Samsung LM561C Build: Testing PPFD

Hi everyone.

As I’m getting my shit together for this build, I’ve been testing a few different things along the way. This weekend, I assembled my strips into 2 lights that would be similar to the Horticulture Lighting Group’s QB288 boards. Each of the lights is comprised of 12 strips with each strip containing 24 diodes, for a total of 288 diodes per light. For now, I just taped all the strips together, then fastened each “light”  to a piece of wood to be able to hang them easier.

One light. 12x strips wired together in a combination of series and parallel.

Both lights attached to a chunk of wood I had lying around.

If you look closely at the wiring on the right, you can see that each light has 3 sets of 4 strips wired in parallel. These 3 sets are then connected in series. When I tested both lights together, I also wired the 2 lights together in series.

Wiring the Lights

The only suitable driver I had on hand for this test was my HLG-185H-C1400B, so I had to do some weeeird parallel/series wiring in order to get the current to each diode right. Here’s how I figured out how to wire it:

  • Each individual strip has 24 diodes. On each strip, the 24 diodes are internally wired in 3 groups of 8 diodes. Each group of 8 diodes is wired in series, then the 3 groups of 8 are wired in parallel. This means that whatever current I put through each strip will be divided by 3, and the result of this will flow through every diode that’s wired in series.
    • E.g. – If we apply 600mA to a single strip, this will be split among the 3 parallel groups and will result in 200mA per group of 8 diodes. Since diodes among each group are wired in series, every single one of them will receive this 200mA.
  • The strips have a sticker on them that gives their forward voltage at max current. At 600mA, which is the max for each strip, you can expect to see 24.2V. I didn’t want to give them max power (nor do I have a driver to do it), so I decided I would shoot for about half of max power (~300mA) to each strip. Since my driver puts out 1,400mA, it’s now just a matter of figuring out how to split up the strips to get the right current through each.
    • 1,400mA/300mA = 4.6 strips. I decided to go with groups of 4 strips since it worked out nicely, having 12 strips per light. This means I will end up with 3 groups of 4 strips per light. If I wire these 4 strips together in parallel, they will split the 1,400mA and get 350mA each. This translates to 117mA per diode (350mA divided by 3)
    • Now, I just need to wire these groups of 4 strips in series. If I wire them in series, every group I add in series will receive the same 1,400mA, and total circuit voltage will continue to add, which is good. Since the 4 strips in each group are wired together in parallel, the voltage for each group will remain the same. At 350mA per strip, I was seeing a voltage of about 23.3V. Since I have 3 groups of 4 strips wired in series, I end up with 23.3V + 23.3V + 23.3V = 69.9V per light. When the 2 lights are combined, I end up with 139.8V. This is perfect, as my driver can do constant current up to 143V.

3 Groups of 4 strips per light.

Measuring PPFD

To get consistent measurements, I needed to clearly mark the area I was taking readings. I measured out some common grow tent sizes and taped them out on the floor. I ended up with a 1×1, 2×2, 3×3, and 4×4 area.

Next step was to suspend my “light bar” to the height I needed. I just used some picture hanging wire and wrapped it around a joist above me.

Leveling the light to ensure even coverage.

Ready to go.

Test Conditions

  • I measured output from a single 288-diode light first, then centered both lights over the measurement area and took readings with both lights powered up.
  • I took readings at 2 different distances from the light to the sensor: 18″, and 12″.
  • This was in an open environment – there is no reflective material around the light or measurement area.
  • After heating up, voltage for the single light settled around 69.2V. Voltage for both lights together settled around 138.5V.
  • Current was measured at 1,380mA.
  • Total power for the single light was 95.5W. Total power for both lights was 191.1W
  • I focused mainly on taking readings within a 2×2 area, as PPFD dropped off to unusable levels outside of this range. Having reflective walls would improve these numbers.

Single Light (288 Diodes) Results

18″ Above Sensor

12″ Above Sensor

Double Light (576 Diodes) Results

18″ Above Sensor

12″ Above Sensor



I think these lights have a ton of potential. Increasing power a little and adding reflective material that you’d typically have if you’re growing in a tent will undoubtedly boost these numbers considerably. Just by increasing power from 117mA per diode to 150mA per diode, the Samsung calculator says I should gain an additional 4,500 lumens per light (for S6 bin – lesser bins will have smaller increases and I’m not positive which bin these strips are). Even at this higher current, each diode still has an efficacy of 172 lumens per watt.

Given the numbers I got, I think you could get away with running a single board in a 2’x2′ grow tent and 4 boards in a 4’x4′. The lights don’t run very hot so having them close to the plant canopy shouldn’t be a problem either.

Check out my final build and results here.






  1. Thanks for publishing your results man. Awesome <3!

    • LEDGardener

      April 18, 2017 at 5:55 pm

      You bet! If ever you think of something else you’d like to see tested, let me know and I’ll try and make it happen.

      • Could you do a center reading from 1 strip @ 12” from the light?

        By the way, reading all this all over again… am i right in assuming you used no heatsink for these lights?
        If that’s so that’s fucking crazy man! can you confirm? If this is true then it’d make no sense to use COBS anymore considering that the heatsinks raise the price of making the fixtures so much.

        Each of the 2 lights you built should be enough to be considered high performance grow lights for a 2 by 2 space with those PPFD measurements.

        And well.. How much would the cost be?
        Well, lets say 110 USD for (12 strips + wires) + 30 for a driver = 140 USD. There are no commercial white LED fixtures offering this kind of output and these temperatures for that price…

        I’ve previously spent between 200-250 USD making a light that exceeds this output, but at 1.5x the price… And that doesn’t translate to 1.5x times the performance.

        This also allows making really, really good grow lights for small spaces, and the perfect spread. It allows for vertical lighting supplementation.. I mean, the uses are endless…

        Oh god, this is such a rant! Thanks for the good content man, keep posting!

        • LEDGardener

          May 24, 2017 at 8:32 pm

          Hey dude!

          Curse this spam filter of mine, I keep finding comments that never made it through. I’m going to make some changes to it tonight to stop this from happening.

          I did not run heat sinks for this test and actually ran 6 strips for 14 hours a day for a week with no heat sinks. This was at 117mA or ~60% of max current. Since I decided to run the strips harder in the final configuration, I did end up using u-channel for heat sinks, as I’m sure you saw in the next post.

          You’re right – these strips are super flexible and there are so many applications they’d do really well in. You could fasten them to a baking sheet with thermal tape and that’d probably be sufficient “heat sinking” for them. Might make a good replacement for my shitty oven light too. I’ll know for sure when my cookies are ready with these bitches lighting them up.

          I’ll have to think of how I can best isolate a single strip to read PPFD @ 12″. I’ll follow this comment up by the end of the week with a result.

Leave a Reply. For help with your own build, please make a thread in the Forum

© 2018 LED Gardener

Theme by Anders NorenUp ↑