Choosing an appropriate LED for ATMGE32U4-MU uC

Discussion in 'Embedded Systems and Microcontrollers' started by pedro147, Oct 24, 2015.

  1. pedro147

    Thread Starter Member

    Jan 1, 2013
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    I am trying to replicate an existing circuit for a quad copter flight control board. It has two SMD LED's fitted but no current limiting resistors are used. I realise that this is very bad design practise but I want to recreate the board exactly as it was sold up until recently. My question is on choosing an LED that has the best chance of survival under these arduous conditions :) I thought that this one with the following specs might have a decent chance of survival and am just hoping for an educated opinion but not one that involves fitting a resistor.

    Forward current 20 Ma

    Forward voltage 3.3v

    Source voltage 3.3v

    This giving a value of a 1 ohm resistor required. Also how crucial is it that the pad size is roughly the same size as the component footprint or is it more a matter of there being full contact .




    Thanks

    Alienwii Schematic.jpg
     
    Last edited: Oct 24, 2015
  2. spinnaker

    AAC Fanatic!

    Oct 29, 2009
    4,887
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    Look at the specs for your mcu. If the max current of your binary out cannot exceed 20ma then it is probably the "safe" way to go. As you mentioned not the ideal way to go. Do you already have the PCB?
     
  3. pedro147

    Thread Starter Member

    Jan 1, 2013
    52
    2
    spinnaker thanks for your reply. As you can probably tell I am well out of area of expertise here. I am in the process of trying to develop a board file in Eagle for this project. The schematic and associated libraries are the only information that I have. I really would not know how to find the max current of the Atmega32u4 binary out. If you might have time to have a look for me I would appreciate it, but if not I understand. I cannot directly link the datasheet just the part specs which include that link.
     
  4. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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    Another way to go would be to put a place holder for a current limiting resistor on any new PCB that you create. If you decide to populate it with a 0 Ohm resistor to match the currentboard you have that possibility. If you find it better to have a current limiting resistor you do not have to change the board again later.
     
  5. pedro147

    Thread Starter Member

    Jan 1, 2013
    52
    2
    StayatHomeElectronics thanks good idea. Yes, after the hopefully successful implementation of this board, I wish to incorporate this and some other improvements like larger FET's and a reverse current diode. The current discontinued production board works without any current limiting resistors so I am just hoping to emulate this in my first version of the board.
     
    Last edited: Oct 24, 2015
  6. dl324

    Distinguished Member

    Mar 30, 2015
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    Unless you have a good reason for using non-standard footprints, I'd go with published specs.

    The datasheet was ambiguous regarding current sink capabilities. I'd check the IV curve for the LED you plan to use and see what it's current will be at 3.3V. If you're only going to make one board, you could cherry pick an LED for specific IV characteristics.
     
  7. pedro147

    Thread Starter Member

    Jan 1, 2013
    52
    2
    Thanks dl324. So I looked at the IV curve and at 3.3v the forward current is 17.5 mA. How do I "cherry pick an LED for specific IV characteristics" by using this info please. Sorry I am pretty green :) I am finding the use of different terms for the same thing confusing here. So R = voltage source - forward voltage divided by forward current which going from the IV curve is 17.5 mA @3.3v. So obviously regardless of at what forward current I calculate for, the top line will always be zero. So does this mean that because the voltage source and forward voltage are the same value, no current limiting resistor is required in this circuit?

    LED Specs.jpg LED IV curve.jpg
     
    Last edited: Oct 24, 2015
  8. dl324

    Distinguished Member

    Mar 30, 2015
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    LEDs in the same intensity group will have a range of forward voltages.
    ledSpecs.jpg
    LED-iv.jpg
    For this LED, if you used one that had a Vf of 2V @ 20mA, at 3V it would draw 100 mA. If you used a diode that had a Vf of 3V @ 20mA, at 2V it would draw less than 20mA.
     
  9. pedro147

    Thread Starter Member

    Jan 1, 2013
    52
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    Sorry, you beat me to it. I am finding the use of different terms for the same thing confusing here. So R = voltage source - forward voltage divided by forward current which going from the IV curve is 17.5 mA @3.3v. So obviously regardless of at what forward current I calculate for, the top line will always be zero. So does this mean that because the voltage source and forward voltage are the same value, no current limiting resistor is required in this circuit? I appreciate you going to the trouble of trying to "learn me" :) but at this stage I suppose I just want to know if this LED has a decent chance of survival without a resistor.
     
    Last edited: Oct 24, 2015
  10. dl324

    Distinguished Member

    Mar 30, 2015
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    The LED datasheet should give a range of values for Vf at a specific current, usually 10mA or 20mA. The IV curve is for a typical LED.

    To cherry pick means to pick a device from a group of devices for specific characteristics. What you're looking for is one that will conduct no more than a certain current at 3.3V

    BTW, since this is a battery powered circuit, I'd be inclined to omit the power on LED.
     
  11. takao21203

    Distinguished Member

    Apr 28, 2012
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    If theres no resistor most likely it's pulsed. Indication leds need a few Ma.

    FYI a single led on atmel controllers used for arduino will draw 60 ma.

    So it could be pulsed at 1/20. Just for a single indication led it's not really cutting it to remove the resistor.
     
  12. pedro147

    Thread Starter Member

    Jan 1, 2013
    52
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    takao21203 I cannot understand the multi-quote system here so "So it could be pulsed at 1/20. Just for a single indication led it's not really cutting it to remove the resistor"You will have to spell it out very simply for me I'm afraid :) So by "pulsed at1/20" does that mean like the duty cycle. Likewise by "it's not really cutting it to remove the resistor" are you meaning that this not best practise, for want of a better description. My whole issue here is that I am after advice on how to pick an LED for an existing circuit that does not have a current limiting resistor. I know that it is not the correct way to do it but that is the way it has been done by someone else.
     
  13. takao21203

    Distinguished Member

    Apr 28, 2012
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    I mean there's no big saving. You could use white or UV led .
     
    Last edited: Oct 26, 2015
    pedro147 likes this.
  14. pedro147

    Thread Starter Member

    Jan 1, 2013
    52
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    Thanks for your help :)
     
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