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I am trying to design small DIY grow light for Chilli Peppers. I found ideal to use a combination of Samsung LM301H (https://www.samsung.com/led/lighting/mid-power-leds/3030-leds/lm301h/) and LM351H V2 (https://www.samsung.com/led/lighting/high-power-leds/3535-leds/lh351h-deep-red/). Now I am not sure what is the best series-parallel combination of these two types of LEDs together. I want to use a Constant Current source. For LM301H(Uf=2,75) I want to use 65 mA and 350 mA for LM351H(Uf=2.2). Is it a great idea to put 6 LM301H together in parallel, series to 1 LM351H? Then I will have a block that will have a voltage drop of 5V and driven with a constant current of 350 mA. Is this correct design, or am I missing something?

enter image description here Thanks! :)

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3 Answers 3

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I would be concerned with manufacturing variances in the devices causing 1 or 2 of the 6 LEDs to flow more current than the others, heating up more, flowing more current still, and creating a vicious cycle.

That could be at least contained by putting a resistor in series with each LED.

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  • \$\begingroup\$ Adding a resistor seems mad for me. Samsung sorts this type of LEDs by Luminous Flux with constant amperage to some groups. So I suppose the difference between individual LEDs from the same group or reel should be that high. But maybe I am wrong. If I take a look onto some professional boards with these LEDs directly from Samsung I don't see any current limiting resistor (samsung.com/led/lighting/led-modules/industrial-light-module/…). How they design work then? Thanks :) \$\endgroup\$
    – Vojta Ermis
    Commented May 6, 2020 at 6:04
  • \$\begingroup\$ @VojtaErmis Are we looking at the same thing? Look at the last page. I see a repeated pattern, 6 LEDs and a little dot. What's the dot? A resistor. Specs say 21.5V nominal, sounds to me like 6 LEDs in series with a resistor. Do you grasp series and parallel? Do you understand LEDs are constant current and that means you series-wire them where able? \$\endgroup\$
    – Harper - Reinstate Monica
    Commented May 6, 2020 at 6:20
  • \$\begingroup\$ You mean this imgur.com/a/Sy5eH1T? Its LM351H I think. \$\endgroup\$
    – Vojta Ermis
    Commented May 6, 2020 at 6:52
  • \$\begingroup\$ Meh, the 21.5V doesn't make sense then. Maybe it's 7 LEDs in series, but still we're talking series-parallel ladders, not what you plan. With 7-series strings it'll tend to even out manufacturing irregularities. Unless you think it's exactly the ladder you propose up top, four of them in series to get 21.5V?? There are 8 groups per unit. \$\endgroup\$
    – Harper - Reinstate Monica
    Commented May 6, 2020 at 7:13
  • \$\begingroup\$ So connection like this is OK? I suppose that's is exactly what Samsung uses. Then is 700 mA (not 1.4A I made mistake on drawing) and around 21,5V nominalhttps://imgur.com/a/5XDdrPo \$\endgroup\$
    – Vojta Ermis
    Commented May 6, 2020 at 7:41
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I found ideal to use a combination of Samsung LM301H(https://www.samsung.com/led/lighting/mid-power-leds/3030-leds/lm301h/) and LM351H V2 (https://www.samsung.com/led/lighting/high-power-leds/3535-leds/lh351h-deep-red/).

The efficiency of the red diodes is no better than the whites. You could get similar results just using white LEDs of the color temperature you want, at least assuming those exist.

Is it a great idea to put 6 LM301H together in parallel, series to 1 LM351H?

You'd have to look at the spec sheet to get an idea of the variation in forward voltage, but in general, no I wouldn't prefer that. Instead I would try to have a few in series so that the variation in individual units averages out and the current is more uniform. Plus the voltage is probably too low to make sense for something like a growlight where you want many watts of power.

Typical grow lights put 10-30 lights in series, and then build parallel strings from that. This way the current is more reasonable. Unless you have a very small grow light, usually you want to be working at somewhat higher voltages to reduce wiring losses.

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  • \$\begingroup\$ It's hard to get quality "white" LEDs with this spectrum. I meant I can then stack these blocks together, so like 4 blocks in series and 4 in parallel. Then I would have around 20 V with 1400 mA totally, this seems like a good light source. Isn't it? \$\endgroup\$
    – Vojta Ermis
    Commented May 6, 2020 at 5:57
  • \$\begingroup\$ You're risking damaging the diodes if you get an odd white diode with a low forward voltage, so I wouldn't do it. Put a couple more white diodes in series behind each red and the problem goes away. \$\endgroup\$
    – user1850479
    Commented May 6, 2020 at 15:08
  • \$\begingroup\$ @Vojta Emris Why is that spectrum important? The reason I said color temperature and not spectrum is that the ratio of red to blue is often important, but the exact spectrum is not. Alternatively the efficiency on the red diodes you have picked is poor. Buying better diodes would save you some electric bill. \$\endgroup\$
    – user1850479
    Commented May 6, 2020 at 15:11
  • \$\begingroup\$ I already designed an array like this imgur.com/a/5XDdrPo with suggestions bellow. Is it better in your opinion? Btw. current is not 1.4 A but 0.7 A on the image. \$\endgroup\$
    – Vojta Ermis
    Commented May 6, 2020 at 15:45
  • \$\begingroup\$ @Vojta Ermis I think it is a bad design for the reasons outlined above. \$\endgroup\$
    – user1850479
    Commented May 6, 2020 at 16:25
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This looks like a reasonable shared current balance for up to 350 mA if you have adequate heat transfer in the design for each part with shared temperature for each parallel LED ( most critical without a series R equivalent for each White LED equal to its ESR of 2 Ohms. ) However, this is a fixed design of < 2 watts.

Always start from the system specs design for radiated power at each wavelength in PPF or PE then design according to the range of system application requirements rather than a potential array and see what it can do or if it works.

Also learn about thermal runaway and thermal balancing and ESR of each part = incremental R=V/I and tolerances. Samsung will have lower tolerances than no-name brands.

https://www.samsung.com/led/about-us/news-events/news/news-detail-49/

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  • \$\begingroup\$ Samsung uses really tight sorting. By thermal balancing, you mean putting LEDs with characteristics into the same series connection to minimize non-constant dividing of current? I was thinking that If manufacturers sort LEDs into groups by their parameters I don't need to worry then about this. PPF and PE I am not sure what is ideal for my plants, so It's more experimental. For good heat transfer, I am planning to use aluminum core PCBs, not FR-4 with copper. Single-sided with large cooling pads. The PCB will be mounted to a passive Al radiator with thermal grease. \$\endgroup\$
    – Vojta Ermis
    Commented May 6, 2020 at 6:25
  • \$\begingroup\$ Since Vf change affects current sharing , not only nearly matched at room temp but all temps. If in doubt, verify the parts on each batch or added 1 Ohm. \$\endgroup\$
    – D.A.S.
    Commented May 6, 2020 at 6:28

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