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  1. #41
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    Re: Driving 140x 3W LEDs

    Without seeing the datasheet for the UV LEDs, I wonder if the 700mA is the absolute maximum allowed current if they have perfect cooling? How will you cool them? With a spray of Liquid Nitrogen? Then the thermal extremes will kill them soon.

    I agree that the You Tube video is completely wrong. The hFE of a transistor that you buy can be 50 or 300 or anything in between and it changes with temperature and current changes. The circuit would probably burn out at least one of the series LEDs then they all would not produce any light.



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    Re: Driving 140x 3W LEDs

    Quote Originally Posted by Audioguru View Post
    Without seeing the datasheet for the UV LEDs, I wonder if the 700mA is the absolute maximum allowed current if they have perfect cooling? How will you cool them? With a spray of Liquid Nitrogen? Then the thermal extremes will kill them soon.
    Yes I get 'excited' too when I think about cooling but really my plan is based on another similar project of someone who already did it using 1W LEds. In 'his' project he used 12 LEDs for a smaller size PCB (unlike mine which is A4) and for the full exposure to occur it took around 30 seconds. Based on this I am assuming I will need less, probably MUCH less then 30 seconds so heat-wise I am not concerned. However I will be doing the following:

    I bought star shaped aluminum PCBs which you can solder the LED onto, and then you can solder wires to the aluminum in specific places where solder is able to stick. I will have long strips of aluminum heatsink which I will mount the star shaped boards using screws. In between the led and the aluminum star I will have thermal paste, and again I will have thermal paste between the star and the large aluminum heatsink. Considering a maximum ON time of 30 seconds I think this is more then enough.

    Alternatively I can do without the screws and instead I can use a 2-part thermal paste which is specifically for the purpose. It hardens and sticks the materials together.



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  3. #43
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    Re: Driving 140x 3W LEDs

    I bought star shaped aluminum PCBs which you can solder the LED onto...
    I believe the LEDs have already some form of heat sink- there is a central pad on the reverse which is expected to be soldered to a board (in this case the PCB). With 70 LEDs dissipating 3W each (200W) on a A4 size PCB, you can just get away with a regular fan. You should distribute the LEDs for maximum uniformity as a short distance (more of less equally spaced or use a hexagonal arrangement). On the reverse side, keep a large pad to dissipate the heat.

    I doubt you will be able to solder to Al plate (perhaps they were anodized and that will make soldering impossible). But you will be able to glue them surely.

    But using the mounted LED will not solve your problem unless you mount them directly on a heatsink.

    If you use 1W LEDs, you can get away using the same number and the light will be about 1/2 but the heat produced too will be reduced. The LEDs will run cooler and at higher efficiency. And you can run them easily ar 300mA without problem.



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    Re: Driving 140x 3W LEDs

    Ok so we are talking 20 of these circuits should do the trick. Assuming the 30VDC are clean, I should have 1.25v across R1, I should have any voltage between 3.2 and 3.6 on the leds and a constant current of slightly less than 700mA as set by the 1.8ohm. Any additional voltage left 'unused' will be dissipated by the LM317, ie: 30v from PSU - (7 leds x 3.2v lower range) = 7.6V - whatever the LM317 uses but lets not take that into consideration for the purposes of leaving some headspace. So on the LM317 there will be a maximum of 7.6v x .7A = 5.32W while on R1 there will be 1.25v x .7 = 0.9W
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    As for the power supply, 20 circuits @ 700mA = 14A so 30V /16A should be enough
    Last edited by Xenobius; 11th September 2019 at 20:30.



  5. #45
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    Re: Driving 140x 3W LEDs

    I assume that 700mA is the maximum allowed current for your LEDs.
    Then what about the range of reference voltage of the LM317 (1.3V max) and the tolerance of the 1.8 ohm resistor (minus 5%= 1.71 ohms)?
    Then the current in the LEDs will be 760mA.
    You are planning too close to the limits.



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    Re: Driving 140x 3W LEDs

    Quote Originally Posted by Audioguru View Post
    I assume that 700mA is the maximum allowed current for your LEDs.
    Then what about the range of reference voltage of the LM317 (1.3V max) and the tolerance of the 1.8 ohm resistor (minus 5%= 1.71 ohms)?
    Then the current in the LEDs will be 760mA.
    You are planning too close to the limits.
    Yes 700mA is the max of the LED so I could do a 2R and that will take the current down to 0.625A with a 5% tolerance the range will be from 0.595A up to 0.657A - so thanks for the tip



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