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Thread: Low output voltage with external power

  1. #1
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    Low output voltage with external power

    Hi y'all, electronics noob here looking to get a bit of support on a small project, using a 3.2 I'm looking to read button presses and use those to light individual LEDs. I had this working initially, however I was using USB power to power the LEDs (13 in total) and I think the current load toasted the board. I've rewired this using a fresh board, cut the line to VUSB and have wired in external 5v to the board. The program logic works as expected however testing against a single LED is very dim, voltmeter only shows 1.2v, the tech spec shows Digital output at 3.3v, am I missing something for the voltage to be so low in my application?

  2. #2
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    Are you measuring the voltage over the LED? Any series resistor ?

  3. #3
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    Yes I'm getting 1.2v from the LED pins, no resistors between the teensy and the LED

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    That can be a great way to fry both the Teensy and the LED. The LED pulls the voltage down to 1.2V and excess current from the Teensy output pin.
    Always use current limiting resistors!!!

  5. #5
    Senior Member PaulS's Avatar
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    Here is how the onboard LED of the Teensy 3.2 is hooked up:

    Click image for larger version. 

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    See this page for schematics.

    Paul

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    (Honest question) The LED is rated for 12v, how would it be fried by the 3.3v coming from the board

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    Quote Originally Posted by PaulS View Post
    Here is how the onboard LED of the Teensy 3.2 is hooked up:

    Click image for larger version. 

Name:	Capture.PNG 
Views:	9 
Size:	17.7 KB 
ID:	22681
    See this page for schematics.

    Paul
    Sorry I'm not sure what I'm supposed to do with that.

  8. #8
    Senior Member PaulS's Avatar
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    Ah OK, you are using a 12V LED. Do you have a link to that LED perhaps? It should have a current-limiting resistor built-in somewhere.
    The schematics above were meant to show you how to hookup a standard LED.

    Paul

  9. #9
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    Quote Originally Posted by PaulS View Post
    The schematics above were meant to show you how to hookup a standard LED.
    Paul
    I'd argue someone who doesn't know how to hook up an LED can't read a schematic

    LED is nothing special, just an ebay unit - basically this - https://www.ebay.com/itm/16mm-12V-Ca...IAAOSwx4ZfiHlS

  10. #10
    Senior Member PaulS's Avatar
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    Well, the Ebay LED is special. Standard LEDs do not come with a current-limiting resistor...
    In your case I guess the Ebay LED [if it is really an LED and not a bulb] just draws too much power from a Teensy pin.

    Paul

  11. #11
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    For something like this I'd always check with a multimeter and 1k resistor whether the LED already has current limiting
    resistor and double check the pin out and polarity (I usually do this for any such component as a double check).

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