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Container farms take a lot of light. Powering it is not cheap. And I have spent a lot of time pondering the different possibilities.

I've developed a PCB to control 4 x 300watt PWM amplifiers. This gives me a simple controller that can handle sets of 4 channels of say 3 x 100watts COBs. The inputs into these are ~24-27v and ~1500watts, so a single PSU at around $150 a pop will do it.

But that is not cheap.

I have some very grunty 5v controlled 20kw SCRs and 240v. If I rectify this with a FWBR and smooth it then divide in series by 12 light controllers, I get my ~27v to each fitting.

Was basically thinking to make a secure enclosure with 240v in and say 12 sets of terminals out. But how do I separate the terminals within the enclosure - i.e. types of resistor - and what happens if the load between terminals is very uneven (lights unused).

And is all this too silly, and not worth the $2000+ in savings for individual SMPS units.

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  • \$\begingroup\$ What's your source of energy? is it "normal" power grid? If so, why not simply cheap construction store LED floodlights? \$\endgroup\$ Dec 17, 2020 at 16:44
  • \$\begingroup\$ Yes grid power. I have gone down the road of COBs and they are not expensive if I fabricate myself, and lend themselves to tight spaces and vertical setup. I also need to control current carefully from a micro to utilise power prices. It is just the power supply that is challenging. Also to be fair, I am keeping renewables as a backdrop hence the DC focus at the individual lights \$\endgroup\$ Dec 17, 2020 at 16:47
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    \$\begingroup\$ Yeah, but the power supply is what will make your solution infeasibly expensive, whereas grid-power 200W LED floodlights from your friendly construction store are cheap for reasons of economics of scale... \$\endgroup\$ Dec 17, 2020 at 16:55
  • \$\begingroup\$ They may help complement the setup. But I will need the COBs for spectrum, and for hanging in the tighter spaces. Will certainly consider. I have a set of HID lights I picked up cheap, but they are a bit thirsty, If the SCR solution can work though it solves the issue. \$\endgroup\$ Dec 17, 2020 at 16:58
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    \$\begingroup\$ @ChrisStratton Most commercial grow lights use ordinary LEDs, often those made for exterior or roadway lighting. There is no special spectral requirement for most grow light applications beyond that they have some approximate ratio of blue to red light, and often that is only very roughly specified. You can use ordinary light bulbs of the right color temperature, but it won't be cost effective due to very low efficiency. \$\endgroup\$ Dec 17, 2020 at 20:57

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Ok so you are talking about 12x "100W" active loads in series at 240V?

First of all if you turn some of them off, the extra voltage has to get dropped by the remaining ones, which means they'll be seeing not just 27V but 30 or 40 or 50V or even 210V, at the same current. Where will that heat go?

Secondly, while putting them in series is great for thing like bulbs, PWM LED controllers produce a constant power output and high efficiency. That means input is also constant power, which means negative impedance on the input (i.e. if you feed it with a higher voltage, it will pull less current). Negative impedance can make things unstable, so you'll have to work that out. However simply adding positive resistance will ruin your efficiency.

Have you considered the old fashioned solution, a transformer? Reduce the 240V to, say, 30V-36V, which your controllers can hopefully tolerate and leave breathing room for the ripple. 240V x 10 Amps will get you in the ideal case 30V x 80 amps = 2400W You'll want to have plenty of margin there, and might want a transformer even bigger than that, if load is switched suddenly / has inrush. You can get you into pretty good trouble, i.e. produce big voltage spike, trip breakers etc, switching loads suddenly in decent sized transformers.

On the plus side, you won't have line voltages on every part of the system.

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  • \$\begingroup\$ Thanks Pete. Yes, this was my gut feel. I have considered a transformer yes. And will revisit this based on your post. Appreciated \$\endgroup\$ Dec 17, 2020 at 20:18
  • \$\begingroup\$ I just checked and my MIG welder puts at 26v min. A quick bit of digging reveals they have a better performance and quality of current than the old arc transformers. Perhaps the solution was with me all along! \$\endgroup\$ Dec 18, 2020 at 3:55
  • \$\begingroup\$ Great! Sourcing the transformer should be pretty easy. Just a big old toroid with the appropriate number of turns in it. Just make sure you have enough current & power rating, and do some reading on how much margin is appropriate on the transformer specs for this type of load. That is really important. \$\endgroup\$
    – Pete W
    Dec 18, 2020 at 15:14
  • \$\begingroup\$ Hi Peter, I've been researching this. The MIGs are inverter welders. I can buy circuit boards off of Aliexpress for < 100 that put out 200+ Amps at up to 90% efficiency. However, I wonder if they can run continuously? Have made some enquiries. \$\endgroup\$ Dec 20, 2020 at 17:05
  • \$\begingroup\$ Ok. MIG welding maybe have other requirements different from LED lighting, maybe can be used. If you post the specs you can get some comments on it. \$\endgroup\$
    – Pete W
    Dec 21, 2020 at 0:56
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First, the overwhelming concern in horticulture applications is usually running cost. As you note, power is expensive. For example, I might pay 50 cents per watt to install capacity, but power costs ~ 3-4 cents a month to run that light 12 hours per day, so by the 14th month, I'm already paying more in power than the entire up front cost of the unit. Sooner in higher cost areas or if you're running more than 12 hours per day.

You're mentioning 100w COBs, which already sounds like a bad idea. You'll need costly heatsinking to cool them, and even then packing that much power into a small area means heat will be higher and efficiency lower. Taking the same diodes and not putting them into a COB will give you higher efficiency (lower running costs) and easier cooling (lower up front costs). What is the efficiency of these lights in umol/J? How much will it cost to keep them at a temperature where you can get that efficiency? Is there a reason you wanted to use those COBs or did you select them because they looked cheap? Your running costs may be different, but a good starting place is to be well above 2.5 umol/J.

As for power supplies, generally you design a constant current driver, one per string of lights as this is efficient/safe whereas voltage control is neither. You don't need super clean power, but you want to avoid PWM or too much ripple because the umol/J drops at higher currents, which adds to running costs.

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  • \$\begingroup\$ Thanks for your wisdom. I wish I had met you over a year ago. The COBs were selected due to the cost, ease of fabrication, and also the recommendation of industry leaders like MyGrow who have done comparative tests with COB against SMD. As they are cheaper to buy and fabricate, I can run more at lower amps to reach a greater efficiency, and yes I was going PWM, but am concerned by your comments. Although we have developed a PWM controller, we have not committed to production, and perhaps the constant current driver is preferable. How about adding a CAP to the PWM output? \$\endgroup\$ Dec 18, 2020 at 3:27
  • \$\begingroup\$ "Another aspect of the COB vs SMD LED Lights difference is in the use of energy. COB is known for better lumen-per-watt ratios and heat efficiency." solarlightsmanufacturer.com/cob-led-smd-led/…. \$\endgroup\$ Dec 18, 2020 at 3:33
  • \$\begingroup\$ @AndrewMcClure Efficiency for LEDs used for horticulture are specified in umol/j, not lumens/watt, but a typical lumen value for horticulture LEDs would be over 200 lm/watt. That website is talking about COBs at 80 lm/watt, which is extremely poor. Post the datasheet for your COBs. \$\endgroup\$ Dec 18, 2020 at 4:29
  • \$\begingroup\$ I will do better than that. I will put up a light and read umols per watt for the warm white lights I have. It's been on my list to do. Do you have a link to the lights you are talking about? \$\endgroup\$ Dec 18, 2020 at 4:42
  • \$\begingroup\$ Ok. this isn't looking good for me. But then it could be a factor of distance? What distance from plants were those measurements taken? So for my warm white I am getting approx 10.5 u/mols per watt at 22cm straight above and 9.3 when off 100mm. The Red is worse at 7.5 straight above and 6.7 below. So yeah, please give details of where you got those figures because I am not in the ball park here. \$\endgroup\$ Dec 18, 2020 at 5:36

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