Author Topic: Quick COB question  (Read 366 times)

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Offline DCooper

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Quick COB question
« on: April 20, 2021, 08:50:09 pm »
Hello! I have just completed a single COB light build and unfortunately I ended up damaging part of the COB. When mounting it into a fixture my screwdriver slipped and touched the COB, and now there is a dark line across the COB which will not light up. The line is approximately 1/8 of the COB. So to my question :

Before the damage was done, the COB drew about 50V and 1 amp. Now it draws 50V and .8 amp. So should I turn down the voltage or the current to compensate for the leds that are no longer part of the circuit? So I dont overdrive the rest of them? Or how does this work? I have a Meanwell HLG psu where i can adjust both voltage and current.

The COB in question : Luminus CXM-22-35
The PSU : HLG-60H-54A
 

Offline thm_w

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Re: Quick COB question
« Reply #1 on: April 20, 2021, 10:25:44 pm »
The datasheet max current is actually 3.5A, but what you can safely set it to depends on your cooling capability.
If you have good cooling, there is no need to adjust anything. Do you have a thermocouple or IR temperature meter you can monitor it with?

If you wanted to be safe and run the LEDs at the same individual current as prior, then you'd adjust current limit down to 1A * 7/8 = 0.88A
PSU can be set to max (54V).
 

Offline DCooper

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Re: Quick COB question
« Reply #2 on: April 21, 2021, 06:29:36 am »
Thanks for answering!
I have ordered a thermocouple but not received it yet. What would be a safe temperature range do you think? The cooling is passive, but has quite a lot of mass.

So if I understand this correctly; even if theoretically only half the cob worked, I should still keep the voltage the same (at manufacturers recommended spec) and only adjust current?
 

Online james_s

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Re: Quick COB question
« Reply #3 on: April 21, 2021, 07:25:19 am »
LEDs are current driven, you don't set the voltage, you set the current. Driven at the correct current, the voltage automatically goes to whatever is needed to maintain the set current, within the compliance range of the power supply.
 

Offline thm_w

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Re: Quick COB question
« Reply #4 on: April 21, 2021, 09:11:09 pm »
Thanks for answering!
I have ordered a thermocouple but not received it yet. What would be a safe temperature range do you think? The cooling is passive, but has quite a lot of mass.

So if I understand this correctly; even if theoretically only half the cob worked, I should still keep the voltage the same (at manufacturers recommended spec) and only adjust current?

Recommended operating Tj is 85C.
Thermal resistance is ~0.37 C/W. So a case temp of around 70C or less would be ideal at 50W.

They have some heatsinks here that might give an idea: https://www.led-heatsink.com/content/files/PDFs/productPdfs/MTX/Overview/Luminus_LED_cooler_overview.pdf
 

Offline DCooper

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Re: Quick COB question
« Reply #5 on: April 22, 2021, 08:21:05 pm »
Thanks for all the help!
 


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