Custom LED flood light - circuit design and advice.

[Cadmoon]

Greetings, humble sparky seeking some advice regarding LED circuit design.

Background on the project: I'm building a UV black light flood for my brother for his birthday, he's into his doofs so it's going to be individual LED's in a timber board with an octopus design routed into it. Looking at 50 3w LED's.

LED specs:
3w
365-370nm
Forward voltage of 3.3-3.6v
Forward current of 700mA

Proposed power supply:
Meanwell XLG-200-24-A ( https://docs.rs-online.com/cab6/A700000007465913.pdf )
24v - 200W - 8.3A

Given I'm planning on using 50 I believe my best option would be a series/parallel circuit design. An online calculator I used suggested several layouts but the one that looks the best fit for an easily available power supply is a 10 parallel legs of 5 LED's with a 10 ohm resistor on each leg. This gives a total required current of 7A.
Am I on the right track here? Is there any major drawbacks to a circuit like this, eg. early failures of chips?
Bonus question around cooling, would passive heatsinks on each chip be adequate cooling or should I look into some form of active cooling?

Thanks in advance.

[John B]

Many of the meanwell drivers, like variants in the HLG series, come with trim pots to adjust the output voltage and current. This would allow you to use integrated linear constant current ICs on each parallel string, rather than a resistor, and trim the output voltage to the lowest possible droupout voltage for the regulator. For example:

3.3v Vf x 7 = 23.1V, 7 strings giving 49 LEDs in total
3.6v Vf x 6 = 21.6V, 8 strings giving 48 LEDs in total

However, with this approach you might as well just go for a higher voltage PSU in the first place, ie a 48V or 54V driver, then reduce the number of parallel branches needed.

[james_s]

Using resistors to limit the current with high power LEDs is not really practical, you end up burning up a lot of power in the LEDs. You are better off IMO to use a constant current driver and wire the LEDs in series, or use several series strings each with their own constant current driver in order to get the voltage down to something reasonable/safe.

[Cadmoon]

Thanks for the advice guys. I would like to avoid resistors as well.
If I understand correctly I can instead use a HLG driver with adjustable current and voltage outputs and set each to the right amount to drive the LED's without any other V or I regulators?

So for example 5 legs of 10 LED's in series giving:
34.5v over each leg and 3.5A total current over the 5 legs
A Mean Well HLG-185H-42A ( https://www.meanwell.com/webapp/product/search.aspx?prod=HLG-185H ) set at 34.5V and 3.5A output.

Will that above setup eliminate the need for resistors as I understand it?
Thanks again.

[jonpaul]

most UV LEDs have very low output, not simple to measure

Simple solution...use old black light lamp, common in 1960s, 70s for d7scos.

Most are many a 15W long tube, with purple filters

Search 15wbl or black Light lamp.

Very powerful spotlight 250W Hg à sœur available from Spectroline in NY.

Jon

[themadhippy]

Simple solution...use old black light lamp, common in 1960s, 70s for d7scos.

Most are many a 15W long tube, with purple filters

To do the job properly  you really should be using a 400w uv cannon,

[james_s]

UV LEDs have improved tremendously. You can get domestic PAR type screw in LED blacklight lamps now that are pretty impressive. I think a few of them would compare favorably with one of those 400W mercury blacklights, I don't know what the efficiency is overall but they put out an insane amount of heat.

[Cadmoon]

Commercially available uv lights are fine in places where more uv light is needed. What I'm making however is just as much an art piece as a functional uv light.

[John B]

Thanks for the advice guys. I would like to avoid resistors as well.
If I understand correctly I can instead use a HLG driver with adjustable current and voltage outputs and set each to the right amount to drive the LED's without any other V or I regulators?

So for example 5 legs of 10 LED's in series giving:
34.5v over each leg and 3.5A total current over the 5 legs
A Mean Well HLG-185H-42A ( https://www.meanwell.com/webapp/product/search.aspx?prod=HLG-185H ) set at 34.5V and 3.5A output.

Will that above setup eliminate the need for resistors as I understand it?
Thanks again.

You can use constant current when driving multiple parallel branches, however you have to trust that each branch will have the same V and I characteristics, and also that the temperature will be closely matched. Ie the current will increase for a given voltage if the temps go up.

I have used this method for LED panels that are driven quite low on their conduction curve, but I would recommend using a current limiting device for each parallel branch normally. Even if that is a dumb resistor, that's going to be better than nothing.

[Zero999]

Greetings, humble sparky seeking some advice regarding LED circuit design.

Background on the project: I'm building a UV black light flood for my brother for his birthday, he's into his doofs so it's going to be individual LED's in a timber board with an octopus design routed into it. Looking at 50 3w LED's.

LED specs:
3w
365-370nm
Forward voltage of 3.3-3.6v
Forward current of 700mA

Proposed power supply:
Meanwell XLG-200-24-A ( https://docs.rs-online.com/cab6/A700000007465913.pdf )
24v - 200W - 8.3A

Given I'm planning on using 50 I believe my best option would be a series/parallel circuit design. An online calculator I used suggested several layouts but the one that looks the best fit for an easily available power supply is a 10 parallel legs of 5 LED's with a 10 ohm resistor on each leg. This gives a total required current of 7A.
Am I on the right track here? Is there any major drawbacks to a circuit like this, eg. early failures of chips?
Bonus question around cooling, would passive heatsinks on each chip be adequate cooling or should I look into some form of active cooling?

Thanks in advance.

That power supply will do constant current. Set it to 7A and use 1R resistors to help the LEDs current share.