Electrical question (resistors)

Discussion in 'OT Technology' started by 91RS, May 21, 2006.

  1. 91RS

    91RS If this flag offends you... you need a history les

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    I figured I'd post this here so hopefully I won't get a bunch of dumbass replies. I'm trying to put LED turn signal lights in the mirrors of my car (I don't want opinions on this, I've already made the decision), and I'm putting 7 in each mirror. I know I need resistors so the 12v from my car doesn't explode them or set my car on fire or something. I bought 20 6000mcd LEDs and he included 20 1/4 watt 470ohm resistors. I asked him if I could run all 7 LEDs of one resistor for each side and he said only to run max 3 per resistor. Is this really true or should I use one resistor for each LED or can I use one resistor for all 7? I want to make this safe so if I have to run one per LED I'll find somewhere to stick the damn things.

    Thanks for any help.
     
  2. 7960

    7960 New Member

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    probably better here.......good luck
     
  3. FyreDaug

    FyreDaug lolswift

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    With 7 LED's if you run them in series off of a 12V signal you wont need any resisters... Theres enough of a voltage drop across each LED that you wont need a resister... However, if you want each LED to be wired parallel you would probably put 1 resister per LED... I would recommend series though
     
  4. 91RS

    91RS If this flag offends you... you need a history les

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    So wire them up like this:

    [​IMG]

    and I should be ok without a resistor? Here's the "specs" on the LEDs I bought:

    Product Description

    Emitted Color : RED
    Size (mm) : 5mm T1 3/4
    Lens Color : Water Clear
    Peak Wave Length (nm) : 465 ~ 470
    Forward Voltage (V) : 3.2 ~ 3.8
    Reverse Current (uA) : <=30
    Luminous Intensity Typ Iv (mcd) : Average in 6000 MCD
    Life Rating : 100,000 Hours
    Viewing Angle : 20 ~ 25 Degree
    Absolute Maximum Ratings (Ta=25°C)

    Max Power Dissipation : 80mw
    Max Continuous Forward Current : 30mA
    Max Peak Forward Current : 75mA
    Reverse Voltage : 5~6V
    Lead Soldering Temperature : 240°C (<5Sec)
    Operating Temperature Range : -25°C ~ +85°C
    Preservative Temperature Range : -30°C ~ +100°C
     
  5. High Voltage

    High Voltage Guest

    google search Ohm's Law. that will help you.

    -Zac
     
  6. FyreDaug

    FyreDaug lolswift

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    Yes wire each of them up like that... The only bad thing is if one goes out, they all go out... but if thats the case of one going out anyways, youll rip apart the whole thing to figure out what was wrong whether its 1 bulb or all 7.

    Make sure you hook them up right though, ive seen some LED's where 1 pole wasnt longer than the other and in order to tell with was pos and neg you had to really look inside... Harder on 3mm led's..

    You will be fine without a resister.. but if its too bright add a resister to it... go 75ohms at a time, for stepping I would say... but I doubt you would need to do that
     
  7. 91RS

    91RS If this flag offends you... you need a history les

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    Alright, I'll try that. I thought about the one goes out they all go out, but I hope that won't be a problem with LEDs. They're going to be behind a double sided mirror so I doubt brightness will be an issue either.
     
  8. deusexaethera

    deusexaethera OT Supporter

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    A quick google search turned this up. I do believe it is exactly what you needed to know. I'm gonna bookmark this one myself; presumably, any device can be substituted in place of the LED in this guy's resistor calculator, because any device will consume electricity just like an LED does. Given that this can tell you exactly what resistance to use to connect a single LED to whatever power source you want to use, I'd recommend hooking your LEDs up in parallel so you don't have to test each one individually to figure out where the problem is if one of your LEDs burns out on you. Hooking them up in series is begging for a "christmas-tree-lights" kind of problem.

    http://ourworld.compuserve.com/homepages/Bill_Bowden/led.htm

    ALSO: if you can't buy exactly the right resistor, you'll need to know how to combine ones you can buy to get the proper resistance. So, combinations of resistors work like this:

    Resistors in series: the total resistance = all of the resistors added together.
    Resistors in parallel: the total resistance = invert each of the resistances (1/x), then add them together, then invert the total again. I don't know why this works, but it does. Such is the way of the world.

    Given this, connecting your sets of resistors in series is obviously the easier way to do the math.

    Interestingly, capacitors are the opposite: they add together when connected in parallel, instead of in series.
     
    Last edited: May 23, 2006
  9. deusexaethera

    deusexaethera OT Supporter

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    Ooh. The double-sided mirror is going to be a big problem. The backing material will almost certainly block all the light from the LEDs. You're probably going to have to punch some holes in the shape of an arrow into a mask of some sort (cardboard, perhaps), then hold the mask firmly in place on the back of the mirror while you scrape the backing material off the spots where the LEDs will be. A small, round brass brush connected to a Dremel or an electric drill should do a good job. Something like this:

    http://www.dremel.com/en-us/attachments-and-accessories/attachment-accessory-detail.htm?H=188544&G=66994&I=66407

    Just go light on the pressure, or you'll scratch the glass too much. Though, it might also give the LED openings a nice frosted look...

    EDIT: If you don't have flat-tipped LEDs, you'll want to buy them or else sand down the LEDs you do have so they have flat tips. It will make mounting them to the back side of the mirror a hell of a lot easier; you can just smear some clear epoxy on the back of the mirror and press the LEDs into place.
     
    Last edited: May 23, 2006
  10. P07r0457

    P07r0457 New Member

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    why not shell out a couple hundred for the real-deal mirrors?
     
  11. 91RS

    91RS If this flag offends you... you need a history les

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    Thanks for that info, I tried that calcluator real quick and it wouldn't work (it said total voltage had to be greater than something). I'll try and mess with the stupid thing later when I have time.

    I've talked to someone who already did this to his car, I have to take the mirror off the backing and drill the holes in the backing and then glue the mirror back in place. He said he used Epoxy on the back of the LEDs to hold them in place and to make them waterproof.

    Because no one makes turn signal mirrors for a 91 Camaro. ;) I don't have a couple hundred to shell out anyway.
     
  12. 91RS

    91RS If this flag offends you... you need a history les

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    If I'm understanding that calculator right, if I hook the LEDs in a series I won't need a resistor, but if I hook it up parallel I will but I don't know what resistor I need because that calculator page is for series wiring. Since the LEDs I bought came with 20 and I only need 14, I'll take 6 of them and hook them up series and hook them to my car battery and see what happens.
     
  13. deusexaethera

    deusexaethera OT Supporter

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    It worked fine for me...

    [​IMG]
     
  14. deusexaethera

    deusexaethera OT Supporter

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    A single LED can be considered a 1-element series, so the calculator is still valid.
     
  15. 91RS

    91RS If this flag offends you... you need a history les

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    I put in 7 under the total LEDs (since that's how many I'll be using on each side) and it told me that the total voltage had to be more than the sum of the LED voltage, which I took to mean that I should be ok without a resistor like FyreDaug said. I tried connecting them up in a series real quick and then to a car battery and they worked fine, I got no spark, explosion, etc. :sad2: :cool:
     
  16. deusexaethera

    deusexaethera OT Supporter

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    I was recommending that you connect your LEDs in parallel, not series. There are two reasons for this:

    1. It's a more reliable setup, in that you won't lose the whole string of lights if a single one burns out, and you won't burn out all your lights if a single one has a short circuit and stops doing its part to control the battery's voltage.

    2. Each of your LEDs runs at a nominal voltage of 3.5v. Multiply that by seven, and you get 24.5v, or more than double the voltage that your battery can provide. (ignore the fact that alternators typically generate 14 volts for the time being.) What that means is that your LEDs, when connected in series, will be somewhere around half as bright as they would be if you connected them in parallel with the proper resistors in place.

    Given a parallel setup, each LED would need 270 ohms of resistance attached to one of its terminals in order to control the battery's voltage properly. Seven LEDs, seven resistors (or sets of resistors, anyway). It's very doable.

    EDIT: Actually, you only need have just three 270 ohm resistors (I just did the math, listed below) connected in parallel to each other so they split the amperage load between the lot of them. Then you'd send out seven wires to connect to your seven LEDs, then recombine the trailing terminals of the LEDs back into a single wire. That makes it even more doable.

    3.5volts * .03amps * 7devices = .735watts per mirror, or slightly less than 3x the rated capacity of a .25watt resistor. Like I said, three resistors in parallel will do the trick, but I suppose you could include a fourth resistor just to be on the safe side.
     
    Last edited: May 24, 2006
  17. 91RS

    91RS If this flag offends you... you need a history les

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    I understand what you're saying, but my problem is that I don't have much space and I figued that I (hopefully) won't have a problem with LEDs blowing anytime in the near future, in any other case I would probably do parallel on all of them since I'd probably have the room. I just went outside with the extras and played with them since it's dark and I could see them better, and whether I have 7 in a series with or without a resistor (470ohm) they are they same brightness.
     
  18. deusexaethera

    deusexaethera OT Supporter

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    Space is a concern, I guess. How big are your LEDs that you don't have room to spare for a pack of resistors half the size of a stick of Trident gum?
     
  19. 91RS

    91RS If this flag offends you... you need a history les

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    Sorry to bump and older thread but I need this again. Sorry for not replying to your last post deusexaethera, I didn't even know you replied since I didn't get an e-mail notification.

    I got all the parts in finally and I found out that I did have a little more room than I thought I had to work with but it's still a tight squeeze because I don't want to get something in the way of the mirror motor to where the mirror can't move properly or all the way in one direction. Anyway, after drilling the holes, I decided that I needed 8 LEDs so it'd look better. I used Epoxy and glued in my new mirror glass on both mirrors and I've only glued in the LEDs on the passenger mirror so I could figure out what I needed to do. I hooked them up in a series at first (I just taped the leads together) and it looked like I didn't get anything at all but when I looked really hard at the back of the LED there was almost no light coming out at all (with and without a resistor). Then I wired them up in parallel and put one resistor before the first LED and they all lit up but not very bright. I thought I was going to have to go parallel and use one resistor per LED, but I tried hooking up just one LED with a resistor to the battery and I could barely see it through the glass. So I need to do something to get them brighter. I'd still rather wire them up in a series because it'll be much much easier to deal with but since I don't know how this stuff works I don't know what I'd need to do to get these things to shine brighter. They're supposedly 6000MCD and I can barely see them. Do I just need to get a bigger resistor?

    EDIT:

    I just read your edit of the post above. So you're saying that the resistors I have will be enough if I put more than one resistor on the wire (one after another)? If so that would definately be doable in the space I have. I could just take one long wire and strip it and wrap it around each positive terminal and solder it on (same with the neg) and then put the resistors in the wire in some shrink tube so they're water proof.
     
    Last edited: Jun 25, 2006
  20. P07r0457

    P07r0457 New Member

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    Incorrect. If an LED goes out, the circuit will be open, and none will light. They will not cause others to go out, as the circuit would be open. Actually, parallel would be the problem -- if one goes out, the circuit remains, but with less current draw, which can lead to an over-volt on the circuit, which CAN damage the remaining LEDs.

    So you have it back-asswards.

    The proper answer is a collection of several small areas of series-wired LEDs with the several arrays wired in parallel.

    doable? Yes. Smart? Not really. Best? Definitely not. A complete waste of resistors, and a definite over-complication of a simple problem.
     
  21. 91RS

    91RS If this flag offends you... you need a history les

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    So you're saying that series is what I should use for this? What should I do about the resistors?
     
  22. P07r0457

    P07r0457 New Member

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    I'd say put 3-4 in series, then use two arrays in parallel for each mirror. Should work well based on approx 13.8V input, which is common to see in a healthy car electrical system. Actual input can vary from 12-14.4V.
     
  23. GOGZILLA

    GOGZILLA Double-Uranium Member

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    jollys got the right idea. from an electrical stand point you could make two series strands of 3-4 of the LEDs and apply 12V across both of them. That should be just about perfect as far as voltage goes and if one of the LEDs burns out your other strand will still work. The purpose of adding in resistors is to limit the current going through the LED. If you apply 12V across one of your LEDs youll have too much current going through it and itll burn out. V = I*R . Since you've got the power ratings youd do P = I*V . From those two equations you can figure out the best resistors to use but itll be the best idea as far as ease and electrical properties to do the two strand series hooked in parallel to the 12V.
     
  24. deusexaethera

    deusexaethera OT Supporter

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    Every piece of electrical equipment -- even a simple LED -- has an inherent resistance that cannot be eliminated (unless you cool it down to superconducting temperature, which isn't likely in this application). Electrical equipment doesn't draw more amperage than its inherent resistance allows. If you increase the VOLTAGE, then the amperage will also increase (because increased pressure results in increased flow), but since the voltage coming from the car's alternator is governed at around 14v, a voltage spike is not going to happen to the thread-starter's LED turn signals.

    If you have LEDs connected in parallel, with a resistor in series for the whole cluster of LEDs (ignore the fact that this guy can't get a resistor that will withstand seven LEDs' worth of load, for the moment) and one of the LEDs burns out, the rest of the LEDs are still going to be exposed to the same voltage because the resistance for each of the remaining LEDs will stay the same; the total amperage for the circuit will drop because the demand will drop.

    If you want a good example of this; shut off a few circuit breakers in your house and marvel at the way all your equipment DOESN'T explode due to the supposed higher voltage that Jolly incorrectly predicts.
     
    Last edited: Jun 26, 2006
  25. Doc Brown

    Doc Brown Don't make me make you my hobby

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    "Electrical equipment doesn't draw more amperage than its inherent resistance allows.

    If you have LEDs connected in parallel, with a resistor in series for the whole cluster of LEDs (ignore the fact that this guy can't get a resistor that will withstand seven LEDs' worth of load, for the moment) and one of the LEDs burns out, the rest of the LEDs are still going to be exposed to the same voltage because the resistance for each of the remaining LEDs will stay the same; the total amperage for the circuit will drop to compensate."



    That really seems to be the only real problem here.

    Added to the fact that led's are pretty durable, last for an ungodly number
    of hours, and he most likely wouldn't have one go bad for as long as he owned the car.
     

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