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Stumped on DIY LED Build

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    Posted: November 10 2013 at 1:47pm
All,

I've hit a wall on my LED build. Here's hoping one of you will spot a stupid mistake...or I'll notice it just by typing it all out. TIA.

Fatman, feel free to chime in here if I forgot any details.

Equipment:
4 heat sinks
32 RB Cree XT-E
16 NW Cree XT-E
Other colors (not really applicable for troubleshooting here)
Meanwell ELN-60-48D x 3 (there are other drivers, but they aren't applicable for troubleshooting)

Ok, so the LEDs are spread evenly across all four heatsinks. So, that's 8 RB and 4 NW on each. One driver controls two heatsinks worth of RB LEDs (so one driver controls the left side, and the other driver controls the right side). The third driver controls all the neutral whites.

I turned down the SVR2's on the drivers, wired everything up in series, and set a pot at 10V, master. The current was 200mA, and the voltage was correct (forget the exact number, but it matched the calculations). When I turned up the pot and the SVR2 pot, the max current I could get was 300mA. This happened with all three drivers and both colors.

I should be getting 1.3A max, but I'm far from it.

Alright, here goes. Ignore the wiring mess. It was beautiful until I had to start troubleshooting.

The LEDs all run...meaning, they all light up. Continuity is good.



Here are two heatsinks showing the wiring. There are 16 Cree XT-E Royal Blues run in series (showing + and -).



Here's the Meanwell ELN-60-48D showing the potentiometer (SVR2) turned all the way down (counter-clockwise):



And, here's the driver hooked up to the pot and LEDs:



Here are my calcs:





But, I found two different spec sheets for the Cree XT-E's.





Are the initial calculations the problem? I even tested just 8 XT-E's and still got 200mA min and 300mA max.

HELP!!!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Erikts Quote  Post ReplyReply Direct Link To This Post Posted: November 10 2013 at 2:49pm
That is the same problem I had. My problem was an initial forwad voltage calculation error. Once I fixed the forward voltage problem my issue was resolved. But it looks that you have already tested that theory with no improvement.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ann_A Quote  Post ReplyReply Direct Link To This Post Posted: November 10 2013 at 3:05pm
I could be wrong, but if I remember correctly isn't total power calculated by finding the sum of the power at each resistor?

So in this case for the neutral whites my calculations come out as this...
(V=electromotive pressure in Volts, A=current in Amperes, R=resistance in Ohms)

For a single LED (minimum calculations):
R = 2.9V / 0.7A = 4.14 ohms
P = 4.14 x 2.9V = 12W

For a whole circuit with 16 LEDs (minimum calculations):
12W x 16 = 192W



Or am I just totally wrong with these equations? It's been a while since I had to do any electrical calculations.


Edited by Ann_A - November 10 2013 at 3:10pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ReefdUp Quote  Post ReplyReply Direct Link To This Post Posted: November 10 2013 at 3:18pm
The power is only calculated across each LED, not "total" like it says. The first "power" column is the LED rated power (3W). P=I*V, so P=0.7A*2.9V=2.03W. P=I*R, so yes, your calculation of resistance is correct. Now, I don't know what my resistance through the setup actually is...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ReefdUp Quote  Post ReplyReply Direct Link To This Post Posted: November 10 2013 at 3:23pm
More info...

I just tested the output voltage of the driver alone...48V on spec. I then added the 8 RB LEDs. The voltage through the system dropped to 24V. What happened?

I used 24 gauge stranded wire as was recommended...any chance this is causing excessive resistance?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Ann_A Quote  Post ReplyReply Direct Link To This Post Posted: November 10 2013 at 3:29pm
Ah yes, I mixed up the equation for power... well I don't see anything else wrong with them.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Erikts Quote  Post ReplyReply Direct Link To This Post Posted: November 10 2013 at 3:46pm
The drivers are auto ranging to load. Did the 8 led test affect svr2?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ReefdUp Quote  Post ReplyReply Direct Link To This Post Posted: November 10 2013 at 3:49pm
I adjusted svr2 on the 8 led test...got about 22-24v, min & max.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Erikts Quote  Post ReplyReply Direct Link To This Post Posted: November 10 2013 at 4:17pm
What amp range did you get on the 8 string test?  Cree XT-E can come from the 3 bins are you sure they are all from the 3v bin? group f.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Krazie4Acans Quote  Post ReplyReply Direct Link To This Post Posted: November 11 2013 at 10:39am
Nikki I'm not sure if you got this solved yet or not. First thing I do on those drivers is set the dim+ dim- voltage at ~1-1.5 volts (this is going to make sure that the output current is at it's lowest value). Then carefully turn SVR2 counter clockwise just until it stops (this pot is very fragile so be gentle). 

Once that is set then power it on. Now check the output voltage from the driver (not the current the voltage.) since you are running 16 LEDs on this driver you may need to turn SVR1 (the one through the hole in the heatsync) up slightly to account for any variation in the forward voltage of the LEDs. I generally set this voltage at .5v greater than the calculated normal forward voltage needed for the string ( this accounts for loss in wires and junction variance in the LEDs). You could also adjust this voltage near the maximum forward voltage for the string just to make sure that the issue is not with the forward voltage clipping the output current. Once this voltage is set with a load on the driver, then you can start to adjust the dimmer voltage and the driver output current.

Keep in mind that in order for this driver to output the maximum current. the output voltage must be equal to or greater than the needed forward voltage for the LEDs. if the output voltage is less than the needed forward voltage then the driver is unable to rectify the output current properly. Also make sure that your meter is set on the 10amp input when measuring the current and that the meter is in series with the driver output and the LED input.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ReefdUp Quote  Post ReplyReply Direct Link To This Post Posted: November 11 2013 at 11:10am
I think you're on to something... Sent you a message on Facebook. :) Thanks!!!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote phys Quote  Post ReplyReply Direct Link To This Post Posted: November 11 2013 at 11:41am
I turn my internal pot all the way up and use the external voltage control to increase output. I noticed that when the internal pot is not adjusted high enough, the amperage output is too low to run my 8 xp-g N whites.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatman Quote  Post ReplyReply Direct Link To This Post Posted: November 11 2013 at 10:15pm
Reef'd Up fixed it. Thanks all for your help and ideas. Just took some extensive troubleshooting and spec reading. I'm still blind from seeing the blues on one of four heatsinks. And that's just 8 of about 30 LEDs on each. The workmanship is fantastic and this will be something to see when she gets it up and running completely. Thanks to all those you took the time to offer suggesting, it really helped a lot.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ReefdUp Quote  Post ReplyReply Direct Link To This Post Posted: November 12 2013 at 5:42am
No Kevin, YOU fixed it!! I finally slept like a baby last night.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Krazie4Acans Quote  Post ReplyReply Direct Link To This Post Posted: November 12 2013 at 9:03am
Really?! That's all your going to put on here? You ask for help on something that others might run into and then you only say it's fixed with no explanation or solution?I hope you left the solution off of your thesis paper too. :)

Come on fill us in. what fixed it and where was the error (admitting you made a mistake makes you relate-able and human :) ). Now spill it. lol
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ReefdUp Quote  Post ReplyReply Direct Link To This Post Posted: November 12 2013 at 11:32am
I was on my phone (still am). Kevin can probably explain it better than I can.

We tried two different pots with no luck on 8 and 16 LEDs. Then, we used a 9V wall wart with thicker wire on 8. That worked. We tried the wall wart on 16 again with no luck. We then tried the wall wart with 14 LEDs, and we were golden.

It sounds like part of the problem was trying to run 16 LEDs, which is at the upper end of the driver limits. But, it also sounds like there was a pot problem since we couldn't get it to work on 8 LEDs.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatman Quote  Post ReplyReply Direct Link To This Post Posted: November 12 2013 at 12:13pm
The explanation is really pretty simple (but perplexing at the time). We knew we weren't getting enough current through to light the LEDs on the string. The voltage was low also. We had enough to light them, but not enough voltage to get them to raise above just lit. I moved the SVR1 pot up and still no increase in voltage. After checking to see if the wire in from the driver was limiting current because it was too small we found the problem. 16 LEDs on a single drive circuit, run in series, are too many. It will light (hits the minimum forward voltage), but doesn't have the voltage to raise the light level. 3 volts times 16 LEDs is 48 volts required (to get to minimum). Once we removed two LEDs on the series we had about 3.4 volts available for each LED which let them light to maximum and the current went up to nearly 1.5 amps (too much for these LEDs, but we used SVR 2 to lower it to 1.35 amps). So the moral of this story is that Meanwell ENL 60-48D drivers will run 14 Cree LEDs, not 16. Another moral is not to let the apprentice engineer (me) do the math on the forward voltage. Pretty simple really,to get the maximum current (and light) from these drivers on these LEDs you need 56 volts and the drivers aren't capable of it. But... they all work now so we are happy campers and they put out a LOT of light.


Edited by Fatman - November 12 2013 at 12:14pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Krazie4Acans Quote  Post ReplyReply Direct Link To This Post Posted: November 12 2013 at 12:46pm
So here is the good news (besides the fact that they are working), I was hoping that my suggestion to Nikki to try adjusting SVR1 would be enough to over come the needed voltage but apparently her CREE's are running a bit about the 3 volts per LED advertised. I suspected that might be the case when I mentioned that the driver will limit the current if it is unable to supply the needed voltage for the load.

Here is the error in the design. Every LED has a small voltage drop across the junction that is not normally accounted for in the forward voltage spec for the LED. This is also the case for a regular diode (~.7v lost in the junction). While LEDs have a smaller loss there is still some and that is what was putting this design over the driver limits. The design was planned for average LED forward voltage, Maximum driver output voltage with no room for any additional losses. Easy mistake and something I suspected when I did the math from the OP.

Here is the strange part. According to the driver specs it should be able to put out enough voltage to drive 16 LEDs (if adjusted with SVR1). So that fact that you were not able to get them to regulate even after adjusting SVR1 tells me there is a bit more loss somewhere than expected.

According to the CREE spec sheet these LEDs will run at 1.5amps as a max. So that shouldn't really be a problem. However, the Meanwells are only rated at 1.2 amps continuous (yes I know they put out more, infact everyone I have ever used does) but running them about 1.2 amps can make them run warmer than usual and could lead to premature failure of the driver. I might turn them down a bit more if it were me.

Kevin don't worry, the spots in your eyes go away after about a month. :) Great work and glad some things were learned in the process.

Just for my information, which posts on the pot were you connecting the wires to? There are generally 3 legs and only two of them are important but which two is more important than using two. Good news here is that it won't matter once you connect to your apex. For future reference I always use and 9v battery on the dimmer wires as I know it will turn them on but doesn't add additional components or connections into my chances of having something hooked up wrong. :)


Edited by Krazie4Acans - November 12 2013 at 12:51pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatman Quote  Post ReplyReply Direct Link To This Post Posted: November 12 2013 at 12:53pm
There is so much light on these that we could move it to a parallel supply line with two sets broken off to be in series. However, I am so overjoyed in getting them running and learning how to times 3.5 volts by 14 and 16 that I don't think we want to mess with success.

The driver will easily handle 1.35 amps. Plus my son has about 5 or 6 spares here so we're covered.

Thanks for the help on this.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Krazie4Acans Quote  Post ReplyReply Direct Link To This Post Posted: November 12 2013 at 1:28pm
I guess that is just the Engineer in me coming out. I definitely not saying it won't work the way it is. So please don't get me wrong there. lol Engineers are notorious for making something work and then finding 10 ways of making it better, faster or cheaper. :)

Glad it's working and there are many more options than just parallel strings that could be made to work. I'll try not to spew them all here. Smile

Oh and I'm not saying that the drivers won't handle 1.35 amps just that if you read the label and the spec sheet they are only rated at 1.2 amps. So running them higher just adds a little risk in that they could fail quicker as they weren't designed to do that continuously for their entire life (chances are Nikki will be running them at less than 70% most of the time anyway).


Edited by Krazie4Acans - November 12 2013 at 1:30pm
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