DIY LED ring – optimal LED configuration, LED quality, LED PSU. - Page 1
Forum section: electronics / projects

author: 3dgeo
author_rank: Frequent Contributor
timestamp: May 03, 2019, 09:19:41 am
content: Hi,I found this very very cheap10W LEDsand decided to make a ring light for my camera. I don't have a doubt that those LEDs will have terrible CRI rating, to be honest for that price I'll be very surprised if the will work at allI let you know how balanced are individual LEDs inside when I'll receive them...I purchased 10 "cold" and 10 "warm" LEDs, also 2 ofthese constant current PSUsto power them all.My plan is to use 1 PSU per 10 LEDs so I can change overall color temperature. Circuit will be 5 Leds in parallel and 2 in series like picture below. What do You think of this circuit design and those LEDs? I was considering buying more expensive and higher CRI LEDs, but 20 good quality LEDs is a bit pricey. It would be way cheaper to buy many of these80+ CRI SMD LEDsand make custom PCB for them, but I don't know recommended circuit configuration for these LEDs, how many in parallel/serial will be optimal while using same PSU (I'm talking about current balancing)? Can I expect those SMD LEDs to be equal quality current wise?One SMD LED is 150mA, so I'm thinking about 5 * 12 in series, but again, is it a good practice?I know in both cases that I will get much more light than I need, but my intention is to undervoltage them to decrease temp.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 03, 2019, 11:45:24 am
content: buy the SMD leds that you mentioned, then use the already existing CC led driver ICs which you can buy for cheap. match the voltage to the number of leds you have in series and slap a resistor (or potential driver, depends on the exact IC so check the manual) you're good to go. all the leds are matched due to matched current and because the chips are so cheap, you won't need any leds in parallel. just add another chip with a string of leds for it to drive. for example if your input voltage is 12v and each led takes 2.1v, you can safely have 5 in series for a given chip to drive + some margin due the voltages not being exactly matched!Edit:This maxim chip for example, which only needs a resistor (and a bypass capactor):you can get switching ones as well but they need more than just a resistor

author: 3dgeo
author_rank: Frequent Contributor
timestamp: May 03, 2019, 12:16:24 pm
content: Hm, if I understand correctly this chip is not what I need – 1. it needs high voltage in order to drive many LEDs, so I need to add extra high voltage PSU anyway, 2. it has PWM dimming which requires additional components as well . My PSUs can step up voltage according to on board pot and has CC pot.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 03, 2019, 02:49:06 pm
content: it doesn't have to be PWM, you can tie it to high to have full power, that's just an additional feature.as I mentioned these chips are not meant to drive like 100 LEDs at once, and they are reasonably cheap! if you choose a standard automotive voltage like 24V, you can easily drive 10-11 LEDs using 1 single chip. If you need more, just use more chips ... even if you use 100 leds, you only need 10 driver ICs and 10 resistors which will cost less than 5$ (not including LED chips). your power supply is not as reliable and also due to the method you used for the driving circuit, your LEDs won't have matched brightness which also means some of them will die sooner (the brighter ones). there is a reason that pretty much all modern circuits use either 1 LED driver with a high voltage supply, or multiple chips powered from a lower voltage supply