Designing and Building a LED Fixture

[Hoppy]

Thank you!

I will be away most of the day, so I can't work on this before tomorrow. While checking out my driver circuit I also found I messed up the current setting resistors. If this had worked it would have burned out the LEDs with twice the current they will take! That is called a "silver lining", I guess. I need to correct that before doing any more testing.

[giga]

Carefull you don't get a short with those screws hitting the + and - pads with your screws

[Hoppy]

Originally Posted by giga

Carefull you don't get a short with those screws hitting the + and - pads with your screws

I got the idea for using flathead screws from Kev at http://www.plantedtank.net/forums/di...tml#post822998 The back of the screw is tapered to fit in a countersunk hole, so only the edges of the taper contact the "star". Not even close to shorting them out.

Saturday I spent the day watching my grandchildren play baseball, out in the hot sun for 4 hours, so between that and the allergy problems from the dust, I haven't been back on this project since then. Probably tomorrow, or Wednesday at the latest, I will get back to it.

The fix for my circuit resistor goof isn't too hard, but I need to remove the circuit board to get to it to do it. And, I will add the ground where it belongs, at the same time. Hopefully this will make it work.

[kev82]

I don't really know what could be wrong, but my tip would be to start small. Just begin with powering a couple of LEDs and work your way up to the full array.

I noticed you stripped a fair bit more than needed off the wire in the pic giga posted - make sure the exposed wire doesn't touch the heatsink anywhere.

Good luck

[Hoppy]

I took the fixture apart, removed the circuit board, and soon discovered another mistake, the one that caused the dim light in the first place. The circuit I'm using to control the current to the LEDs relies on the voltage drop across a low resistance resistor to set the current. I'm using 5 watt, both leads at one end, resistors for this, .75 ohm each. They mount in the circuit board side by side. But, I left a Murphy's Law problem when I laid out the circuit board. It is possible to install the resistors at right angle to how they should be installed, so of course that's what I did. And, that left them shorted out, thus providing nearly zero resistance to the current, and near zero voltage drop to set the circuit current. So, I had near zero current flowing to the LEDs. This could be called a "eye dee ten tee"* mistake.

That's corrected, the other two mistakes corrected, and I'm ready to reassemble the fixture. I expect to see the light before tomorrow is over!

"eye dee ten tee" = ID10T

[evilc66]

It's not the arrow, it's the Indian Don't worry, it happens. I expect to see action shots though.

[Wasserpest]

The suspense becomes unbearable. National Guard on standby.

[Hoppy]

Originally Posted by Wasserpest

The suspense becomes unbearable. National Guard on standby.

I will turn off all the smoke alarms first

[Hoppy]

WARNING: Wear welding goggles to view the following photo!

I got it reassembled this morning and plugged it in. First only 4 LEDs in one string lit up. So, I quickly pulled the plug and rechecked every LED for a ground to the heatsink. No problems found. After lunch I plugged it in again, and all of the LEDs lit up, instantly ionizing all of the air in the garage! OK, maybe it didn't, but WOW, that is some bright light! My guess is it will be way more light than I want, probably more like 200+ micromols than the 50-100 I want. I used my power change switch to try to dim it a bit, but I couldn't see a difference. It looks as painful to see as a HQI light does. After just a few minutes I hear a faint pop and 3 LEDs quit working. That is 3 adjacent ones in a string, but all of the rest continue to work. I'm trying to understand what could cause that.

Anyway, if you all have the welding goggles on, here is the pic I just took:
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If you find yourself blinded, see my lawyer!!

[kev82]

Now we're getting somewhere If the LEDs burned out they were probably fed too much current. Are you sure the drivers are feeding the correct current?

[CL]

I knew that that would be incredibly bright!
Get a couple of actinic t5s and set up a reef lol

[redman88]

hmmmm, my next project guess i will take that basic electronics class in the fall and build a light bar for my 29 gallon

[Hoppy]

Originally Posted by kev82

Now we're getting somewhere If the LEDs burned out they were probably fed too much current. Are you sure the drivers are feeding the correct current?

I'm not sure the drivers are feeding the correct current, but they should be, and both parallel strings should have the same current, about 670mA each, or half of that if the switch is in the other position. My multimeter doesn't measure current more than 250mA, so I can't use it for this. At one time I was going to install a little panel mount ammeter, and now I wished I had done so. I may look into doing that now.

If a LED in a string of LEDs wired in series burns out, do the remaining LEDs remain lit? I would expect a burnout to open that circuit. This seems more like a short across LEDs. But, I can't see how that happens if they are all isolated from the heatsink. One effect of a short circuit across 3 LEDs should be overheating and possible failure of a transistor in the driver for that part of the circuit. (Because the FET transistor would have to dissipate more of the 48 volts with 3 missing LED voltage drops.) Unfortunately I can't use the multimeter to measure the voltage drop across each string, with the lights on - it is too blindingly bright to even see the connections.

[kev82]

Yeah I think the remaining would stay lit - but they would probably be given the extra current/voltage.

This is pretty much the problem with parallel strings - if a LED dies in either string, the rest of them need to take on the extra electricity. Unless you're using a current mirror.

I'm not an electrician so I'm really bad at explaining this stuff, but basically you want to avoid parallel strings and aim for series if you can. Or go parallel and have a current mirror installed.

[evilc66]

Hoppy, repost your circuit and source voltages so we can check it out.