tak tady je pár důvodů pro seriové zapojení :

• http://members.misty.com/don/ledd.html
Do not put LEDs in parallel with each other. Although this usually works, it is not reliable. LEDs become more conductive as they warm up, which may lead to unstable current distribution through paralleled LEDs. LEDs in parallel need their own individual dropping resistors. Series strings can be paralleled if each string has its own dropping resistor.
• http://www.bivar.com/eLetter/driving-la.htm
You can also connect LEDs in parallel. However, variations in the forward voltage requirements of individual LEDs will result in non-uniform current distribution, and non-uniform current distribution results in non-uniform brightness.
• http://www.kpsec.freeuk.com/components/led.htm
Avoid connecting LEDs in parallel! Connecting several LEDs in parallel with just one resistor shared between them is generally not a good idea. If the LEDs require slightly different voltages only the lowest voltage LED will light and it may be destroyed by the larger current flowing through it. Although identical LEDs can be successfully connected in parallel with one resistor this rarely offers any useful benefit because resistors are very cheap and the current used is the same as connecting the LEDs individually. If LEDs are in parallel each one should have its own resistor.
• http://www.otherpower.com/otherpower_lighting_leds.html
It's important that each string has its own resistor.... putting them in parallel with a single resistor is bad practice. We didn't know this when this article was first written....thanks to all the folks that pointed this out!
• http://home.nikocity.de/andymon/hfg/ledlamps.htm
If you want to operate LEDs in parallel with higher power levels, you must select them for equal voltage/current characteristics! It won't do to buy four LEDs if you want to build a four-LED-Lamp! Buy ten and measure the voltage of each LED at 20mA.
• http://www.help.ip3.com/s.e.design/C...ng-soLip.shtml
{both sides of an argument on this subject}