Multi LEDs and resistor Q.

Can I just use 1 resistor on the main +12 line, and the use the output from the resistor and split it up to however many LEDs I want to run?

Thanks
Roman

Just how many LEDs are you talking about?

30 all together...I just don't want to put a resistor on each one. Like maybe 1 resisor for every 5 would be nice.

It should work, but if I remember correctly  the more current you draw through the resistor, the more voltage will be dropped across that resistor, possibly not leaving enough to power the leds.

Lets do some basic Ohms law. P(watts)= E(voltage)x I(amperes). So for 12 volts going into 30 leds, which are usually 20 milliamperes each, would be: 12(volts) x .5(500 milliamperes) = 6 watts.
So, in order to put all 30 leds in line with one resistor, you would need at least a 6 watt resistor.
with putting a resistor every 5 leds, then the resistor would need to be at least 1.2 watts.
It could be done, but finding these large resistors are not as easy as 1/4 watt or 1/2 watt ones.
You could put three 1/2 watt resistors in parallel with each other, but you need to increase the resistance to compensate for the drop in resistance.
As you can see, it is not as easy as you wanted to know. It would be easier to put a 1/4 watt resistor in with each led.
I hope this helps,
Tom

OK. You need to know:
1. The supply voltage, in this case approx. 13 volts
2. The LED forward voltage. 1.7 volts for average red leds
3.The LED rated current, approx 20 mA (milliamps)

You can arrange the LEDs in 6 paralell strings, each having 5 LEDs and 1 resistor in series.

Then, to find the resistor value we use the formula:
R= 13-(5x1.7)/0.02
R= 225 ohms
The closest largervalue being 240 ohms.
The resistor will be dropping approx. 4 volts,
dissipating some 80 milliWatt (4x0.02), so a 1/4w resistor will be ok.

Good luck!

Thanks guys...good info here. My LEDs have a max v. of 3.8 and 30ma draw. I think I'll just get 30 resistors :)

Thanks.
Roman

Try to get a batch from the same manufacturer, same model number, same specs.

Do not try connecting them all in series.

well, all these guys are getting a little technical for this simple question. they are all right.they do sell led's  that have the resistor built into the led and run on 12volts they sell them at radio shack for about $1.00 each. that's pretty expensive for 30.these hook directly to 12vdc nothing else needed .they have colored wires even so you hook them up the correct way . easy. but if you dont' want to buy these then you do have to put 1 resistor per led or all the led's will not light up the same brightness. the resistors limit how much milliamps that flow through the led. if they are high briteness led's ,the kind that about blind you looking directly at them.,handle 50 milliamps. i have' pushed this limit with no problems. now if you havea normal led they are 20 milliamps maximum for full brightness.  go and buy yourself a bag of 100 resistors 1/4 watt (about $3.00 per 100) and solder the resistor on to one of either leg of led and shrink tube it. it does not matter which leg that you solder it to just as long as there is one soldered to one leg otherwise you will burn up the led. if you look close at the led next to one lead coming out of the led you'll notice a flat side on it . look at it from the bottom and you'll notice it. this is the + side of the led  also known as the cathode side.hook 12vdc positive + to this side.  ground to the other. pick a value resistor from between 240 ohms and 3300 ohms 1/4 watt is fine. 240 ohm being the brightest and 3300 ohms being the dimmest. as long as you got the series resistor on either lead on the led you can't blow up the led by hooking it up backwards (+ and - wrong) it just will not light up. this is a way of determining which side is the + and - on the led if there is no flat side or the thing just won't power up. use a 9 volt battery and a 240 ohm resister soldered to either + or - wire from the battery and then you have a led tester that will test any led to see if it lights. it should be very bright. if it does not light reverse the leads. if still doesn't light led is bad. keith  e-mail: uhyeahlarry@aol.com if you have any questions e-mail me.