LED Electric circuit help

Discussion in 'Replica Props' started by Jedifyfe, Nov 6, 2011.

  1. Jedifyfe

    Jedifyfe Master Member RPF PREMIUM MEMBER

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    If I had a small 12v battery and wanted to run (4) 5mm LED's and (12) 3mm LED's plus a small EL sheet, what would the circuit need to look like? I need this in laymen's terms please...maybe even a picture. Thanks!
     
  2. Delmustator

    Delmustator Sr Member

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    Run them all in parallel. But you will have to step the voltage down with resistors unless everything is rated for 12 volts.
     
  3. Delmustator

    Delmustator Sr Member

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  4. exoray

    exoray Master Member

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    Need forward voltage and forward mA of the LEDs to answer that properly...
     
  5. Jedifyfe

    Jedifyfe Master Member RPF PREMIUM MEMBER

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    Thanks Del!
     
  6. Jedifyfe

    Jedifyfe Master Member RPF PREMIUM MEMBER

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    Looks like 3.3-3.8 ma
     
  7. Dave Porter

    Dave Porter Sr Member

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    You may need a lot of resistors there.

    I'd swap out the battery for something smaller.
     
  8. Jedifyfe

    Jedifyfe Master Member RPF PREMIUM MEMBER

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    the 12v is a small battery. What would you suggest?
     
  9. Knightjar

    Knightjar Sr Member RPF PREMIUM MEMBER

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    Did you choose 12v to suit the power requirements of the EL inverter? If so, it may be easier to power the EL from a separate supply.
     
  10. robstyle

    robstyle Sr Member

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    if your a cheap ******* and good at scavenging hit up the local dollar store for a cheap LED flashlight/flasher. Ive seen some with 24 super bright LED's and others with 12, 8, 4... Depending on the size of the EL sheet you should be able to run it as is off the existing scavenged parts. You may need to desolder, add lengths of wire to the board then resolder things but its a cheap and efficient jump off.
     
  11. Dave Porter

    Dave Porter Sr Member

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    Small battery in size, yes, but with a lot of juice.

    I have used SR44s in the past. They are 1.5V each, so you chain them together.

    You can do it with a 12V but you will need to bring it down or you will fry those LEDs in a second.
     
  12. Troy Downen

    Troy Downen Active Member

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    No, no - the 12V is perfect for your needs. Simply run 4 parallel arrays of 4 LEDs in series and you won't need any resisters. Shoot me a PM and i'll send you a quickie diagram of the circuit.

    Why does everyone always want to run individual LEDs in parallel and use all of those resisters? Not necessary...
     
  13. Delmustator

    Delmustator Sr Member

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    If you need another power source, I have about 50 of these..
    18650 Li-Ion batteries

    [​IMG]

    Soon, I'll have enough to power my house.. :confused
     
  14. Troy Downen

    Troy Downen Active Member

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    Okay, I had a request so I'll just post some sketches here. Sorry for the sloppy work, tho. :$

    First attached sketch is for running 4 LEDs in an array. The specific request is to run a total of 16 LEDs and an EL sheet. So I sketched two complete LED arrays (the two vertical strings of 4 upside down triangles, which are called diodes - that's what LEDs actually are) and then I got tired of sketching and just drew three more arrays with (...) in each array to represent the LEDs and the EL sheet. See? Sloppy work...

    In this first circuit you do not need resistors. Each LED is rated at between 3.3 and 3.5V. So each 4-LED array "wants" to be "fed" between (3.3 x 4 =) 13.2V and (3.5 x 4 =) 14V. You're only supplying 12V, so each LED will run slightly less bright than if you had a 14V battery. EDIT: note that for some LED types you may experience a noticeable degredation in brightness running at less than 3.3-3.5V. For this reason, your preferred choice may be the 3-LED array discussed next. I will confess that I don't know what the spec is for your EL panel, so I don't know if you might require a resistor for it or not.

    In the second attached (sloppily drawn) circuit diagram I show a 3 LED array with a resistor. If you're REALLY worried about running your LEDs without enough voltage, you can step down to only running 3 LEDs in each array. Now you'll need to construct 5 arrays each containing 3 LEDs plus another array with only 1 LED, plus another array with your EL panel.

    So in this second circuit you DO need resistors. Each array requires a 75 to 100 Ohm resistor. This is because the 3 LEDs in each array only "want" to be "fed" up to 3.5 x 3 = 10.5V. You have to find some way to "consume" the remaining 1.5V (or slightly more if your LEDs are rated at 3.3V) that the 12V battery is supplying, otherwise you'll burn up the LEDs. So that's why you insert the resistor in this circuit. 75 to 100 Ohms running at 20mA current will produce the needed 2.1V or 1.5V. Realistically you can simply choose 100 Ohms (a common rating) and be done with it. Note that I've assumed that each array will be running a current of 20mA. That's typical for LEDs and should be typical for the circuits that you'll be constructing here.

    For the array where you only have 1 LED at 3.5V, you'll need a resistor for that array that can provide 8.5V (12V battery - 3.5V LED). So you'll need a 425 Ohm resistor (or thereabouts; anything from 400 to 500 Ohms should work well; your LEDs can run a *little* "hot"). Again, I don't know what your EL panel will require.

    For those new to electronic circuits, I've uploaded a third diagram which shows how the second circuit would be wired. Note the polarity of LEDs: the longer lead from the LED is the positive lead. The peanut shaped things at the top of each array are the resistors.

    PM if you have more questions. It's not difficult - it just takes a bit of experience and a few "smoked" LEDs to get the hang of things! Good thing LEDs are cheap! :lol
     
    Last edited: Nov 7, 2011
  15. Troy Downen

    Troy Downen Active Member

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    Tip: don't use that heavy 22 or 24 gauge wire that everyone seems to use. It's bulky and gets in the way and really requires that you solder it to the LEDs. Ugh. Use "wire wrap" wire instead - it is more than sufficient to handle the mili-amps of current that you'll be using to run LEDs. EDIT: Don't run more than approximately 140 mA current in the 30 AWG wire wrap. No problem for these little 20 mA arrays, tho.

    If you're in the States then go to Radio Shack and pick up a wire wrap tool (see attached photo) and a spool of "wire wrap" wire. It will look too thin, but trust me - this is exactly the stuff that you'll need. Use the wrap tool to produce nice, snug wraps of wire around your LED leads (don't forget to use the included wire stripper to strip off the insulation on the wire wrap!). See the second photo for an LED with the wire wrap around the leads. No soldering required - this is a good, tight fit!

    I'm extra conservative in building my circuits, so after wrapping my LEDs I also put some 1/8" heat shrink wrap around each lead and shrink it into place to prevent any accidental shorts in the circuit. (see attached photo) You can get the 1/8" heat shrink wrap at Radio Shack or, a bit more economically, at Lowes or Home Depot in the electrical aisle.
     
    Last edited: Nov 7, 2011
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  16. Jedifyfe

    Jedifyfe Master Member RPF PREMIUM MEMBER

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    Thank you Troy. So you are saying I won't need ANY resistors?
     
  17. Troy Downen

    Troy Downen Active Member

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    If you run the 4 LEDs per array, that is correct with the exception being the array with your EL panel. You'll need to look at your spec for that element and see what it's upper voltage limit is. If its less than 12V then you'll need a resistor for that element only.
     
  18. Troy Downen

    Troy Downen Active Member

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    I'm going to revise my thinking on this after reviewing some current - voltage diagrams for LEDs this morning. Some LEDs can experience significant degradation in current draw with reductions in supplied voltage. Thus, running the array of 4 LEDs at 3V each might produce a measurably dimmer LED as a result depending on the LEDs used. In light of this, I would recommend the 3-LED arrays with a 75-100 Ohm resistor just to ensure that you get the bright lights that you want. Sorry for the misdirection!
     
  19. Rebelscum

    Rebelscum Sr Member

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    I love the wire wrap tip. I'll definitely use that in the future.
     
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  20. Troy Downen

    Troy Downen Active Member

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    Wire wrap makes it SO fast to wire up a circuit. Or un-wire it if needed.

    One note, which I've amended to the above: don't run more than approximately 140 mA current in the 30 AWG wire wrap. No problem for these little 20 mA arrays, tho.
     
    Last edited: Nov 7, 2011
  21. DrewSmith007

    DrewSmith007 Well-Known Member RPF PREMIUM MEMBER

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    This site will tell you all you need to know about your LED circuits, however you need the datasheet of the LED's you're using.

    LED series parallel array wizard

    Forward voltage is sometimes listed as voltage drop.

    Below is an explanation of how that calculator arrived at its value. You don't need to know this stuff, but I thought I'd put it in there for those of you who want to know it.

    The voltage drop is important because it tells you how many LED's can be run on the same strand, called "in series". Voltage drop adds together with each LED in series. If you have a voltage source of 12v, 5 LED's, and your voltage drop is 2.2 for each LED (11V total), then you have a net voltage of 1 volt. This net voltage is what is used to calculate the resistor needed for that strand. If the LED's are rated at 20mA (.020A), with your 1V (net voltage) you would need a resistor value of:

    Ohm's Law: Volt (V) = Current (A) X Resistance (Ohm)

    Rewritten to solve for Ohms: Ohms = V/A = 1/.020 = 50 ohm resistor

    Since 50 ohms is not a common value, you would move to the closest higher value available. This will reduce the current a little, but it shouldn't affect the brightness of your LED much, unless you go much higher in resistance. You don't want to move down, as this will allow more current to flow than the rated 20mA, and could cause the LED's life to shorten.

    So, in this case the most LED's you can run in a single strand is 5. If you need more than 5, you just put them on another strand. Remember that if you have fewer than 5 LED's on the new strand, the net voltage will be higher and therefore need's a higher resistor to limit the current to 20mA.

    Following these rules will give you the brightest LED's possible, that will last the longest and give the longest battery life. Sure you could do 5 strings of single LED's with a 560 Ohm resistor for each LED, but that requires 5x20 = 100mA of current, which will last 1/5as long for the same result.
     
    Last edited: Nov 7, 2011
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  22. Delmustator

    Delmustator Sr Member

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    I hadn't seen anyone do wire wrapping since my days in a telco terminal. This is how ALL wire connections are made in a central office on wire wrap blocks. Back in the day, we had thousands of these. It definitely works if you can find a wrap tool. I used to have a wire wrap gun that was electric (kinda like a drill). I could completely wire wrap ABAM cables between a DSX panels and M13 shelves in about 2 hours. If you never seen an ABAM cable, it's 30 pairs of 22 gauge wires per cable.
     
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  23. Troy Downen

    Troy Downen Active Member

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    Teleco is about the only place where they still use wire wrap, I believe. But they use the heavier gauge wire. Hard to believe, but some of the Apollo program computers on the ships that went to the moon were connected entirely with wire wrap. Maybe it's a lost art? I dunno... but I'm just now finishing up a Star Wars Star Destroyer lighting project using 30 AWG wire wrap to hook up all the LEDs. It sure makes for fast work.
     
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  24. exoray

    exoray Master Member

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    High quality PC boards that can better and more reliably route the signals are more cost effective and can be made smaller and quick, so yeah it's a dying art... When you toss in very high quality stab-lock connections it even pushed the death faster...

    It has it's place, but new tech has simply replaced it in almost all application...
     

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