this is a noobie question

[patscavalier]

hello everyone
this is a noobie question for the masters of the led world
now i have spent some time reading and rereading and now i have myself lost
i have used a led calculator  http://ledcalculator.net/

now heres the question
this calculator says i need to put a  resistor  after every five leds
ok sounds simple and i have done that
but can i tie all of the led together
this is what im using
35 5mm red leds
Power supply voltage  12v
voltage drop  2.1v
LED current rating  20mA
Number of LEDs   35
and using The 75 ohms resistor after every 5 leds

so it will look some thing like this
led-led-led-led-led-resistor-led-led-led-led-led- resistor
+- +- +- +- +-  resistor +- +- +- +- +-  resistor

or do i have to have a start point of the leds
and a ground point for the  resistor 
this is what i have right now
start 12v in  led-led-led-led-led-resistor and one leg of the resistor going to ground

thanks for the help
pat

 

[minster9]

  I don't consider myself a master,but each series of 5 leds w/resistor should be wired in parallel between + & - busses.  It doesn't matter if each string of 5 leds has its resistor on + or - end of the string. I.E., 12v into each string w/it's own resistor, ground the other ends of each string. You'll need a power supply rated at an amp @ 12 volts,maybe more. Those resistors are gonna waste a lotta current. Lemme check something quick,I'll get back to you.

[minster9]

I'm back. Go to LEDSupply.com & check out their led drivers. Click on the 2008b-700 power puck & read the application notes(pdf). It'll safely run 35  5mm leds @ 20ma wired in parallel across outputs regardless of a widely ranging input voltage(5-32v DC). If you're using a wall-wart for power,you'll need an amp of dc output current. I don't mean to sound solicitous,but I've been buying led stuff from them a couple years now. The driver is $18,I've got a couple of 'em running some benchlighting myself.  The resistor scheme creates heat & wastes current,but 1st & foremost won't save your leds from a voltage spike! I fried a Luxeon III just by turning it on one night,apparently line voltage was high at that moment & the resistor I was using permitted an excess flow of current.   Since you're gonna be doing a lotta soldering anyhow,you can simplify things greatly by just running all the leds from 1 driver. And if you really want all of them to point straight ahead,make yourself a "die" to hold them in the pattern you want(round,square,etc) by drilling 5mm holes into a block of wood & drop them into it w/leads sticking up.  Pay close attention to led polarity! One wire is longer then other & one edge of plastic base has a flat molded into it.  For some odd reason,the lead spacing of 5mm leds will not line up with your average "rat-shack" pc board holes(diag or perp). I've never planted 35 5mm leds on a board b4! Get a board with alternating strips of conductor & drill holes so the board falls around all the led leads sticking up from the "die" holding them. You don't want to introduce bending (mechanical) stresses into the leads of the leds. I modded a headlamp using 10 5mm leds using this method 3-4 yrs ago & a "micro-puck".  Post again,let us know what's goin on. Welcome to linear1! I just got off work 3 hrs ago,sorry. Still wound up,felt like trying to help someone out. Check out ledsupply's 2008b-700 driver specs;that'd be the route I'd go. And don't be shy.

[cadstarsucks]

for me it would depend on the actual power source. 

if your 12V is from a car electrical system even a power puck is potentially risky, but the part says it is ok for automotive lighting even though it does not say it is automotive rated.  it runs with 32V but an actual auto EMI spec calls out a 90V peak, how often that kind of spike makes it to the puck beats me.  these survivability specs are notoriously severe for obvious reasons.

if it is a battery source or supply source the pucks are great if a bit overkill for someone with more technical finesse.

for someone with technical finesse off the power line it is better to roll their own than to buy a supply and a puck.  HV9910 chips are quite easy to work with and run almost right off the line

[patscavalier]

well thanks for the welcome
and the help
i guess i should have said what i was doing
this is going to be a set of tail lights for my car
and i have recounted the leds and there is 38 of them
and i have them in a Radio S*ack predrilled board
here is a picture of what i have done

and this is the back of the board


i have run some thin copper wire around the outside edge  and that is 12v in
then i did the same thing in the center of the bowtie and that is the ground

the resistors im using is 75 ohm
so i have 5 leds and a resistor  and then it starts all over again
what i want to do is have less resistors

now before this question is asked i have figured out what resistor i need to use for the
just the tail light and what resistor i need to use for the brake light and the blinker
oh this set up is for my car which has everything in one place
the car has one bulb with to fillments that do all the lighting
one for the tail and one for the brake / blinker

i hope this helps you try to understand what im tring to do
thanks for the help
pat

[minster9]

Looks like something the cops might give you a rough time about.