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What is wrong with my LED circuit.

zionosis

Oct 23, 2013
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Well I am far from good at any of this and haven't really read into it much.

But I have two cheap Chinese lights and one was faulty so I just decided to take it apart and now I have the LED's from it.

So now I want to use all 6 and make a grow light and use an LED driver for a constant current.

Here is a kindergarten style diagram (I know I didn't use the proper symbols).
 

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zionosis

Oct 23, 2013
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So how will this not work. I mean does the voltage have to add up and be really close or will the fact that the supply is 12 and the light series path add up to only 10.2 cause a problem.
 

Harald Kapp

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LEDs are current controlled. Even LEDs of the same type tend to have slightly different voltage drops at the same current. Thus paralleling LEDs will lead to an uneven distribution of current and therefore uneven lighting.
You also need to limit the current through the LEDs. You should never connect an LED to a voltage source without current limiting.

Read "got a a question about driving LEDs?" in the tutorial section.
 

zionosis

Oct 23, 2013
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LEDs are current controlled. Even LEDs of the same type tend to have slightly different voltage drops at the same current. Thus paralleling LEDs will lead to an uneven distribution of current and therefore uneven lighting.
You also need to limit the current through the LEDs. You should never connect an LED to a voltage source without current limiting.

Read "got a a question about driving LEDs?" in the tutorial section.

Well isn't that what the LED driver is for. It's for current regulating. That's it's whole purpose unlike say a PC power supply which is a Voltage regulator. In LED drivers the voltage can vary but the current is regulated to stay within a certain range. That is why they were made.

So you say that hooking up LED's in parallel isn't a good idea because LED's even of the same type have different voltage drops. Well wouldn't that be more of an issue in a series and not parallel. After all series circuits are where your voltage drops but your current doesn't and in parallel their are many branches so the voltage doesn't drop but the current must divide. Or are you saying this is wrong also. Seems everyone just conflicts the next person and nothing is meant to work.

So your are saying that you can't have different voltage LED's in a parallel circuit. Ah I am nearly 100% sure you can. I even have evidence to back that up also if you need it. You are aware that each branch of a parallel circuit still has the same amount of voltage as the source output.
 
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zionosis

Oct 23, 2013
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parallel%20circuit2.GIF
 

zionosis

Oct 23, 2013
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So now tell me if I had 3 10V LED's in that picture up there why it wouldn't work. Especially when the source would be from an LED driver which regulates the current so it's always at the specified amount for the LED. That is just an example, Please don't argue saying stuff like lol 10V LED's.

Maybe you guys on here don't know what an LED driver is. I have heard that many people in electronics don't really know much about them because they have a very specific purpose and most people in electronics work with and know about voltage regulating power supplies like a PC has.
 

zionosis

Oct 23, 2013
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Plus as far as I know about circuits it's perfectly fine to have 2 of those parallels having a 10V light and on the 3rd you could put 2 5V lights or any other combination, Just so long as it doesn't require more than 10V and runs at the same current.
 

zionosis

Oct 23, 2013
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Alright you guys claim this isn't possible but the electronics basics I have right in front of me prove otherwise. I am just going to build it anyway since thousands of other people build setups just like this and all LED's are evenly powered. Otherwise no one would buy an LED driver if they were that crap.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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The problem is stated in the link you were given with numerous references.

The short explanation is that if the LEDs have a slightly different Vf then the one with the lower Vf will draw more current and heat up. The heat causes Vf to fall, so it's Vf is even lower, so it draws more of the current. Eventually it (in this case one string) may draw the full current available and he other one will go out.

In some cases this will lead to the destruction of LEDs in a cascade.

Bulbs do not share this problem because increased current and heat lead to a higher resistance rather than a lower one.
 

BobK

Jan 5, 2010
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Alright you guys claim this isn't possible but the electronics basics I have right in front of me prove otherwise. I am just going to build it anyway since thousands of other people build setups just like this and all LED's are evenly powered. Otherwise no one would buy an LED driver if they were that crap.
There is nothing wrong with an LED driver when used correctly. You are using in incorrectly. A constant current driver works with a single string of LEDs because the current must go through each LED in that string. As soon as you put two strings in parallell, there is no guarantee that the current will divide equally, thus it is no longer controlling the current through the LEDs.

Oh, and bad circuit designs will often work, for a while. A good circuit design will continue to work. The cheap LED lights that you can get on Ebay often paralllel the LEDs and they also often fail after a short time.

Bob
 
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