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Passing High Voltage/Amperage Through Tri-State Buffer (car LED project)

  1. Dec 18, 2014 #1
    Hi guys,

    I'm a total greenhorn so please bear with me. I'm designing a circuit in which I use a lot of tri-state buffers. All ICs in the circuit will be powered with 5V, but I need to pass 12V (and up to 15A) from the input to the output of the tri-state buffers when the buffer is enabled. Are there buffers I can use that can handle this type of power?
     
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  3. Dec 18, 2014 #2

    berkeman

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    Welcome to the PF.

    Can you tell us a bit about the application? Why so much power? And why the need for the Tri-State feature? What switching speeds for the propagation delay through the buffers as the state changes Low-High-Low? What bandwidth are the digital signals you are sending through the buffers?

    In the end it may be best to use Analog Switches, but we'll need to know more first...
     
  4. Dec 18, 2014 #3
    Basically what I'm making is a controller for LED's (in an automotive application) and it has to be able to power a lot of them! I selected tri-state buffers as a way to connect multiple inputs to the same output and select which input is used based on input to the device. I'm really not sure what I need for switching speeds, I probably don't need anything particularly fast. I've never designed anything like this before and a lot of this is completely new to me. I'm learning as I go. In terms of signal bandwidth, only one bit will be passing through any buffer at any given time. That is what you mean by bandwidth, correct? I hope I answered your questions adequately, I appreciate the help!
     
  5. Dec 18, 2014 #4

    berkeman

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    Thanks, that helps. So you have an LED drive application that will involve moderate power. Have you done much reading yet about LED lighting and controllers? To save power, you would control the LEDs typically with PWM current sink circuits. You would do your logic stuff before the final drive circuit, and control the PWM signal for each string of LEDs using the final logic information.

    Can you say how many LEDs you want to control, and in what kind of groups?
     
  6. Dec 18, 2014 #5
  7. Dec 18, 2014 #6

    jim hardy

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  8. Dec 26, 2014 #7
    I'm building a digital device for controlling LEDs. I need some help figuring out the correct transistor for switching use. The control circuit will run on 5V (and that will be the voltage applied at the transistor base when current is to flow through) and the transistor will have to be able to pass 12V and up to 15 or 20A from the collector to the emitter. I haven't worked with transistors before (I'm an electronics greenhorn) so any and all help is appreciated!
     
  9. Dec 26, 2014 #8

    phinds

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    How many LEDs are you controlling that you would need 15 to 20 amps ??? And why 12V? LEDs activate on much lower voltages.
     
  10. Dec 26, 2014 #9
    That is the typical first response! Ideally I want to be able to control about 1,000 LEDs. This control unit will be used for automotive applications and so the LEDs being used (having also been set up for automotive use) are run at 12V.
     
  11. Dec 26, 2014 #10

    meBigGuy

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    It's meaningless to say "I want to control 1000 12v LED's and need a transistor".

    You want to control them how? Turn them all on at once? Turn 1 at a time? Groups of 100? Flash? Variable brightness with PWM?

    Also, Post a link to the LED spec sheet. Why do people think we are telepathic?

    If you really want meaningful help, post a sketch of what you are trying to do. Show you have thought about it a bit.
     
  12. Dec 26, 2014 #11

    phinds

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    OK, makes sense on both counts although your statement that the LEDS "run on 12V" does NOT sound correct ... what you mean, I assume, is that the LEDS have to operate in a circuit with a 12v source.

    I second BigGuy's suggestion that you think we are telepathic.
     
  13. Dec 26, 2014 #12

    jim hardy

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  14. Dec 26, 2014 #13

    berkeman

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    << Two threads on same subject merged... >>
     
  15. Dec 26, 2014 #14
    You trust correctly, phinds asked the logical first question. Thank you for the links, the first one looks particularly helpful. I'm going to spend some time going through it.

    This is a link to the data sheet for the LEDs: http://www.oraclelights.com/Images/Interior/specsheets/oracle_lighting_flex_5050_spec_sheet.jpg.

    What I said, although it wasn't particularly specific, was that it will be used for switching. Basically I just need it to turn all of them on and off.

    You are correct about the voltage.

    I appreciate the help and patience, I've taken on a bit of an ambitious first electronics project!
     
  16. Dec 26, 2014 #15

    phinds

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    Yeah, I took it as being something like "yeah, that's what everyone says 'til I explain it more"
    Shouldn't be too hard ... big thing is to find the right power transistor and in case you are not aware of them, might want to read up on transistor heat sinks.
     
  17. Dec 26, 2014 #16

    meBigGuy

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    The spec sheet tells very little about what is actually on the strips. There are "bridgelux 5050 RGB" LEDs used in the RGB strips, but there are no specs for a 5050 RGB at bridgelux, and there are many different possible varieties other than RGB listed at the bottom of the sheet.

    As for bridgelux, there is no spec for a 5050RGB, no results for 5050, and no pertinent results for RGB.
    http://www.bridgelux.com/products/led-chips/
    They are sold binned for voltage, so I expect they are parallel on the strip?, and there is no resistor?. But, I am not at all sure.
    I looked at 1 Blue power chip and it was rated for brightness at 350ma (3.4V forward voltage)
    http://www.bridgelux.com/wp-content/uploads/2013/10/BXCD4545-45mil-x-45mil-or-1.14mm-x-1.14mm-Datasheet-DS-C15.pdf [Broken]

    300 LED's per strip on the spec sheet at the link provided.
     
    Last edited by a moderator: May 7, 2017
  18. Dec 31, 2014 #17
    I apologize, one of the downsides of typed communications unfortunately. I'll look into heat sinks. Once I think I've found the right transistor I will post it up!

    I agree that the spec sheet isn't very informative. I believe that the LEDs are parallel in groups of three on the strip (which seems to make sense given the forward voltage on that Blue chip). I also don't think there is any resistor in the strip.
     
    Last edited by a moderator: May 7, 2017
  19. Dec 31, 2014 #18

    jim hardy

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    since they tell us to connect to a 12V 10% 'class 2' supply, it's a fairly safe bet there's something on the strip to set current. Could be an active current controller or a simple resistor.... Will they sample you one to examine ?

    i looked up 'class 2 supply' and it means, per US NEC, current limited to lesser of 100VA or 5 amps...

    interesting project.

    old jim
     
  20. Jan 1, 2015 #19
    If there is something on the strip it has to be a resistor. I am supposed to be getting a sample in the mail some time soon.

    Does anyone have a good resource where I can read up and really get a good grasp on transistors, (types, selection, etc.)?

    What I have right now is this:

    My control signal to the base of the transistor will be 5V and 5mA. The load will be 12V and up to (ideally) 20A. It is my current impression that I should use an NPN transistor, but I'm struggling to find the correct transistors for my project. I have never worked with transistors so I thought finding a good educational resource might be a good move.
     
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