Im looking to us a Cree MTG2 to power a Princeton tec shockwave 2 flash light that I use for diving. The light currently runs off 8 C batteries. Cree lists the specs (voltage=6, forward current=1100). I plan on using a constant current driver to run everything. If I use say a 2a constant current driver would the 12v from my battery blow the led because the forward voltage is over 6? Or does the forward voltage no matter as long as it is being supplied over 6v?
Question on Forward Voltage
- Thread starter neon822
- Start date
Help Support Candle Power Flashlight Forum
Well, I'm no diode expert but from what I can tell, the forward voltage doesn't matter much as long as you're higher than the spec for the diode..
However, the 2amps from your driver is going to fry that little sucker..
Option 1: Get a driver with a constant current no greater than 1100
Option 2: Forget the driver, remove half the batteries so you cut the power to 6 volts.
Option 3: Put two of the LED's in parallel with a 1.1a fuse in front of each. (not the best idea).
I'm no expert however...
However, the 2amps from your driver is going to fry that little sucker..
Option 1: Get a driver with a constant current no greater than 1100
Option 2: Forget the driver, remove half the batteries so you cut the power to 6 volts.
Option 3: Put two of the LED's in parallel with a 1.1a fuse in front of each. (not the best idea).
I'm no expert however...
poolman966554
Newly Enlightened
- Joined
- Oct 20, 2010
- Messages
- 77
If im looking at the same datasheet, your 6v version of this mtg2 has a max current of 3a. i think the 1100 your seeing is referring to the forward voltage example.. saying that the 6v version of this led running at 1100ma will run at 5.7v
to run this led at 6V, you will run around 2200ma current through led
DIWdiver
Flashlight Enthusiast
You said you plan on using a constant current driver. That's the optimal way to run LEDs, and if you get the correct driver, you should be good to go on that point. What the driver does is to drop the voltage to 'whatever is necessary' to get the set current. If that's 2A, the driver will apply just enough voltage to get 2A in the LED. That voltage will vary a bit with temperature, from LED to LED, etc.
"The correct driver" will be one that can be set to the current you want, and can handle the range of input and output voltages you will give it. A switching driver, particularly a buck type (AKA step-down) should do that fairly efficiently. There are lots of these around, from a few bucks at DealExtreme, KaiDomain, or other catalog outputs, to $30-40 for top-end ones from Taskled.
I think the big problem you're going to have though is heat. If you put 12W into the LED, and the driver dissipates 2W, around 2.5-3W is going to come out the front as light, the rest will be turned into heat. The light is a plastic body, which isn't a great conductor of heat, so the air in the light will get pretty warm. The incandescent bulb can handle this without a heatsink, but the LED can't. And you probably can't fit a big enough heatsink into the light. The designers at Princeton Tec who built the Shockwave LED only chose around 4W of LED, almost certainly for this reason. And they had access to lots of money and if not seasoned LED designers, at the very least some decent engineering staff.
"The correct driver" will be one that can be set to the current you want, and can handle the range of input and output voltages you will give it. A switching driver, particularly a buck type (AKA step-down) should do that fairly efficiently. There are lots of these around, from a few bucks at DealExtreme, KaiDomain, or other catalog outputs, to $30-40 for top-end ones from Taskled.
I think the big problem you're going to have though is heat. If you put 12W into the LED, and the driver dissipates 2W, around 2.5-3W is going to come out the front as light, the rest will be turned into heat. The light is a plastic body, which isn't a great conductor of heat, so the air in the light will get pretty warm. The incandescent bulb can handle this without a heatsink, but the LED can't. And you probably can't fit a big enough heatsink into the light. The designers at Princeton Tec who built the Shockwave LED only chose around 4W of LED, almost certainly for this reason. And they had access to lots of money and if not seasoned LED designers, at the very least some decent engineering staff.
Wow you guys are a huge help! So either go the way of running 6v to the battery unregulated or get a 1100ma driver. I plan on making a rechargable battery pack for NiMh, would one be better than the other (6v unregulated vs 1100 driver)? I also have one more question for everyone, at what volt does the battery have to have before the led will shut off? I always thought that, that is what forward voltage meant.
The forward voltage is always a function of current and temperature. But when listed only as a single number it is usually the forward voltage at rated current.
Often the voltage is specified at rated current and at a low current, near the knee of the curve.
Often the voltage is specified at rated current and at a low current, near the knee of the curve.
Ok I think I get what you are saying. So based on the Electrical Characteristics graph would the led fail to light at 5.125v?
http://www.cree.com/LED-Components-...XLamp/Arrays-Directional/XLamp-MTG2-EasyWhite
http://www.cree.com/LED-Components-...XLamp/Arrays-Directional/XLamp-MTG2-EasyWhite
Awesome! You've been a huge help! I'm thinking of using 6 nimh wired to 7.2v, i've been looking around for driver for about a week but not sure what to get (something around 1100ma range). Modes dont matter, but high+low would be best. Is there anything you would recommend? This is the only thing ive found
http://www.ledsupply.com/led-driver...a-constant-current-led-driver-without-dimming
http://www.ledsupply.com/led-driver...a-constant-current-led-driver-without-dimming
DIWdiver
Flashlight Enthusiast
LEDs generate light based on the current through them. Lumens per amp is somewhat constant, but drops some with increasing current, and with increasing temperature. Most LEDs will run down to 1% or less of their rated current, and will even have better efficacy at low currents. Be aware that Luminous says their LEDs should not be run below a minimum current, which is a significant portion of the rated current. In my experience however, most LEDs work fine down to 0.1% of rated current or less.
But amps per volt varies a LOT, so lumens per volt and watts per volt vary a LOT too. This is a really important distinction. If you want to understand running LEDs, you have to understand this. If you don't understand the volts vs. current graph in the data sheet, PLEASE keep asking questions until you understand it.
This is why it's so much better to run LEDs with constant current than with constant voltage. You will get much more predictable and consistent performance if you go with higher battery voltage and constant current driver, than if you go with trying to match the battery voltage to the LED voltage.
But amps per volt varies a LOT, so lumens per volt and watts per volt vary a LOT too. This is a really important distinction. If you want to understand running LEDs, you have to understand this. If you don't understand the volts vs. current graph in the data sheet, PLEASE keep asking questions until you understand it.
This is why it's so much better to run LEDs with constant current than with constant voltage. You will get much more predictable and consistent performance if you go with higher battery voltage and constant current driver, than if you go with trying to match the battery voltage to the LED voltage.
I think i'm getting the gist of everything. Based on the volts vs current chart slightly increasing current slightly will produce a slightly higher lumens. Increase volts even by a small amount and lumens will become extremely high... Is this correct?
I think you have it. That is exactly why it is better to use a constant current (or controlled current) driver circuit instead of a constant voltage driver.
DIWdiver
Flashlight Enthusiast
The other thing about running directly from batteries is that the battery voltage drops quite a bit during discharge. And now you know what the consequence of THAT is!
Cool I'm really glad that I'm getting it all. I honestly couldn't have done it without everyone's help! Before I order everything could everyone tell me if this setup is ok?
http://intl-outdoor.com/noctigon-mtg20-mcpcb-cree-mtg2-p0-5000k-6v-led-p-727.html
http://www.all-battery.com/8pcsofcsize5000mahhighcapacityhighratenimhrechargeablebatteries90419.aspx
http://www.ledsupply.com/led-driver...a-constant-current-led-driver-without-dimming
Heat sink would be a custom 20mm by 25mm solid piece of copper round stock made on a lathe
Host light: https://www.google.com/search?q=pri...oDw&ved=0CAoQ_AUoAg&biw=1092&bih=514#imgdii=_
I'm really looking for more flood than throw. I know plastic is a poor conductor of heat, but the light would be run for short periods of time in water 50-65 degrees f. If anyone has a better option, please I am all ears!
http://intl-outdoor.com/noctigon-mtg20-mcpcb-cree-mtg2-p0-5000k-6v-led-p-727.html
http://www.all-battery.com/8pcsofcsize5000mahhighcapacityhighratenimhrechargeablebatteries90419.aspx
http://www.ledsupply.com/led-driver...a-constant-current-led-driver-without-dimming
Heat sink would be a custom 20mm by 25mm solid piece of copper round stock made on a lathe
Host light: https://www.google.com/search?q=pri...oDw&ved=0CAoQ_AUoAg&biw=1092&bih=514#imgdii=_
I'm really looking for more flood than throw. I know plastic is a poor conductor of heat, but the light would be run for short periods of time in water 50-65 degrees f. If anyone has a better option, please I am all ears!
