Rule #1 Lets start with the obvious, remember FRED (F**king Ridiculous Electronic Device)
FRED will fail sooner or later, no matter how perfectly designed the system is.
FRED always does two things: 1)whatever he is supposed to do to 2)generates heat.
In the case of LEDs the heat is small in our opinion but the LEDs themselves don’t think so and they will usually
fry long before they were hot to the touch. So try not to make your table impossible to destroy if/when you need to
replace LEDs. You can thank me later 
As for wiring...start by referring to rule #1 above. Then see the existing sticky at: http://nuigroup.com/forums/viewthread/1030/
which details the electronics behind it quite thoroughly, and no it doesn’t matter where in a series you put
the resistor, + end, - end, in the middle...just make sure you use them. If you see a weird value presented by
the calculator you use, just increase it to the next higher standard value, more power rating in a resistor
means it’ll run cooler in use, and a slightly higher resistance won’t degrade your performance noticeably,
lower values in either rating can be regrettable.
Everyone posts links to the online calculator they think is best...here’s the one I used:
http://led.linear1.org/led.wiz
What I like is it doesn’t assume you are going to do a straight parallel or series wiring, instead it shows
several methods to accomplish the result.
Obviously you will need some specs to make it work, but they should be readily available from your purchasing
source...and when in doubt, some ‘normally safe default numbers’ for unknown LEDS are: forward diode voltage 1.3V
and diode forward current 20mA. Those numbers carry no warranty, but very few LEDS will fail to handle them…
of course those will rarely be optimum values either, and if you have a robustly designed LED you would be
shortchanging your output available with these numbers. For the record, 12V LEDs do NOT exist, there are some
high voltage LEDs that DO exist, but most things marketed as higher than 2V are simply an LED and a resistor
in one package. If you don’t know anything about a specific LED, but have a variable voltage power supply, an
infrared sensing spectrophotometer, and know how to use them...you don’t need me.
The downsides of several wiring schemes:
Series: If you put all of your LEDs in a series, and 1 device fails, your entire array is compromised at best.
Depending on the electrical definition of how it fails, you will either have too much voltage at the survivors
stressing them towards failure (if it burns into a short) or none at all (if it burns open, which usually
happens soon after burning into a short anyway). Once it burns to an open, you will have to test each LED
until you find the offender because you suddenly have no blobs. Unless you are only using 5-10 LEDs or are
really fond of your multimeter...don’t do it. (again, thank me later)
Parallel: If you put all of your LEDs in parallel, your entire array normally depends on one resistor or
resistance network to keep it stable. All of the current goes through that resistor. It most likely WILL get
hot if you have a large number of LEDs. So IF you decide to do this, at least get a resistor that is rated
for much somewhat higher power than what is recommended by the calculator as it will run cooler. Aside from
that, it’s nice & straightforward and offers a reasonable degree of predictability...Until you remember that
off the shelf electronics are typically considered to be ‘close enough’ with a tolerance of 10% from the
published specs. Some of your LEDS will be brighter, and fail sooner, than the rest. My first 6 attempts at
wiring LEDS to 12V were with 6 LEDS each time, following an online calculator that recommended a straight
parallel wiring scheme. The longest living LED kacked in slightly over an hour. At least when one fails the others
stay lit though, so troubleshooting is easier.
Series-Parallel combinations: You are basically making short series chains that are separately resistively
loaded to the voltage specified and then connecting them in parallel. Anyone really need it explained that
you are dividing the stress of regulating the current across several resistors while spreading the voltage
drop across several smaller LED subsets? Anyone not detect a distinct bias towards this on my part?
Didn’t think so. Realistically, I do suggest that when the calculator poses you with a dizzying array of
choices as to the different ways you could do this...consider if you are using ‘land-line’ power, or batteries.
If NOT batteries, scroll through and look for the one design that has the same number of LEDs in each series
(5 & 7 are common numbers with 12V supply) as it will only require purchasing one value of resistor & you
won’t need to remember which resistor type goes with which number of LEDs...consider adding a few LEDs to the
design to get to a multiple that looks lucky to you. Then if you want to add more LEDs at any point, just make
sure your power supply has ample current available, and build more series modules just like the rest & add
them to in parallel. If you are on batteries (or are concerned about efficiency for other reasons) look
through the choices for which has the lowest actual power consumption from your supply and accept the fact
that it will be a touch more complex.
If you are designing a mosfet driver for your LEDs...of course your way is best.
