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I am a high schooler with an interest in growing special plants.

I made this array of LEDs with each diode using 700ma and a voltage drop of 3.55V across each of them. The array has 30 of them in a 3x10 configuration powered by my variable lab bench power supply at 10.6V 7A and has no resistors whatsoever to control the current flow as I just vary the voltage supply to change the current.

Is it possible to power this array using a 12V 5A power supply from ebay without the need to add resistors to each series against voltage spikes? Also, because the power supply will be at 100% utilization if it works, it will be submerged in oil to help cooling.

If the ebay power supply wouldn't work what would you suggest I do?

Array with bulky cooling and 3D printed shroud:

enter image description here

LED array powered with low voltage:

enter image description here

The array will be wired something like this - but without the resistors:

enter image description here

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  • \$\begingroup\$ Interesting moniker! It's not entirely clear from your writing... but are you hoping to just place all the LEDs in parallel with each other? It reads like that's what you are doing, but I want to be sure. \$\endgroup\$
    – jonk
    Aug 6, 2021 at 10:13
  • \$\begingroup\$ Sorry if i wasnt that clear but yeah on the image my array looks like that as a schematic but without the resistors imgur.com/8zOTn1H . Currently the growlight works but only with the variable power supply at 10.6V. I hope using the ebay 12v 5a power supply without the need of adding resistors like on the schematic. Tnx in advance \$\endgroup\$ Aug 6, 2021 at 10:31
  • \$\begingroup\$ It won't work directly (without resistors or a driver) from 12V. You have to limit current across them. \$\endgroup\$
    – NStorm
    Aug 6, 2021 at 10:36
  • \$\begingroup\$ @DymethylTryptamine LEDs are quite different, one to another, and some can hog current. Placing more in series helps because of how statistics works. But even that isn't a sure thing. It would be best if you used individual current control for each series string. But if you are willing to test each string, mixing and matching as you go, then you might reasonably get all the strings balanced enough. That still won't mean they use a standard nameplate voltage when all is said and done. \$\endgroup\$
    – jonk
    Aug 6, 2021 at 10:39
  • \$\begingroup\$ @NStorm but by using a power supply that can only give 5A of current isn't going to limit by itself ? \$\endgroup\$ Aug 6, 2021 at 10:50

2 Answers 2

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Simply put, no, this specific power supply will not work. Your array requires (10x0.700A) x (3x3.55V) = 7A x 10.65V = 74.55 Watts of power. This 12V 5A is only capable of delivering 60 Watts, regardless of not being able to deliver the 7A current that you require. You could put some DC-DC regulator in between, if for some reason you really don't want a resistor (for example, for power efficiency reasons).

Best suggestion I can give you (or at least the most cost effective or simple, I suppose), is to get a 12V power supply capable of delivering at least 7A (more current is okay as well, less isn't), and do put in the resistor. This resistor then needs to 'bridge' from 12V to 10.65V, so across this resistor would then be a voltage of 1.35V. Given the nominal current of 7A, this means a resistor of roughly 0.2 ohm. This resistor will then burn about 10W of power, so you need to make sure that resistor is capable of handling that.

Other alternatives could be some linear voltage or current regulator, but they will still burn away the excess in the form of heat.

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    \$\begingroup\$ It's not a good idea to put a single resistor in series with parallel LEDs. They don't share exactly the same characteristics. As the schematics posted in question one should put 1 resistor across each row of LEDs in series, i.e. 10 resistors for each of 3 LEDs in this particular example. \$\endgroup\$
    – NStorm
    Aug 6, 2021 at 11:37
  • \$\begingroup\$ Yeah, I thought about while compiling my answer. But I thought the answer would become unnecessarily complicated or confusing by including all kinds of variations on where you could place resistors. However, after I posted the this answer, the schematics were uploaded. Anyway, when implementing the given schematic with a 12V supply, the resistors should be ~2 ohm each, each capable of handling 1W of power (1.35V/0.7A=1.93 ohm). \$\endgroup\$
    – Robert
    Aug 6, 2021 at 11:45
  • \$\begingroup\$ OP wants to pick 5A max supply, so he needs to aim for a little lower current. I've already offered to pick 3 Ohm / 1W x 10 pcs resistors to get a 450 mA / 4.5 A of current in the comments above. \$\endgroup\$
    – NStorm
    Aug 6, 2021 at 11:49
  • \$\begingroup\$ In that case, if the 5A max supply is a necessity somehow, I would only connect 7 of the 10 rows and still go for the 2 ohm ones, instead of under-supplying the LEDs. In either case you get less light output, but when LEDs are supplied by a too low voltage their color or output spectrum may change as well, which seems relevant in the given application. Also they might be less efficient than nominal (if so, you get even less light, relative to the connecting 7/10 case). \$\endgroup\$
    – Robert
    Aug 6, 2021 at 12:18
  • \$\begingroup\$ My suggestion wasn't about the voltage, but the current. Of course we don't know which exact part OP used to check it's parameters. But I assume OP means that 0.7 is the maximum rating and driving these with 0.45A will only make it better in terms of lifetime due to lower heat. More than a half of maximum rated current should be just fine for almost every LED. Anyways this is the just the guess without actual datasheet. \$\endgroup\$
    – NStorm
    Aug 6, 2021 at 12:45
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Most grow lights do work like in your diagram with no resistors. However, they require a constant current power supply. Constant voltage will simply blow up the LEDs since nothing is limiting current.

By the way, your general idea is good. You should not use resistors with high power LEDs. Constant current driving is more efficient and much more practical at higher current. You just need to buy the right type of power supply.

One final note: you can buy high quality LED arrays wired for constant current driving from many vendors under brands such as "Quantum Board" or similar. These will be more cost effective and probably better designed then trying to build the same thing in small volume.

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