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LED Design Info

Old 01-28-2007, 04:18 AM
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Default LED Design Info

I posted this on another forum and thought I would paste it here as well to answer some of the questions people ask. The first part gets into a mild amount of technical detail (not too deep, but deep enough that many may just want to skip to the end) and the end has some examples:

With everyone doing so many led mods these days, and asking so many questions, I thought I would put up some quick info on leds and thier operation. As always, I dont mind answering questions, but these days it is hard to keep up with the pms I get for that and led swap stuff, so maybe this will help. Forgive me if it is already there, I didnt see it when I searched.

LED concepts:

An LED is simply a diode that emits photons of a particular frequency when a current is passed through it. The various frequencies related to different colors. The color is dependent on the material the internal junction is made from. Which brings us to how a diode works.

This is a simplified description, but should get the basic principle across. The junction within a diode is what makes it work the way it does. There is a P and N side to the diode. If the P side (anode, or positive) is connected to a positive voltage, and the N side (cathode, or negative) is connected to a negative charge in relation to the anode, current will flow. This is why they are polarity dependent.

An led can be thought of for the most part as a constant voltage device. There is a minimum voltage required to start current flow. Once this is met, the device acts almost as a short. That is why it is more accurate to design around current with these devices rather than voltage. The voltage needed depends on the junction materials (which, if you think back, relates to the color).

Blue, super green and white require a higher voltage to start current flow than do red and amber leds. This is why we have to mod some circuits based upon the color used.

When designing with leds, you can think of them as a point of constant voltage.

For example, lets say you have a blue led requiring 3.2V @ 15mA to provide the output you need. IF you are connecting to a 12V source, you effectively have 8.8V left after accounting for the LED voltage drop. That means the resistance in the circuit has that much voltage applied across it. Using ohms law, you see that if you have no resistor in line, you basically have a short. Current equals voltage/resistance, so here you have (8.8V/nearly zero ohms).

To state ohms law, with I equalling current, V equalling voltage and R equalling resistance: V = IR . So you can see you can re-arrange to get what you need.

In our example, you have 8.8V (after taking out for the LED) and you need 15mA. So, you have: 8.8 = .015 * R => R = 586 ohms (or the next closest value you can find).

If you have leds in series, their voltages add up.

Here is one warning. If you place LEDs in parallel, ALWAYS design so that each has its own load resistor. NEVER connect leds in parallel to one another and tie them all to one resistor. A failure of one can cause the others to operate improperly, or damage one of them. And due to the characteristics of leds not being 100% matching, they will not operate exactly the same way. That is why I do not reccomend the HVAC mod that simply jumpers some leds in parallel without adding a resistor for each one.

I realize that those with no electronics experience may not get all of this or want to for that matter, but the basic understanding will help you choose what mods to do and how they should be done, as well as explain why the HVAC needs to be modded for the other colors.

A note on that part. Stock, the HVAC has every 3 leds in series with an 8V supply from the board powering them via a load resistor. When you switch to blue (requiring about 3.2V) the issue appears. If you take the added voltage (9.6V) of the LEDs and subtract from the source voltage, you see there is nothing left to provide current flow. So you end up with them terribly underpowered and dim (or not working at all). The purpose of the mod is to re-arrange things to make it work.

One last example to make it clear:

Lets say you have some blue leds rated for 3.2V at 20 mA typical. You need to run a bunch of them off of your 13.8 V battery source. If you run each of them separately (in parallel. usually if they are not going near each other) then you figure this way for EACH led:

Subtract the led voltage from the source: 13.8 - 3.2 = 10.6V
Divide the remaining voltage by the current needed: 10.6/.020 = 530 (size of resistor needed)
Chose nearest common resistor size UP from value. This is 560 Ohms in this example. This means each led has power connected through a 560 ohm resistor, then to the LED, then to ground.

If you are placing the leds near each other (like on a project board) and can run some of them in sets (series), then you first need to determine how many in series you can run. I like to leave at lease 1V under the source voltage, but if they need to run when the car is off (12V) then you may want to leave more room for variation. So for me I would put them in sets of 3. So every three can be in series with a connection to battery and ground.

So: 13.8-(3*3.2) = 4.2V
Current is the same throughout each series branch, so if you need 20mA, then you have 4.2/.02 = 210 Ohm or nearest typical match UP in value. Then connect battery positive to a 210 Ohm resistor, then the other side of the resistor to the first LED in the series, then from the third led cathode to ground. Each branch of three will have its own resistor.

I will try at some point to make up some schematics to demonstrate if needed.


Anyway, I hope this will help some in the modding they do, or at least help some understand what is involved when they have thiers modded.
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Old 01-28-2007, 04:24 AM
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yay!

lol... good info Dave
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Old 01-28-2007, 05:02 AM
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Great post Dave! Might clarify some confusion for some uhmmm... people.
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Old 01-29-2007, 01:04 AM
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This should be made into a sticky so that others will not ask about it!
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Old 01-30-2007, 03:28 AM
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It's really nice to have professionals in forums! :D
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Old 02-25-2007, 03:37 AM
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Default Re: LED Design Info

Originally Posted by engifineer
To state ohms law, with I equalling current, V equalling voltage and R equalling resistance: V = IR . So you can see you can re-arrange to get what you need.


"I" = current ?

I thought ohms law refers current to "A" for amps.
I know that's the term for "current" in automotive applications.
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Old 02-25-2007, 03:43 AM
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A does equal amps. An amp is a unit of current. So I = current, A = the unit used to specify a unit of current. Ohms law specifies current, which is denoted by I. To actually be 100% correct, it is E=IR, with E being voltage potential. But people have substituted the unit for potential, V, for so long that it just gets written that way most of the time.

Sort of like mass and gram. Gram is a unit of mass.
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Old 02-25-2007, 03:56 AM
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^Gotcha.
You know your sh*t.
Sorry for challenging your intellect.

Great stuff.
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Old 02-25-2007, 04:01 AM
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Oh, wasnt taken as a challenge at all, people should always question if they have doubt. Glad I could help!
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Old 09-03-2007, 01:45 AM
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Very nice post indeed !!!
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Old 12-15-2007, 12:51 PM
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Are you good at installing LED's? I'm trying to install LED's in my cup holder...which way would be the easiest way? This is my first time doing anything like this but I want to give it a shot...
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Old 12-15-2007, 05:04 PM
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hmm.. drill holes, mount leds, and for convenience, the cigarette lighter is right there to tap power from.

wire to switch if preferred, and tada.
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Old 12-15-2007, 05:35 PM
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thanks for the detail [email protected]$$
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Old 12-15-2007, 05:38 PM
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hey... you asked.

here's a finger in the right direction if you're still having issues.
https://www.scionlife.com/tech/

for future reference, i'd tone that attitude of yours a bit down, as there are plenty of other people on this site that will put you in your place alot more blunt than i have.

welcome to the site.
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Old 12-18-2007, 08:57 PM
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well if you have nothing else better to do go ahead...I'm just looking for info so I can learn how to do this stuff...
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Old 12-30-2007, 10:05 PM
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Hey...Jus need to know how to install the LED lights with the interior lights, so when my door opens, they will go on with em. I tried to search, but nothing explains how to tap into the wire or anything! Any help would be greatly appreciated!!! Thanks!


Oh and dboyd4, no need to be smarty w/ these guys...they will help jus as long as u dont come off like an a$$. Give respect to be respected!!!!! Jus a lil friendly advice!! Have a good day!
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Old 01-28-2008, 09:03 PM
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If you want the LED to be a bit brighter, is it safe to lower the resistance a bit, or will that cause the LED to fail/break?
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Old 01-28-2008, 10:39 PM
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Originally Posted by DarkOmegaYoshi
Hey...Jus need to know how to install the LED lights with the interior lights, so when my door opens, they will go on with em. I tried to search, but nothing explains how to tap into the wire or anything! Any help would be greatly appreciated!!! Thanks!
Tap into the dome, or any light that turns on then the doors are open, for your power source
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Old 01-29-2008, 01:38 AM
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Originally Posted by nominal_asu
Originally Posted by DarkOmegaYoshi
Hey...Jus need to know how to install the LED lights with the interior lights, so when my door opens, they will go on with em. I tried to search, but nothing explains how to tap into the wire or anything! Any help would be greatly appreciated!!! Thanks!
Tap into the dome, or any light that turns on then the doors are open, for your power source
Hey guys I remember by b/f saying it was harder than he thought when he did this for me, here he is...

I thought it was that simple too when I went out to do it for her (see sig- custom footwell LED w/ 3 way switch on-off-door).

I started taking random readings and found a wire that would read 0vdc while I held the door switch closed, and would slowly build up to 12vdc when the dome light would fade off.

I was confused there. Then I googled the tC wiring schematics and saw what they had going on. There is constant 12vdc at the dome light, hense being able to turn the light off with no key in the on possition. The door switch tells the body ECU the door is open, and the wire I had been reading (now the igntition LED) reads 0vdc, so I went to ohms, and it read 70ohms to ground.

The body ECU grounds to dome light and the ignition light/LED to ground (with 70ohms resistance) and to slowly dim them to off starts to turn this ground to 12vdc (rather than increasing resistance to the point of being an open). So the LEDs I used need 220ohm resisters for 12v, so I bought 150ohm to put on the constant 12vdc side of the LED, and the car handles the rest (70ohm) automatically. I then used a 3 way switch that looks like an OE switch to give the LED lights the option of working when the door is opened like teh dome light, or on all the time, or off all the time.

This was the reverse of how I thought it would work (countering the 12vdc with 12vdc to create what one would think is an open), but it makes sense I suppose, with electronics there is always more than one way to do what you want.

Hope that helps. I guess if this hasn't been covered around here before my girl is the first to sport this, or at least one of the very few?

B.
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Old 01-31-2008, 04:46 AM
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good info but man this led stuff is hard to do
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