LED Question

[99Grand/\mSE]

resistor is connected to negative LED terminal right ? :)

[Jaysin]

positive dude

[tenspeed]

It doesn't matter but most people wire it to the positive (longer) LED lead. The voltage will still be dropped if it's on either side.

[magyver]

actualy, it should be on the negative side. DC current flows from negative to postive... so you want the V knocked down before it reaches the led.. but yes, it dosent matter, it will work on both sides...

[Merlyn]

the voltage is knocked down regardless of where the led is placed. it doesn't matter if you put it on the positive or negative, just make sure you have one.

[Bjornboy81]

the resistor isn't there to reduce the voltage at all...it's too limit the current going throught the led...so it's doesnt' matter which side.

BTW, 99Grand/\mSE...Vash the Stampede rules!!!

[99Grand/\mSE]

Heh yeah he does... Thanks for the replies, your helping a noob out, been a while since i had those electonics classes back in highschool couldn't remeber anything just hope i can still solder *sp*. I would like to try to do the third brake mod with Leds and a circuit board that i've seen in a few other posts and th dome light mod :)

[tenspeed]

If you're using a bunch of LEDs, you can skip the "current limiting" resistor and wire the LEDs in series. When I made my third brake light, I wired six in series and hooked them up to the 12 VDC source.

[Merlyn]

with my 3rd brake light, I still put the resistor on there, but I did the resistor in series, with the led's in parallel. Either way, it all works.

[Jaysin]

whoops my bad...its just ive been puttin the resistor to the positive and it always has worked well so i thought that was the way it went...oh well, lesson learned

[99Grand/\mSE]

For the third brake light did you use a strip board to hold them in place or is there a simpler way to do it ( i haven't taken the third brake light apart) :)

[magyver]

Originally posted by Bjornboy81
the resistor isn't there to reduce the voltage at all...it's too limit the current going throught the led...so it's doesnt' matter which side.

BTW, 99Grand/\mSE...Vash the Stampede rules!!!

hey squirrl nuts for brains, please dont speak anymore, you have absolutely no idea what your talking about. if you had any common sense, any light will only draw what it wants too current wise... the resistor knocks down the voltage... PERIOD!

so what your tellin me is that i cant hook up a led to my battery w/ a 1/4W resistor? your tellin me that 1/4w carbon film resistor is holding back 600+ amps? your tellin me that if i have a 900A capabale 3.3V power supply that i have to have resistors on my led's when my led's are 3.3V otherwise they will burn up?

what you yack about makes absolutely no sense...

your fired from LED knowledge answers!

[tenspeed]

Originally posted by 99Grand/\mSE
For the third brake light did you use a strip board to hold them in place or is there a simpler way to do it ( i haven't taken the third brake light apart) :)

I used the stuff I used to call bread board but it goes by many names. It's a fiberglass material with small holes in it. The LEDs fit into the holes and I used a small dab of clear RTV to hold them in place.

I cut the board so that it fit inside the third brake light housing. I removed the bulbs and soldered two wires into the copper bars on the inside. I used two quick disconnects to power the LED board and I can revert back easily if I need to. The red lens holds it in place.

Saying all that, I think it's easier to get a strip of LEDs already made up and put them in there. JCWhitney has several motorcycle LED bars available. There is also satisfaction in making it yourself and perhaps positioning the LEDs into a pattern that no one has.

[magyver]

or goin to a junkyard and grabbin a 3rd brake light from a GM suv, blazer, burbin, ect ect... the ones that are on top.. take it all apart, boom, you got 30 led's w/ power and ground pigtails... trick is its 3 led' stoo long, so ya gotta cut the board and jumper the wires you cut... easies way i think... shouldnt cost hardly anything... looks sexy

[Bjornboy81]

Originally posted by magyver
hey squirrl nuts for brains, please dont speak anymore, you have absolutely no idea what your talking about. if you had any common sense, any light will only draw what it wants too current wise... the resistor knocks down the voltage... PERIOD!

so what your tellin me is that i cant hook up a led to my battery w/ a 1/4W resistor? your tellin me that 1/4w carbon film resistor is holding back 600+ amps? your tellin me that if i have a 900A capabale 3.3V power supply that i have to have resistors on my led's when my led's are 3.3V otherwise they will burn up?

what you yack about makes absolutely no sense...

your fired from LED knowledge answers!

Dude, take it easy! I know my stuff when it comes to electronics. I'm not saying the resistor is "holding back 600+ amps", because the LED does have some internal resistance. But most LEDS are rated for 20mA or so, and you need to limit the current to this with a resistor. Here's (http://www.leesspace.com/LEDs.htm) a link to how LEDS work and why you need a current limiting resistor. There's no need to start throwing names around because you disagree with someone!

[99Grand/\mSE]

Thansk for the input on the third brake guys, i finished wiring the LEDs under the dash came out well, very happy i did it myself too :)

[Bjornboy81]

good job man...you got pics?

[99Grand/\mSE]

yea ill take some tonight when u can actually see the light :)

[tenspeed]

Originally posted by Bjornboy81
But most LEDS are rated for 20mA or so, and you need to limit the current to this with a resistor.

Here's a link to how LEDS work and why you need a current limiting resistor. There's no need to start throwing names around because you disagree with someone!

I agree that Magyver may have been a little harsh with his retort, it was not totally unfounded.

In your link to how LEDs work, under "How to Calculate etc" example three refers to the voltage drop across the resistor. The LED will limit the current draw as shown in the series circuit shown in Example #4. The only danger to LEDs is hitting it with too many volts which will lead to too many amps. It turns the device from a Light Emitting Diode to a Smoke Emitting Diode.

I do like what you have found and I am saving the tutorial for furture use.

[magyver]

Originally posted by Bjornboy81
Dude, take it easy! I know my stuff when it comes to electronics. I'm not saying the resistor is "holding back 600+ amps", because the LED does have some internal resistance. But most LEDS are rated for 20mA or so, and you need to limit the current to this with a resistor. Here's (http://www.leesspace.com/LEDs.htm) a link to how LEDS work and why you need a current limiting resistor. There's no need to start throwing names around because you disagree with someone!

no, you dont understand, things just dont draw as much current as they want... if an led is rated at 30ma, its gonna draw that much.. the only way its gonna draw more is if its shorted... so your still saying that i need a resistor if i have a power supply w/ the V rating the same as the led... IE if i have a 3.3V power supply and my led is 3.3V your saying i need to put in a resistor? that is false... i have 3V power supplies all over my crib running direct to LED's cause what i say is true.. 3+ years and there still runnin...

^ i too know what im talking about! AS degree in electromechanical (electronics, robotics, phneaumatics, hydrolics.. ect ect..)

[95GAinFLA]

if current is resisted voltage goes up.

or is it the amps that go up? whatever, where the pics?

[Bjornboy81]

Diodes (among other devices) are incapable or limiting thier own current due to that fact that in forward bias they are essentialy a short except for the .7 volt drop across the depletion region...(sorry for the tech talk but Magyver should know what i'm talking about here). And tenspeed, the voltage drop that they talk about is because there is a current flowing through a resistance (ohm's law). The resistor isn't there to produce the voltage drop, it's there to limit current and in doing so, produces a voltage drop. I too know what i'm talking about...I have (in about 8 credit's;) ) an electrical/electronics engineering degree! I'm not trying to start a pissing contest here, but I've been dealing with diodes (rectifying and leds) for sometime now and use them all the time at work. Since is doesn't look like either one of us are gonna agree...I'm done with the issue...and you still haven't apologized!!! :)

[99Grand/\mSE]

My fault forgot to take them they'll be up tonight.. was to busy watching the presidental race.

[tenspeed]

Originally posted by Bjornboy81
And tenspeed, the voltage drop that they talk about is because there is a current flowing through a resistance (ohm's law). The resistor isn't there to produce the voltage drop, it's there to limit current and in doing so, produces a voltage drop.

It looks like were looking at the same thing from different angles. When I'm using a resistor in a LED circuit, I'm measuring the voltage across the LED because it gives me the brightest output without smoking the diode. The current takes care of itself if the voltage is right.

[Bjornboy81]

Originally posted by tenspeed
It looks like were looking at the same thing from different angles.
That's a pretty good bottom line! You're looking at it from the voltage stand point, I'm looking at it from the current stand point...same $h!t different smell. Looking at the current is a more accurate description of what going on especially when it comes to diodes. Thanks for being civil and mature about the discussion too!

[magyver]

your not answering my question, how cum i can put an led on a 3.3 V power supply that cant produce 10A if need be...? i wont apologize, maybe for the squirrl nuts remark, but nothing else... from what you say, if i put that led on a 3.3V powersupply, it should blow up according to you... but it never does!!! thats why i am stubborn..

[Bjornboy81]

what kind of power supply (specifically)?...a lot are current limiting

[magyver]

wall wort... a small one, can handle like 750ma or so... but they dont limit the current based upon what load is on there... too cheap for that... its just true.. led's dont need resistors if using the same voltage supply...

[LordNor]

Originally posted by magyver
wall wort... a small one, can handle like 750ma or so... but they dont limit the current based upon what load is on there... too cheap for that... its just true.. led's dont need resistors if using the same voltage supply...

Granted they don't need it, but it is best practice. If for some reason you do have a spike it will help keep your led's and your powersupplu alive longer. Best practice is to put them in. It may cost a little more but can help if you're not producing an exact voltage.

[Jaysin]

heh, about 7 cents more if that