Power led array - horticilture light - Page 1
Forum section: electronics / projects

author: Ermowich
author_rank: Newbie
timestamp: May 10, 2020, 10:05:26 am
content: Hello!I am trying to design small DIY grow light for Chilli Peppers.I found it 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 serial-parallel combination of these two types of LEDs together. I want to use a Constant Current source. Samsungs original light usesdesign AI think (https://www.samsung.com/led/lighting/led-modules/industrial-light-module/horticulture-linear/). I want to avoid putting resistors into the circuit to reduce power loss.Design A)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?Design B)I was also thinking about four LM301H in parallel and then six of these groups in parallel with 4 LM351H diodes. I read that you should place as many as possible diodes in series to reduce their difference in the ESR.-edit 10.05.2020 12:06: added design B sketch

author: Siwastaja
author_rank: Super Contributor
timestamp: May 10, 2020, 11:29:56 am
content: Not difference in ESR - difference in Vf, and not only the lab-measured Vf, butactualin-circuit Vf, which is a function of the LED die temperature, in a really bad way - higher temperature lowers the Vf, increasing the current of the already hottest LED, until it is destroyed.Hence, avoid paralleling LEDs. This being said, it's still normal to do that, but you need to really know what you are doing. So if you end up doing that, make sure you thermally couple them, i.e., the thermal connection to the heatsink must be not only good, but also consistent between the LEDs, and the heatsink should be thick enough (or use a heatspreader) so that there isn't temperature differential between the LEDs. In addition, derate the current a bit from the maximum rating. Also, the LEDs should be of the same production batch / grouping.Putting them in series lets you use maximum ratings because now you can regulate the LED current inallLEDs, guaranteed, within the accuracy of your current regulator circuit of choice, easily within a few %; completely regardless of Vf. You can even use different colors in series.Series connection can mean cost savings if the LEDs are the most expensive part, you can better utilize them with less derating.The downside, or upside, depending on the power source, is the requirement of higher voltage. Thus, the power source and your preferred current regulation scheme are important parts of the choice and should be discussed before you can choose whether and how much to parallel.If the power source, for example, is 12V, one way to do it is to use a boost current regulator, but in order not decrease the boost efficiency too much, the voltage cannot go arbitrarily high, so you end up somewhere 30-40V max. If you have more LEDs than that, then you can create multiple strings, but each need their own regulators. Easy to design by copy-pasting, and scales well, but consider the cost. For example, if a $2 boost drives ten $1 LEDs, that's likely acceptable, but if a $3 boost drives five $0.20 LEDs, it's likely a no-go.