LED circuit

Discussion in 'The Projects Forum' started by FFtravism, Apr 18, 2009.

  1. FFtravism

    Thread Starter Active Member

    Aug 21, 2008
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    This is a led panel I am making. It's 49 red super bright LED's. The idea is series lines of 4 and 3 led's. the 4 led series will have a 100ohm resistor at bottom and the 3 led series will have a 270ohm resistor. My question is how is the circuit look? Supply is 12v. It looks rough but I hope it understandable.
    Thanks
    Travis
     
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  2. Audioguru

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    Dec 20, 2007
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    You forgot to say the voltage or the colour of the LEDs so we don't know their current.
     
  3. FFtravism

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    Aug 21, 2008
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    The led's are red 2.5Vf and 20ma
     
  4. SgtWookie

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    If it's for an automotive application, better re-check your voltage. When the engine is running at a fast idle, you'll have more like 13.8v to 14.2v in your electrical system.
     
  5. Wendy

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  6. FFtravism

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    Aug 21, 2008
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    Yes It is automotive. I will have 6 of these in a LED light bar (Firefighter/EMT). I also have a IC/regulator circuit I am just finishing the design on. It has a PIC16F88 with an IC driver and two regulators one 5v for the IC and one 12v for the driver to make sure it's at 12v.

    Thanks
    Travis
     
  7. SgtWookie

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    Jul 17, 2007
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    What regulator(s) are you planning on using?

    Post your schematic.

    What IC driver? If it's a ULN2003 or ULN2803, they'll have a voltage drop from the collector to emitter depending upon the current.
    You can probably use a 78L05 100mA 5v regulator for the microcontroller. However, a 7812 regulator has a dropout of at least 2v (it "loses" a minimum of 2v across itself), so it wouldn't be very suitable. Even the venerable LM117/LM317 has a 1.7v minimum dropout voltage. While it could provide decent regulation while the engine was running, as soon as you turned it off the output would drop to around 11v. (battery V = 12.7 - 1.7v dropout = 11v).

    If the LEDs you're planning on using have four leads, you really should plan on wiring up all four leads. It's not just electrical; it helps in keeping the LED's cool by dissipating heat more rapidly.

    You need to post the typical Vf (forward voltage) @ current for the LEDs you're planning on using.

    Note that you should use RED-ORANGE LED's that conform to DOT's specifications.

    Phillips SuperFlux HPWT-RD00 (44°x88°) or HPWT-MD00 (100°) AlInGaP Red-Orange would be good choices; they have a typical 2.6Vf @ 70mA. The degree specification is the viewing angle. Note that the HPWT-RD00's viewing angle is more or less oval-shaped, and focuses the light in a desirable direction.
     
  8. Audioguru

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    Dec 20, 2007
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    I made a very bright night-light with 36 Super-Flux LEDs in a case from a compact cassette tape. I drilled many holes for cooling but since the LEDs have a current of 53mA they dissipate a total of 5W and get very hot. On a car in the sunshine in summer they might melt.
    You have 2.45W which is also making a lot of heat. If your LEDs are flashing then maybe they will be cool enough. My night light dosn't flash.
     
  9. FFtravism

    Thread Starter Active Member

    Aug 21, 2008
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    OK I just finished the power unit.
    I used LM7812 to limit to 12v and a LM7805 to limit to 5v for the IC. The circuit is designed off something I have working on my breadboard minus the ULN2803AN and only a few leds.

    I see you said the ULN2803 will have a voltage drop what would be the best solution to getting around that? Let the voltage from the car go unregulated or go from 12v reg to say 13v or 14v?
    Thanks
    Travis
     
    Last edited: Apr 19, 2009
  10. FFtravism

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    Aug 21, 2008
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  11. SgtWookie

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    At around 350mA current, the ULN2x03 will drop about 1.1v across the collector-emitter junction.

    It's right in the datasheets.

    Have you made a decision on your LEDs yet? That's the first step.
    [eta]
    These LEDs?
    http://www.superbrightleds.com/specs/hf5-r5590.htm

    Next, you'll need to state if you're going to flash them, or hold them on steady.
     
  12. FFtravism

    Thread Starter Active Member

    Aug 21, 2008
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    SgtWookie: How would I go about hook up all the pins? Just run down each line hooking up all the + and then same on neg ?

    Travis
     
  13. FFtravism

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    Aug 21, 2008
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    The IC is programed to flash the leds.

    Travis
     
  14. FFtravism

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    Aug 21, 2008
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    Looks like at 10.7v supply I can use 39ohm for 4led and 180ohm for 3led.
    Travis
     
  15. SgtWookie

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    What's the duty cycle? What's the pulsewidth?

    If you don't know, stick with under 30mA current per string.
     
  16. SgtWookie

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    LEDs will have their Vf vary over temperature; the higher the temp, the lower the Vf. So will transistors.

    350mA is a practical limit for ULN2x03 drivers. Even though they're rated for up to 500mA, that's maximum, and pushing limits leads to short service life.

    You're planning on 14 strings. If your current is 25mA per string, that's 350mA; right at that "de-rated" limit.

    Let's go with not using the 12v regulator, and plan on your electrical system putting out 14v while the engine is running.

    Let's also plan on a typical Vf of around 2.2v @ 25mA.

    Let's not forget about the 1.1v Vce (collector-emitter voltage) of the ULN2x03 Darlington driver when you're sinking 350mA. We'll take that off the top first.

    14v - 1.1v = 12.9v remaining.
    2.2 x 3 = 6.6v; 12.9v-6.6v = 6.3v, 6.3v/25mA = 252 Ohms. You could use two 510 Ohm 1/4W resistors in parallel to get quite close (255 Ohms).
    2.2 x 4 = 8.8v; 12.9v-8.8v = 4.1v, 4.1v/25mA = 164 Ohms. You can use two 82 Ohm 1/4W resistors in series.

    It's going to be a bit tight with two 1/4 Watt resistors per string on your board the way you have it currently designed.
     
    Last edited: Apr 19, 2009
  17. SgtWookie

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  18. FFtravism

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    Aug 21, 2008
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    Well the duty cycle could be from 5 minutes to 30 minutes. But I doubt 30 mins would be much more then a few times a year. More of 5 minutes max. As for pulse width I have no idea. It will have some fast flashing..

    Reading your post on the resistors is good info. I been out of electronics for a while and I am very rusty. But my question is the spec sheet says the typical Vf is 2.2 @ 20mA. Would we not use that?
    Thanks
    Travis
     
  19. SgtWookie

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    OK, so without further info, what I've already calculated should keep you out of trouble.

    You could go with 20mA if you'd like. You'll need to re-calculate the resistor values though. 40mA continuous is the specified absolute maximum. I used a Vf of 2.2v at 25mA, which won't be too far off. You really should be OK with the values I've already calculated.

    If you decide to use single resistors, you'd have to go with 1/2 Watt resistors for 252 Ohms. That's not a standard E24 value though, and 1/2W resistors may be hard to find in E192 values.
     
  20. FFtravism

    Thread Starter Active Member

    Aug 21, 2008
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    Thank you for all the info.

    How would I go about hook up all the pins? Just run down each line hooking up all the + and then same on neg ?
    Travis
     
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