vMaster0fPuppets

iTrader: (6)

I installed Blue LEDs in my car today and moticed something strange.

I only installed one resistor in the line for these lights, and both LEDs light up without burning out. So, I assume these lights must be wired in series... right??

Rock-Star

Need more info. Are they raw LEDs or an LED kit that is pre wired?

vMaster0fPuppets

iTrader: (6)

Raw LEDs. I installed what is supposed to be a 470Ohm resistor in the wire going to the first LED

vMaster0fPuppets

iTrader: (6)

O I think I figured it out.

By putting the resistor behind both LEDs, in a wire shared by both, I actually ended up running both LEDS in parallel and sharing the resistor. RIght?

Rock-Star

If the resister is on the positive of one led and the neg goes to the positive of the other LED thus leaving the neg to ground then YES. Positive and neg can be determined by looking at the LED leads. Long= (ANODE)positive Short = (CATHODE)negative.

Rock-Star

CORRECT!!

vMaster0fPuppets

iTrader: (6)

Thanks. Its funny. I'm a going into my third year as a physics student, and I aced the electricity and magnatism class. But this stuff came really slowly to me.

Thanks again for the help Rock-Star

vMaster0fPuppets

iTrader: (6)

Why is it that everything I read stresses that the resistor must go in the positive wire before the LED. But everythign I've read, and what I've done hear shows that it doesnt matter.



So I guess I'm overpowering these LEDs by using a resistor meant for one LED run in series. Right???

WhiteBird00

iTrader: (4)

LEDs have essentially no internal resistance so they act as a short when wired in a circuit. Resistors are used to limit current flow. Since it's a closed circuit, it doesn't matter whether the resistor is on the positive or negative side of the LED - as long as the resistor is in the circuit somewhere it will limit the current flow throughout the circuit.

No, the current flows equally through both LEDs and through the common resistor which limits the current for both equally. IOW, the current flow in each "branch" of the circuit (the separate LEDs) is the same as the current flow through the resistor-limited common part of the circuit.

vMaster0fPuppets

iTrader: (6)

I've been doin all this wiring diagram stuff, and in all my calculations I've used 200Ohm as the resistance for the LED based on the information on the site I bought the LEDs from. I guess I was wrong


So no matter how many LEDs I ran in parrellel, I would need to same resistor.

Based on what I worked out, to run 4 LEDs in parrallel I would need a 180Ohm resistor. Thats also what one of the calculators on the internet said. What should I buy?

WhiteBird00

iTrader: (4)

Not exactly...although the same resistor will be (more than) sufficient to limit current flow to a safe level for the LEDs, every additional LED drops the voltage potential (and hence the brightness) at each one. That's why the recommended resistor value goes down as you add more LEDs to the circuit.

I would go with the value you got from the online calculator in order to maximize the brightness of your LEDs.

vMaster0fPuppets

iTrader: (6)

What would I use to determine how much each LED would drop the voltage.

As far as I can tell, each one of these LEDs have a resistance of ~180Ohm. Because...

A 14.5V battery can run 4 of these in series outputting 20mA. So each LED runs around 3.62V pushing .02A. R=V/I = 3.62/.02 = 181Ohm No????

WhiteBird00

iTrader: (4)

LEDs (like most semiconductors) don't follow Ohm's Law. They have almost no resistance in the forward direction and large resistance in the reverse direction (they act as a conductor in one direction and an insulator in the other - hence "semiconductor"). There is a voltage drop across LEDs so you can't use more LEDs than can be handled by the power source - the combined total of the voltage drop across the LEDs plus the drop across the resistor(s) must be less than the voltage of the source. You get the voltage drop of the LEDs from the package they come in although 2V for red and orange, 2.2V for yellow and green, and 3.0V to 3.6V for blue and white are pretty standard specifications.

The formula for your single resistor when connecting multiple identical LEDs in parallel is:
(source voltage - LED voltage) / (LED current * number of LED's)

Now, you have to be sure that your LEDs are identical - preferably even from the same manufacturing lot - when using a single resistor. The reason is that LEDs with the same specs might have slight differences in conductance. If that happens, the one with the highest conductance will pass all the current (and burn out) while the others remain dark. That's why it's always best to use individual current limiting resistors for each LED when wired in parallel.