Quick question hope someone will be able to answer this as I just can’t seem to remember. But here we go…
I have 28 Cree 3W LED Chips I have arranged on an aluminum heatsink connected in series/parallel in a 7x5 configuration. Except the middle row has only 5 LED as seen in photo. I’m trying to figure out the correct constant current power supply to power my fixture. I appreciate any and all assistance
I think @Mr.Sparkle should know a thing or two regarding lighting.
would need to know the specs of the chips your running, that aside add 2 more chips to make it easy as other wise that center row will be forced to run at a higher voltage as the other chains will be running what they need and that will either cause that center string to run harder or even burn out.
So I changed how I have the LEDs wired and they are the 3w XPE with max drive current of 1000ma. I’ve wired them in one long chain + to - starting with + and ending with - it’s a total of 28 LEDs on the fixture. I’m not sure what info you may need.
Do you have the datasheet for the LEDs or a complete part number? In order to help you, we have to have all the specs. For example, you stated you have 28 LEDs all wired in series. If the forward voltage of those LEDs is 3 volts, you will need a power supply with a minimum voltage of 84 volts just to turn them all on, but you will only need 1 amp of source current. But, if one “opens” you will lose them all since the current path will be broken. If you wire them all in parallel, you will only need a 3 volt power supply, but it would have to source 28 amps if you want to adjust them up to maximum current (which you don’t). Obviously not ideal. But if one “opens” the rest will stay on. You need a combination series parallel circuit so that if you lose a string, you don’t lose all your light. It’s a balancing game.
Hope this helps.
I agree he should add the additional two for even light distribution, but if he uses a constant current sourcing power supply, it will detect the lower current draw if he loses a string and automatically lower the current.
Would need to know the forward voltage of the individual LED, also max and typical amp ratings. With those two values, you can figure out how many volts and amps you’ll need for your power supply.
Are these the ones you have? Cree XLamp XP-E High Power LED Star
Nominal looks to be 350ma with 850ish the max id go which is 2.9 to 3.2v roughly respectively.
id wire them one of two ways with how i know some drivers are
either 4 parallel chains of 7 in series which would require 22.4v or round up and say 24V which is common for some driver and an amperage up to 3.5a or 4a max (xlg-100-24) or similar driver
or 7 parallel chains of 4 in series which is 12.8v or so and 6a or a bit more so a driver capable of say 13v and 7a which you may fine some “12v” driver capable of that.
Not quite it will push the set current through the smallest chain it wont detect it, and like lt fry it in the process if too much
I’m not sure what you mean here. The sense circuit and closed feedback loop are usually pretty tight. I could see this happening with an oversized power supply possibly, but they should (in theory) all see the same current.
No matter. You summed it up well.
most drivers dont have sensing circuits in them, you set them at a current output and they will give out that current output, 4a on 4 parallel strings you lose one then the remain 3 will see 1.33a each vs the 1a they were running at, loose another chain now the remaining 2 chains see 2a each.
I did some digging and you’re right. The majority of manufacturers count on the large number of LED strings light manufacturers use to distribute the current so there is less impact in the event of a failure on one or more strings. That, coupled with keeping the LED current at nominal.
This is all the info I have on the LED’s my apologies everyone I should have already located the information and given such in my initial post. I appreciate all the help really I do. I’ve always had issues with Drivers.
I appreciate the recommendations I’m gonna follow your instructions and go for 4 parallel chains of 7 in series. One last question so I don’t have to ask it later, if I were to double the amount of LEDs I’m guessing it would be smartest to just run the second group of 28 on a secondary identical driver? Thanks again
Thanks @Coda for breaking it down for me, this helped me to understand much more clearly.
