HVAC Lights Wired in Series???
HVAC Lights Wired in Series??? 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??
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??
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Joined: May 2008
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From: Cypress
Need more info. Are they raw LEDs or an LED kit that is pre wired?
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?
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?
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Joined: May 2008
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From: Cypress
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.
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From: Cypress
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
Thanks again for the help Rock-Star
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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???
So I guess I'm overpowering these LEDs by using a resistor meant for one LED run in series. Right???
Last edited by vMaster0fPuppets; Aug 6, 2008 at 11:56 PM.
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Joined: Nov 2003
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From: Jacksonville, FL (originally from Toronto Canada)
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.
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
Originally Posted by WhiteBird00
No, the current flows equally through both LEDs and through the common resistor which limits the current for both equally.
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?
Joined: Nov 2003
Posts: 11,331
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From: Jacksonville, FL (originally from Toronto Canada)
I would go with the value you got from the online calculator in order to maximize the brightness of your LEDs.
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????
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????
Joined: Nov 2003
Posts: 11,331
Likes: 346
From: Jacksonville, FL (originally from Toronto Canada)
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.
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.
