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I am planning to build a high luminosity lamp to act as a growth lamp during winter for my plants.

For this, I manage to recycle (upcycle?) a big LED bulb with the following inscription:

LED Corn Light E40
Volt: AC 35-265V
CC1: 6000K
Power: 54W

The bulb was not matching my needs as is (especially since it was providing an almost 360 degree light, while I need a directional light source), so I took it apart.

I saved 8 nice aluminium strips with 19 SMD LEDs each. I plan to power these from an ATX power supply, so I have a 12V 15A (probably I should say something like 10-12A to be safe) source.

The problem is that I can't identify these LEDs (I couldn't find a datasheet), so I'm not sure how much power they need. Based on similar LEDs I've seen around, I think 3V and 250 mA per LED would be a good guesstimate. Which means I should form groups of 4 LEDs in series and add a 3/0.25 = 12 ohm resistor per pair, everything nicely stuck to a solid piece of aluminum for cooling.

Are my calculations correct?

Side question: I was measuring the LEDs with a diode meter and it seems they very dimly light up. Also, when touching them in certain conditions, they produce a very dim light (the LEDs being disconnected from any circuitry). Is there any explanation for this?

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  • \$\begingroup\$ "I was measuring the LEDs with a diode meter and it seems they very dimly light up" That's normal... The meter needs to use voltage and current to determine a resistance so it is partially powering the LED. \$\endgroup\$ – Ron Beyer Nov 8 '19 at 16:32
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If the whole lamp is rated at 54W, and it contains 8 strips of 19 = 152 LEDs, then the power per LED it's been designed for is 54/152 = 355mW before losses.

If you run each LED at 3V 250mA, that's 750mW, more than twice what is was designed for.

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    \$\begingroup\$ 100 mA is probably closer to the correct value. \$\endgroup\$ – Dave Tweed Nov 8 '19 at 17:03
  • \$\begingroup\$ In the absence of knowledge of the original device's rating, how would you recommend going about determining a reasonable drive current? (I'm just curious, but I think this would potentially improve the answer as well.) Would there be any method better than "slowly turn up the current until they look 'bright enough'"? \$\endgroup\$ – Hearth Nov 8 '19 at 18:14

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