If you open up a cheapo 9 LED flashlight you will find that they put 9 LEDs all on the same PC board and they are all connected in parallel. How do they do this without having the LEDs burn out? For decades, the engineers have warned us that you should never connect LEDs in parallel, because of unequal current distribution. Also, one LED starts to hog the current because its forward voltage is lower than the others, so it gets hotter. Then as it gets hotter, the forward voltage decreases, so it hogs more current, and so on until the LED is damaged by the excessive current and heat.
If so, then how do the 9 LED flashlight makers manage to do it and not have these problems? I have a couple theories as to why they can do it and it doesn’t damage the LEDs. One is that the LEDs all come from the same maker and have a forward voltage that may not be matched, but they are close enough so that the current is fairly evenly distributed. Another reason is that all of the LEDs are mounted close together on the same PC board, and if one gets hotter than the others, its heat is distributed to the other LEDs, so they start to draw more current, too. This evens out the current draw somewhat.
I drive down the streets and stop at many stop signals that have the red and green lights made out of LEDs. I see many of them that have a section of the LEDs that are not working, or that flash on and off intermittently. This leads me to believe that the large arrays of LEDs are getting too hot and damage is the result. So I don’t think that putting a lot of LEDs close together is such a good idea.
There are LED makers that mount several LED chips onto the same frame inside of the LED. The frame is metal and distributes the heat well so that if one LED chip gets a little hotter, its heat is spread out to the other chips. These multichip LEDs may have very high power – up to 30 watts.
Does this mean you can build a multi LED project and connect all of the LEDs in parallel? I would not do it unless I knew for certain that all of the LEDs are from the same maker and are from the same batch, so their forward voltages are very close. Instead I would give each LED its own current limiting resistor and eliminate any possibility that the LEDs could be damaged by excessive current and heat.
I have put several LEDs in parallel on Joule Thiefs. I think that the high current pulses cause a voltage drop across the LED’s internal resistance. This causes the current to be distributed more evenly over all the LEDs.