# XPG vfd at 1500?



## docb (Mar 31, 2010)

Cree XPG specs say:
3.3v at 1000ma 
and 
3.2v at 700ma, so I can calculate resistor. 
(5v supply means I want a 2.7 ohm for 1000ma) 

But, specs say nothing for 1500ma
If I want 1250 or 1500ma, what vfd voltage do I use to calculate the resistor?


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## edroedro (Mar 31, 2010)

http://www.cree.com/products/pdf/XLampXP-G.pdf

This data sheet shows about 3.55Vf at 1500mA.


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## vaska (Mar 31, 2010)

Some info for you


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## docb (Apr 1, 2010)

So my supply is 5.09 with LED attached

5.09v
3.4vfd
1500ma desired
=
1.15 ohm resistor
(really a 1.2 ohm, 3 watt)

I have a $5 generic LED with a 750ma driver and it *looks* about as bright as this Cree with a 1.5 ohm resistor.

Am I missing something? I guess I'll try a 1.2, but it seems like it will be very close.


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## gcbryan (Apr 1, 2010)

docb said:


> So my supply is 5.09 with LED attached
> 
> 5.09v
> 3.4vfd
> ...



It depends on what you are expecting. Are you looking at bare emitters? We can't know what generic LED you are talking about or what it's specs are but assuming it's similar to a XR-E the differences in brightness may not be that great.

The XP-G should have more lumens and a larger hotspot but it probably won't be any brighter. The increased lumens are going to a larger hotspot rather than a brighter one.


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## VegasF6 (Apr 1, 2010)

docb said:


> So my supply is 5.09 with LED attached
> 
> 5.09v
> 3.4vfd
> ...


 
What is your power source? Can it handle that current draw without voltage sagging? Check voltage at LED while burning. Also, check current flow to LED, average vF is just that, only average. A great figure to start from, but not a guarantee. How about temperature? Light output will sag greatly if you aren't properly cooling the LED and 1.5 amps is quite a bit of current. 

Then, compare output of the 2 leds after a couple hundred hours of burn time. I imagine the Cree will "outshine" the generic led. What is the color temp of the generic LED, and how many watts is it being fed?

There are so many factors.


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## docb (Apr 1, 2010)

It's far from scientific, but I just turn one on and then the other, and subjectively decide if the room is any brighter with one or the other. The color isn't quite the same, but subjective brightness of the room seems the same. These $5 generics are drive by generic 750ma drivers. They claim 200L, but as cheap imported no names, I assumed that was an exaggeration.

I have the same type of power supply on each which is 5.09v under load at least with the resistor one. It's a small switching supply that claims 2.2a at 5v. I tried to check current, but was confused by the result. Putting DC ma meter in series bewteen v+ and the resistor, it says 032 on the 2000 range. I assuming that means 320? This with a 1.8ohm resistor, so that part confuses me. 

I have them each mounted to 3x3" heatsinks. The actual LED seems too hot to touch, but the star themselves seems not hot at all, and the heatsinks not warm at all. The thermal paste is the basic stuff from Radio Shack.

I assume that the LEDs will get hot. I can touch for about 5 seconds, and after that is too hot for my fingers.

Any comments about any of this are very welcome! I'm new to LEDs so all of this very nice to read.


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## docb (Apr 3, 2010)

Nobody has any input?


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## VegasF6 (Apr 3, 2010)

On 2A scale put your meter inline between resistor and led positive, or from led - back to p.s.

There isn't much to input really, I don't see how the generic led can be brighter than the Cree, but maybe. Brightness is all subjective, but then, that isn't the only thing that makes for a quality LED as stated above. What are your needs, which LED meets it, buy the one that makes you happiest.


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## docb (Apr 4, 2010)

I'm on 2a scale, ad have a DMM set to DCma in series between the resistor and the LED and it says 400

(If I eliminate the resistor completely, voltage drops to just over 4v and current goes to 700, but that seems like its not a good idea.)

I'm using a 1.5ohm resistor now, so that low ma measurement definitely confuses me. 

Shouldn't it say close to 1000ma? The power supply seems to still be at almost 5v when using a 1.5ohm


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## VegasF6 (Apr 4, 2010)

Something isn't right. Either your power supply, or your meter. Check your battery on the meter. Also, check voltage on something like a brand new alkaline battery or a known good car battery and see how it reads. 

If not, then it is your power supply. What do you mean by "The power supply seems to still be at almost 5v when using a 1.5ohm" have you checked voltage across the terminals of LED while lit? As you stated in your first post, average voltage at your emitter should be 3.2 or so at 700mA and closer to 3.0 at 400mA. 

As I am sure you have figured already, 
5V (power source) -3.3 (led drop) = 1.7
1.7/1.5 (ohms) = 1.133

When you state "(If I eliminate the resistor completely, voltage drops to just over 4v and current goes to 700, but that seems like its not a good idea.)" that sounds to me as though your power source can not handle the load and is dropping voltage. When you add the resistor in the load is even higher and voltage dropping even more. Try a different one, even if it is just 4 alkaline batteries and a 1.8-2.7 ohm resistor.


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## docb (Apr 4, 2010)

Using a different 5v supply - this one is linear - I measure about 600ma now. About 3.45v at the emitter. Supply voltage stays at 5v.

Still I'm still not sure whether to lower the resistance further. This is using a 1.5ohm, which is what I think should measure 1100ma draw.

Seems a little brighter than the generic LED now.


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## vaska (Apr 5, 2010)

docb said:


> I measure about 600ma now. About 3.45v at the emitter.



Too much voltage drop for XP-G at such current. Are you sure you've identified the LED right?


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## docb (Apr 5, 2010)

vaska said:


> Too much voltage drop for XP-G at such current. Are you sure you've identified the LED right?



I ordered the XPG's from Kaidomain so can only I assume that's what was sent. They look like the picture.

Do you mean there is too much voltage? Can you rephrase? What are you suggesting?


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## vaska (Apr 5, 2010)

docb said:


> Do you mean there is too much voltage? Can you rephrase? What are you suggesting?



If you follow the link in my first post you'll see that measured Vf at 600 mA was 3.13V. It correspods with my own experience with several dozens of XP-Gs: Vf is always less than 3.2 at such a current.


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## docb (Apr 5, 2010)

vaska said:


> If you follow the link in my first post you'll see that measured Vf at 600 mA was 3.13V. It correspods with my own experience with several dozens of XP-Gs: Vf is always less than 3.2 at such a current.



I do see that, but no idea what to make of those numbers.. 

Could it be a defective LED? 
Maybe my DMMs are inaccurate?
What else could be going on?
What's the significance of my numbers being so odd?


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## vaska (Apr 5, 2010)

I don't know. If the measurements are right, irregular Vf means some extra resistance wihtin the LED itself, that decreases efficiency.


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## bshanahan14rulz (Apr 5, 2010)

What voltage does your DMM measure a brand new alkaline cell? Mine measures around 1.65V. Try that, see if it's your meter.

Also, why not just take the 700mA driver and hook that up to the cree and see if it looks brighter? 

"small switching power supply" I assume means one of those wall adapter plugs? Those can be strange to deal with, and hard to predict what voltage it can generate at specific loads.


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## TorchBoy (Apr 5, 2010)

*Re: XP-G Vf at 1500 mA?*



docb said:


> It's far from scientific, but I just turn one on and then the other, and subjectively decide if the room is any brighter with one or the other. The color isn't quite the same, but subjective brightness of the room seems the same.


The human eye has a logarithmic response which copes very well with huge differences in input. Testing lights sequentially they have to be significantly different (I think it's a factor of around 2:1) for us to notice any difference.

Try taking a photo of each with your camera settings fixed, and compare the photos side by side.


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