What's wrong with No Drivers

Robert5489

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Joined
Feb 18, 2014
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1
I am trying to go always the simplest way. I need a light for my backyard and what I did, is simple:
I have a 12Volt 2A transformer a bunch of 5mm LED's 3~3.4volt that's it.
I soldered always 4 in series, made 4 rows on each board, solder the 12volt to it and turn it on. No resistors, no drivers and it seems to work fine.
Led's are 5mm round white, 27000mcd.
Each board seems to take 52ma therefore I should be able to hook-up ~36 boards before the transformer would shut down.
Question, can I get more out of the LED's?
Would the Cree's be the better solution doing the same thing?
Thanks, appreciate if anybody answers.
P.S. Does image verification are terrible for me, had Retina surgery on both eyes and I have to go about 4 times before I get it right. Brrrrrrrrrr
 

LEDAdd1ct

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Jul 4, 2007
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I am absolutely not an expert in this field,
however, there are 5mm LEDs from the likes of Nichia and Cree,
and then there is the nameless variety found on "that popular auction site."

Nichia and Cree both have some very good products out there.

One of our experts in the 5mm field would be JohnR66.

He has done lots of "playing" with the 5mm diodes,
and maybe he can help guide you.
 
Joined
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Yes, your setup will work fine with no resistor and no driver, because you have 4 LED in series and a (regulated?) 12v source, so each LED is seeing 3v. This is the on the low end of the voltage range so the LEDs are just loafing. As you increase the voltage, they will get brighter and brighter until they melt into slag.

This is why you need a current limiting driver to get the most performance, and also for safety for unregulated voltage sources like batteries. If you don't want that complication, you can run 3 LEDs in series (4v per LED), then add a resistor to drop the voltage to 3.3v. You would get more light but still be safe.
 

Canuke

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Stuck in California again
The big problems with such "direct" designs are as follows (based on my own understanding):

1. Very sensitive to voltage variations. Even a mild voltage spike may poof your diodes. Inline resistors act as a simple buffer, taking up some of that at the cost of efficiency.
2. Bad power factor. Again, not much of an issue for low power setups like yours and/or in home installations, but there's a reason why power factor correction is a feature for 1000W computer power supplies.
3. Possible line noise/RF emissions?

Except for (1), I imagine that this sort of simple setup does not scale well.
 

RetroTechie

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Oct 11, 2013
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Hengelo, NL
Welcome to CPF, Robert5489! :wave:

This setup only works for 2 reasons:
1) Like schizeckinosy said, poster must be using a regulated 12V supply. And
2) The LEDs used seem to stabilize at a useable current level @ around that 3.0V each LED is getting.

So a LOT depends on how well that 12V supply works, and the exact properties of the LEDs. And temperature... :)

Read: thread starter got lucky. Other than that, nothing to see here. :p Other color LEDs have different Vf. Temperature changes, voltage spikes do happen, a series-resistor setup works well if # of LEDs and resistor value matches supply voltage well. All the thread starter is doing, is making it easy for his LEDs to die. Well, 5 mm. LEDs are cheap too... :laughing:
 

JohnR66

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Aug 1, 2007
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SW Ohio
While the Cree 5mm LEDs are the best bang for the buck going in 5mm LEDs, I think you'd be better off using a COB LED solution like these:
http://www.cree.com/LED-Components-and-Modules/Products/XLamp/Arrays-NonDirectional/XLamp-CXA1304
You can use the 9 volt version and a driver or dropping resistor. They are only around $3 a pop depending on which one you choose. Various color temperatures are available and the 115 Degree light is good for floodlight use.

I tend to be conservative and underdrive the LEDs. This makes thermal management easier.
 
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