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Thread: Help calculating correct power supply?

  1. #1

    Question Help calculating correct power supply?

    Hi, hope someone here can help. I got this cool looking 84-LED array at a garage sale for $5 and thought I could use it for a photo light or grow light. No power supply came with it, and the power supply connector is a non-standard size, which leads me to believe the voltage may be non-standard. I cracked open the case and it has 5 banks of 15 LEDs + 1 bank of 9 LEDs connected parallel to each other. In each of the 5 banks there is an array of 5 nodes (3 nodes for the bank of 9) also connected in parallel, each with 3 LEDs connected in series with a 169 Ohm resistor. There is also a small 12v fan connected to a 149Ohm resistor. The PCB is stamped 94v -- 089 LED.

    I tested with a 12v power supply, and it gives good light and virtually no heat. I tested with a 24v power supply and it gives great light but gets too hot to hold and gives off slight fumes.

    I'm thinking if it was designed for 12v and no heat, why encase in aluminum and install a fan? Must be somewhere in between, right? Can anyone help me with the math needed to reverse engineer this design for optimum light output and longevity? Or should I just go with the 94v? Obviously trying to solve for voltage and current rating. Photos below.

    Thanks in advance!

  2. #2
    Join Date
    Oct 2008

    Default Re: Help calculating correct power supply?

    Those look like typical 20mA white 5mm leds. So if they are all connected in groups of 3 leds in series with a 169 ohm resistor then all paralleled together, using ohm's law I get a supply voltage of about 13.5 volts "about average for a cars voltage when the engine is running. I say stick to the 12 volt power supply you tried first. As far as power goes, 28 sets of 3 in series leds at 20mA each gives 560mA total current draw for the whole LED circuit, does not include the fan through. I would think a 12VDC 1 amp wall wort would power the whole unit just fine....

  3. #3
    Join Date
    Jul 2011
    West London, UK

    Default Re: Help calculating correct power supply?

    Based on what you've said, and the fact that the 12v fan is connected to the supply rails, I'd only be running at 12v per board. Whether the board are run in parallel or series shouldn't matter, but isn't clear from your description. However, parallel is more ideal imo. The lack of great but with a fan could just be a Chinese design quirk, or a cautious designer!

  4. #4

    Default Re: Help calculating correct power supply?

    Thanks for looking at this for me!! I appreciate your advice.

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