Help needed with LED driver :)

[AndersRasmussen]

Hello.
I really need help, as I have to hand in my semester project on friday, which contains a prototype. The prototype is a lamp made for seeing embryos in eggs, and therefore I am working with high power LED's (CXA1304-0000-000C00A40E7 ), 9V ,1A. I am an production engineer , so this is not my normal field of expertise. I just can not seem to figure a soulution to the following problem:
I have made a PCB (see picture 1 and 2).
The PCB consist of 6 LED's wired in parallel. Original intention was getting an LED driver, with a dimmer. Unfortunately it got lost in the mail, and I have no time to order a new one. Therefore I kinda fixed it, with stuff I had at home(see wiring diagram, picture 3(and sorry for the very bad schematics, hope that you get what is going on)). The schematics of the PCB can be seen in picture 4.

So my problem is that there is a significant differens in temprature and lux between the LED's placed furthest away from each other. Can I do something to fix this, or shall i just get used to it? I have a feeling that the amps might be playing tricks on me, but i do not have a ampmeter...
The lamps works, and is sufficient for what I am using it for, but I like working with electronics, and therefore would really be happy to learn what is going on.

Any advice or tipsto a noobie who has a deadline in a few days? xD
Thx, and nice day to your all!

Picture 1 Picture 2

Picture 3

Picture 4

 

[Diver300]

LEDs are supposed to be driven with a fixed current. The voltage can vary a bit between LEDs. Also the current change a lot for a small change of voltage, so the ones at the end away from the power input will have a slightly lower voltage and possibly a lot less current.

It won't make much difference what circuit drives the strips of LEDs.

You could try the connecting one of the power leads to the other end of the strip. The alternative is to add some resistance in series with each LED. I don't know how you would arrange that.

 

[AndersRasmussen]

Thx for the answer.
Those thoughts also crossed my mind, I was just afraid it would be a "ghetto" fix?
I didnt try it out yet, first thing i'll try tomorrow.

Maybe I could mill off some of the protective layer of the PCB boards in between the LED's, then drill holes for mounting a resistor in serie with the LED?
Would that be a viable solution or just a quick fix?
Once again, thx.

 

[Diver300]

With a 12 V supply, 9 V LEDs and 100 Ohms, the current will only be about 30 mA total. That is about 5 mA per LED, so only about 0.5% of the maximum current. The graphs that Cree publish only go down to 250 mA. That doesn't mean that the LEDs won't work at 5 mA, it just means that there may be more difference between adjacent ones that you might expect.

 

[Diver300]

As a quick fix you could try this:-

You need to cut the track either side of each LED. You need to scrape off the solder resist layer that is on the copper, in three places for each LED. Then solder a resistor, shown in green. Then link the tracks up with wire, shown in blue.

You should not have your 100 Ohm resistor at all, and each of the resistors that you add in should be around 600 Ohm, but the exact value isn't important.

That will keep all of the LEDs having the same current.

I'm not sure if 5 mA is enough. Are you sure that it's 100 Ohms at the moment?

 

[AndersRasmussen]

That's very helpful. I'll try that quick fix
Yes, found and old post with a smiliar issue suggesting I used 100 Ohms ressitor, and must admit in my desperation I was hoping for a miracle to happen.

I'll keep you updated on how it worked out.

 

[Diver300]

If you haven't got the 100 Ohm resistor at the moment, you will need a MUCH smaller resistor value than 600 Ohms. The Cree LEDs have a resistance of around 2 Ohms. If you are putting 1 A though each one, with a 12 V supply, you can only go up to about 2 Ohms for the resistor, which won't make a lot of difference.

It would really help to get a good estimate of the current. Even a reasonably accurate measurement of the voltage across the LED when running would help

 

[Diver300]

Also your driver circuit isn't correct. The wiper of the potentiometer should be connected to the MOSFET gate. The way you have shown it would make the potentiometer burn out at one end of its travel. Also you will lose some voltage in turning on the MOSFET, because the source voltage will always be around 1.2 V lower than the gate, if I've understood your diagram correctly.

 

[AndersRasmussen]

The wiper is connected to the gate, lucky for me I am better at reading schematics than making them . And yes it's dropping some voltage, but it was the most suitable MOSFET I had around, do you think it is giving me issues?
Now when I am going to cut open the copper lanes I can measure the current, so hopefully everything will make more sense to me.

 

[Diver300]

If you use the MOSFET to switch the negative wire not the positive, there won't be any significant voltage drop. However, with that MOSFET, you won't get good control. It will be just on or off, as it is a MOSFET that can deliver a huge current.