Diy led meet?

Would it be beneficial if I set up a WIKI site for URS and more specifically LED builds? I think it would go a long way to have the info in one spot and updated by people who stay current with the tech.



I run a small hosting company and can easily drop this on one of the shared servers.
 
I stopped out at Daves place tonight to drop some frags off and he showed me the basement setup for the meeting.

Perfect setup.

I wont be around but hope it goes well.
 
Can someone shoot me a PM with an address of where this will be? Trying to decide if I want to make any other stops while I'm in town. I haven't been to TRS or CF in two or three years.

Also if any of you are interested in DIY drivers based on the LM3409 I mentioned above, let me know ASAP what your target Vin, Vout, and drive current are, and we can put together an appropriate BOM to get it ordered in time.
 
How do we calculate or know what our Vin and Vout are?

I want to run 36 Cree XP-E Royal Blue LEDs somewhere between 660 and 850 mA. I'm not 100% sure what mA I will end up running them at. It depends on cooling, lighting color, etc.
 
Vin is what your supply is providing, Vout is the Vf of your LEDs. The LM3409 is so flexible you can basically shop for the best value in power supplies with the features you want, then plan your string length to land a few volts under that.

For instance, I have a supply running at 28.5v that's powering strings of 8 XM-L at ~2.2A each. The LEDs need about 27v. So, Vin is 28.5v and Vout is 27v.

24v supplies are the cheapest and easiest to get so many people plan around that, which leaves you with strings of 6 or 7 LEDs (depending on the Vf of the LEDs you're using). But if you already have, or find cheaply, a supply with a different voltage rating, you can just plan for that. I would never go below 24v unless you had a very small tank. Sometimes, 32v or 48v supplies turn up at a good price on eBay or other sources, so sometimes people use those.

Higher voltage means fewer drivers which might or might not be a good thing depending on how you want to control the strings.
 
Is Vf the thing labeled "Forward Voltage" on page 5 of the spec sheet for royal blue here? http://www.cree.com/products/pdf/XLampXP-E.pdf

It says forward voltage is 3.5 at 1000mA, and 3.2 at 350 mA. Is forward voltage linear? So at 675mA, the forward voltage or Vf would be 3.35?

In your example, your extra 1.5v is handled by resistors and dissipated as heat right?
 
Dwzm - very interested in the lm3409 drivers. I just finished reading the thread but still not 100% sure what I need. I'll shoot you a pm.
 
Nate is there a maximum amount of voltage you can use through a power supply? I mean you couldnt run a 110v driver could you?
 
Has anyone checked out one of the aquastyle retros yet? thinking about playing with one just to see how they run. only thing is all their heat sinks arent drilled and tapped nd thats one part of the job I am really not into so i would have to use the thermo compound
 
NirvanaFan said:
Is Vf the thing labeled "Forward Voltage" on page 5 of the spec sheet for royal blue here? http://www.cree.com/products/pdf/XLampXP-E.pdf

It says forward voltage is 3.5 at 1000mA, and 3.2 at 350 mA. Is forward voltage linear? So at 675mA, the forward voltage or Vf would be 3.35?
Click to expand...

Yeah, Vf is forward voltage and is typically listed in datasheets. For the LEDs we're using it is usually between 3 and 3.5v. On a large scale it's extremely non-linear but in the region we're talking about you can kinda assume it is linear. The only real catch is that it's a very "loose" spec in that the tolerance is poor. So you want to give some headroom above the calculated Vf in case they all trend high. The typical "industrial" style DC power supplies most people are using have a trimpot on them to tweak the exact voltage so most people don't get bent out of shape trying to figure out the exact Vf, since you can just tweak the power supply if it's not exactly what you expected. This is way more of a potential issue with the CAT4101 chips, since they're linear, and dissipate the extra power as heat. The LM3409 is a switching regulator, and is reasonably efficient over a much wider range.

In your example, your extra 1.5v is handled by resistors and dissipated as heat right?
Click to expand...

Kinda not.

At a very high level, these LEDs need a constant current power supply. The way we achieve this is by providing more power than we need (via the DC power supply) then putting some sort of regulator between the power supply and the LEDs to "chop" the power down to what we want.

There are two basic kinds of circuits that typically are used - passive and active. Passive circuits are "dumb" in that they don't react to conditions. Resistors are passive. You can drive an HPLED with a carefully chosen resistor, just like you do with a tiny little "normal" LED, but the drive current will change with temperature and other fluctuations, and it's inefficient, so it's almost never done. The resistor works by turning the extra power into heat. You use basic math to guess at the resistor's size, then trim it to get the exact current you want under a given set of conditions. In the end, the resistor is just turning a static amount of power into heat.

An active circuit can react to changes in the environment (temperature is the big one) to keep the current constant no matter what. There are two kinds of active regulator circuits - linear and switching. A linear regulator is kind of like a smart resistor. It still does nothing but turn extra power into heat, but it has a feedback mechanism that allows it to adjust to conditions to keep the current stable. When using a linear regulator, efficiency is basically the difference between Vin and Vout, so you want to keep the DC power supply's voltage close to what you actually need in the LED strings, with the caveat that most regs have a minimum difference (for the CAT4101 it is half a volt).

A switching regulator works by turning the current on and off very rapidly, then smoothing the chopped-up current into a constant current at the correct level. The smoothing is usually via a diode and inductor on the output. The losses (and hence efficiency) are mostly related to the switching action itself, plus some small losses in the passives (inductor, diode, etc) not to the difference between input and output voltage. There is no linear relationship between Vin-Vout and efficiency. Switching regs are generally efficient over a wide range of conditions.

The CAT4101 and LM3409 are very typical examples of a linear design vs. a switching design.

davidpesce said:
Dwzm - very interested in the lm3409 drivers. I just finished reading the thread but still not 100% sure what I need. I'll shoot you a pm.
Click to expand...

Saw your email, I will shoot you a note back.

jfl14609 said:
Nate is there a maximum amount of voltage you can use through a power supply? I mean you couldnt run a 110v driver could you?
Click to expand...

In theory, more or less there is no limit. However, there are some very serious safety concerns with high voltage DC, and there are diminishing returns as voltages get bigger. Plus, it starts to get really hard to design drivers for higher voltages because your choices for components get really hard. If you read the recent pages in the DIY driver thread you'll see that even for 48v, component selection starts to get really hard for the diode, inductor, and mosFET used by the LM3409 design.
 
jfl14609 said:
Has anyone checked out one of the aquastyle retros yet? thinking about playing with one just to see how they run. only thing is all their heat sinks arent drilled and tapped nd thats one part of the job I am really not into so i would have to use the thermo compound
Click to expand...

They use junky low-efficiency LEDs and drivers with a poor track record for reliability. You can pick up XP-E for close to $3 each these days so IMHO it's not worth it to save another buck or so and end up with low-quality stuff.
 
One more thought - it would still probably be beneficial to have a camera and projector even if there are only a small number of people - some of the soldering stuff, particularly if we are looking at an LM3409, is very hard to see with the naked eye - trying to get ten people to look at a soldering job on a tiny little part is gonna be hard!
 
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