darrick

if I by leds from the shack what do I use to step down the voltage ?resistors ? what size? did a search or LEDS but found nothing that answers this ,thanks guys

pocketrocket

iTrader: (1)

V = I*R
(Voltage = Current * Resistance)

You figure you're running say 12 - 13V in your car (I believe it should be 12.8 V).

Assuming you're planning on running LEDs directly off your 12+V volt system:
You find the LED you want and look at the voltage rating & current on the package.
LED volltage, Vled = x
Led current = Iled = i

Resistor voltage, Vres = 12.8 - Vled

Resistor value, Rres = Vres/Iled

darrick

what size resistor? the leds are 3.7 volt

Ramius83

Well, it depends where you are gonna put the LEDs at. IF you are gonna replace existing green/amber LEDs in your A/C Module or Window Switches, than a resistor is not needed since there are already some in place on the boards. Where are you gonna be putting the LEDs at.

darrick

in my cig lighter,key ring light thing and the small lamp over my radio

pocketrocket

iTrader: (1)

You're probably replacing regular bulbs (12V).

If you use Radioshack's 3.7V 20mA LED, I believe a 330 ohm resistor should do the trick.

Or may be a 470 ohm? Ahh, decisions decisions.

darrick

some has to know for sure come on guys

«§»Craig B«§»

iTrader: (5)

those aren't LED's, there minilamps like you said

radio shack part 272-1092 or 900-xxxx (I forget the second part of that number)

no resistor needed

search for "1092" and you'll find lots of info about those

darrick

listen Iam installing leds to replace the minilamps I just want to know what size resistor I need the car is puting out like 13 volts or so the leds is going to be 3.7 volts I am trying to do a custom install I want to do it with stuff I can get from radio shack ,i dont want to order stuff online and wait for it to get here I have read threads that ppl say they put leds in and changed the leds in there so i know somebody out there has thought of this before.

MIXXMAX

iTrader: (1)

I believe the LED's are a direct swap, if your putting them in the climate controls. No need for resistors because they are already in the circuit board. Beware, when you take the unit apart, make sure you have some patience, because this takes some time if you do it right. In fact, I just posted a thread the other day on this in the 4th generation.

White93SE

why dont you PAY ATTENTION. all the information you need has been presented to you.

the bulbs you are replacing with your LEDs do not require resistors. if you were to tap in to a 12+ then you'd need to use a resistor or a voltage regulator.

read damnit, do you want us to install the leds for you too?

darrick

you read dumb a$$ they are 3.7 volt leds they are replaceing the 12 minilamps they need resistors. Dont talk when grown folks are talking .

White93SE

Pull out your multimeter and check it. I've replaced my minilamps without the use of resistors or voltage regulators. When you figure out V=IR means, then you come talk to me.

Ramius83

I second that, please use a multimeter and run it across the voltage area of the lamps. I temporarily replaced my lamps behind my A/C lighting when they burned out and they ran fine for over 5 months. But I replaced them back with the lamps because the LEDs didn't disperse the light that was needed to even light up the A/C module. I assure you that the lamps you are wanting to replace are already regulated down to a safe voltage to use a LED. And even if they are not, you can go on eBay or any "electronic store website" and buy 12V LEDs that are designed specifically to use in cars, ie. www.radioshack.com or others of that nature. Please do not get upset with us, we are only trying to help out here.

darrick

but I explaned I wanted to use parts I could get localy and 12v leds are not ,I am sorry to rest of you to have to see me rost White93SE in front of you guys but I am sick of seeing folks get flamed by guys like this he thinks since he knows It all everyone else should so why is he here In a forum?this should be a place where ppl who need a little help can come ask a question (one they have did a serch for)and get some positive input .I think that if you humbel yourself enuff to ask a question then you should be treated fairly but there are some ppl here that see asking a qustion as a sign of weekness,witch is by no means true I come here often,a few times a day,and I love this board but it justs get old when ppl who think they know it all and keep rosting newbies dont have much of a life if you dont know the answer to the question dont respond I just wanted to replace the minilamps In the key-light thing, cig lighter,and the other small lamp above the radio (not the one in the ac unit) but I gess that its impossable to get a strait answer so here so you guys win I give up.

DanNY

iTrader: (14)

well here's the thing...i sat back and waited to see how many people would know this answer.
no one would know the answer because you didn't give enough information about the actual LED you're using. you want to run LEDs..you need to give us the actual voltage (which you said 3.7) and it's power draw (amps). that lamp you're speaking of is a 12V lamp. for all you 4th gens guys..it's NOT a LED in there..it's a mini 12V light bulb...love how u guys beat on him even thought you were dead wrong.

darrick...now don't go and take the "newbie and we get abused stance"..because you well know that's BS. how can we give u a answer if you don't give us enough info. which leads me to believe you really don't have a clue wtf you're talking about in terms on LEDs.

the best answer that was given was from pocketrocket..he gave u the answer RIGHT IN FRONT OF YOUR FACE...he put down the formula on how to figure out the the resistor you need...but you just fail to understand it. you just wanted someone to give u a flat out answer like "oh it's a 10K resistor..part number blah blah at $.15 cents a piece at radio shack"..guess what...you need to get that yourself. we can only help the ones who help themselves.

now take this post as u may..but you need to do a little work for people to help you on here.

Ramius83

Like I said, then try to get a 12V LED to use in place of those bulbs. I did say that they were bulbs, ie. lamps. What would be the best way to mount them in there is to get a 5mm LED light and take out those bulbs. Then "force" the LED in the "hole" where you took the bulbs from. It will be a perfect fit. But do know that the light will not be evenly distributed across the module. So the areas right in front of the LED will be much brighter than the areas away from the LED. That was the reason why I replaced my LEDs back to the original lamps. But I did give a correct answer, try using 12V LEDs.

darrick

Ramius83 thank you for your help but im not replaceing the lamps in my ac unit just the cig lighter ,key ring lamp . now DanNY we dont know each other but we do know each other I read something really smart you wright everyday you are right I dont know a GD thing about leds voltage,resistance nothing at all if I did WTF would I be asking this ,a simple eletronic math question if I did ? come on man you are a really smart guy and I know you know better then this.that is what this place is for right? not askin a question thats be asked a million times am I? I just wanted a little help .
White93SE just got under my skin and I am sorry if any one other than White93SE took what I was saying to heart it was all directed his way I dont have a freaking clue about the resistor needed of the math needed to do what I wanted done I hope he takes comfort in knowing I did not get helped and I will have to go pay some ripoff artist to do this mod for me ,Thanks guys

maximus75

iTrader: (5)

Guys, that formula isn't going to tell you what size resistor to use to make a 3.7V LED work with a 12v system. That fromula will help you find values for an allready functional circuit. You can use it to find a missing value (V,I,R) in a circuit.
Darrick, EM me @ Maxximus75@msn.com There are a few different ways you can wire your LED.

pocketrocket

iTrader: (1)

I disagree. LEDs come with current ratings (just as DanNY said).

I know this because I was in Radioshack just last weekend working out resistor values needed to use LEDs in my car.

If he had the current rating (e.g., 20mA), you could determine how much resistance value you needed.

Not trying to be a smart as*, I just believe I'm correct.

darrick

ok

Albertt

As quoted from Stan Jirman or Audiworld.com:

"How do I pick the right resistor?

Good question. First of all, a resistor is specified for a given wattage (W), current (A), and / or voltage (V). And of course the resistance. The reason why I don't specify W, A, and V is that pretty much any resistor will do. We will be drawing so little current at such a small voltage, that the tiniest resistor will absolutely do (we will be dealing with less than 200mW, less than 1/4W). The tolerance is also negligible for our purposes.

The only thing that matters is the resistance. This value of the resistor depends on two things. First, the resistor must have a minimal value because otherwise you will fry your LED. This value can be computed from the voltage which we are supplying, the voltage the LED can handle, and the current the LED takes. We will be tapping into the sunroof power wire, which leads around 4V (3.8 - 4.2) as strange as it sounds. The general formula is:

R = (U - UL) / IL

where R is the minimum resistor value, U is the voltage we are supplying, UL is the maximum voltage of the LED (marked on the box), and IL is the current of the LED (also on the box). For instance, my Radio Shack super bright LED lives at 2.4V at 20mA. If we are feeding it 4V, the minimum resistor value is (4 - 2.4) / 0.02 = 80 ohm. If you go below this resistance, bad things will happen.

Now this is the minimum resistor. Thing is that the super bright LED's are really bright, and we probably don't want that much light. So by increasing the resistance we decrease the current through the LED and it gets dimmer. My LED wanted a minimum of 56 Ohm, and I ended up with 270 after finding 560 too dim. You should experiment."

Hope that clears things up a bit.

maximus75

iTrader: (5)

Hey, thats ok. Mybe you know somthing I don't. I've always used this formula to find Resitance or current in a funtioning circuit. I don't know how it can be applied to a circuit that can't operate because the load is to small for the supply. Maybe you can help then. LED voltage is 3.7V@20mA. What value resistor is needed to have it operate in a 12v system?
Thanks.

maximus75

iTrader: (5)

Well now, that's a whole different formula This is the way I've alway's done it too and I've come to the same conclusion that the resistor value the formula gives you usually needs to be tweeked a little to obtain the correct light intesity.
BTW the 3.7V LED can also be wired 3 in series without a resistor and there not too bright.

Albertt

When you wire components in series, you are dividing the voltage by the sum of the components.

pocketrocket

iTrader: (1)

Vcar = 12V
Vres = 12V - Vled = 12-3.7 = 8.3V

Rres = Vres / I = 8.3/0.020 = 415 ohms

Pres = VI or I^2*R = 8.3*0.02 = 0.166 W (get a 1/4 W resistor so you don't fry it).

maximus75, I'm not too sure what you mean by "a circuit that can't operate because the load is too small for the supply".

A word though, car voltage is typically above 12V, I believe it's regulated just around 13-14V. If you've got a voltmeter, start your car and check the voltage in your cig lighter outlet. If you change the 12V number for the car's voltage (Vcar), you'll be able to recalculate the resistor you need.

Also, the LED has a normal voltage (propably the 3.7V) and a maximum voltage. Don't exceed the maximum or you'll be looking at $2 LEDs burning out quickly.

pocketrocket

iTrader: (1)

Albertt's right.

If you want, you can limit the current, thereby reducing the voltage & current and therefore, brightness.

I suggest this though: calculate the MINIMUM resistor value needed to prevent damage to the LED (i.e., full brightness), then get a potentiometer (variable resistor), a cheap breadboard and increase the pot value UNTIL you have the brightness you want.

Then, measure the pot's resistance value, add it up with the minimum and you've got the resistance calue you want. A lot of work, but if you want to get the ideal intensity level, that's what you got to do.

maximus75

iTrader: (5)

I just meant that a 3.7v LED (load) would not operate on a 12v system (supply)

Albertt

I'm not sure I follow your train of thought.

nubiannupe

iTrader: (9)

This is beginning to sound like my MECP certification all over again...

But I'll admit, it's ALL fairly good info, if read correctly.

pocketrocket

iTrader: (1)

It would (for a very short while until it fries) or
it would if you limit the voltage drop across the LED so it's close to the 3.7V it was designed for. You do this by introducing a resistor to basically divide the 12V between the LED and resistor. The value you pick for the resistor will determine how the voltage is divided between the two components.

It's not that hard a concept to grasp but I'm not planning to make this post an electrical lesson. Search the internet or pull a circuit analysis book from the shelf. I'll even explain it by e-mail if anyone's interested though I think a few of the previous posts did a good job of explaining it.

No point frying $2 LEDs every single time. My 0.02

Now, I only wish that Radioshack wouldn't run out of those 311 LEDs

multiplexor

hmmm this is such a long thread for something so simple... yikes!

to down 12 volts to 3-4 volts, you'll need something like 450 ohm resistor... around that area.

R=V/I

R=(12-(Voltage of led)3V)/.02
12-3=9

R=450 ohms

Thus R=9/.02
4Volt Led: 8 /.02 = 400 ohms
3.5 volt led: 8.5/.02 = 425 ohms


.02 is for the led... it needs 20milli amps i believe...

then you go here: http://webhome.idirect.com/~jadams/e...esist_calc.htm
and you can check out what colors to look for for the resistor.
The resistor can be a 1/4 watt resistor.

you can goto any electronics shop and ask for a 450 ohm resistor.
then just wire it up before the led, and you have yourself a led whatever you want...

pocketrocket

iTrader: (1)

But if we kept the knowledge to ourselves, it would slowly fade away and no one would know why the LEDs keep frying. Heck, people wouldn't even know what LEDs stood for, kinda like RSVP.

Thanks for the pics multiplexor, I'm waiting for my LED bulbs in the mail so I can put them in my meter.

darrick

thats what you were trying to do,ok ,oh well thanks multiplexor you da man

multiplexor

hehehe hmm... true true... (deletes post previously made.....)



you're going with the autodynamic led bulbs?
getting all 5 i assume

dwapenyi

iTrader: (3)

I don't know if someone suggested this, but you can get 3 LEDs and wire them up in series. 3.7 + 3.7 + 3.7 = 11.1 volts or 4 LEDs at 14.8 volts. I would lean towards 3. No need to add a resisitor.

DW

Albertt

Wouldn't the first LED see 12V, the second (12-3.7) 8.3V and the third (8.3-3.7) 4.6V?

I figured the best way would be to wire then three LEDs in parallel, with one resistor in series to the three LEDs. This way, the voltage would be dropped down by the resistor, provide the potential to the three LEDs and allow them to divide the current amonst themselves.

multiplexor

exactly... you can't have 3 leds and hope the power drops per led...
all the leds will blow... I've tried on **** leds i had lying around my house, as an experiment... sure enough they blew. 12 volts will go to each led. you would have to have a resistor before the leds. Then you can wire up the leds anyway you want cause you know theres only going to be 3 volts coming from the wire, not 12

dwapenyi

iTrader: (3)

Yes, and the voltage difference, 12 - 8.3 = 3.7 volts, is the voltage the 1st LED would work with. the 2nd LED would have 8.3 volts on one end, and 4.6 volts on the other, 3.7 volts being the difference going across that 2nd resistor. Putting the LED in series divides the voltage.

What made me think of this was my very basic EE skill and construction in NYC subways. The voltage that the trains runs on is 600V. You ever notice how the workers have these lighting units, a light bar that is just 5 standard 120V light bulbs? Same ones we all use in our home. Well, if you add it up, 120 x 5 = 600V

DW

multiplexor

In a led scenario, led1 would blow, and then led2 would blow, and then led3 would blow


easiest thing to do in wiring up multiple leds, is to have the 2 main wires, with one wire having a resistor attached to it. then wire up the leds anyway you want, and use the ~3 volts coming from the wires. otherwise 3 leds will burn.


i think with the light bars you were mentionning, it might work differently with AC, since each connecting device might lower the power. But I think with DC, the voltage will remain the same no matter how many devices are connected to it.