charge will always be higher. Then it will taper.

A set of cheap AAA batteries are the key to doing a direct drive that sort of works. 3 alkaline AAA batteries in series have such a serious voltage drop that it is often within the safe range for a lot of LEDs. If you use good batteries with low internal resistance ( or rechargables ) the voltage does not drop as much and you risk burning out the LED.

Dan