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Thread: Teensy 3.2 external power 9Volt problem

  1. #1
    Junior Member
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    Dec 2017
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    Teensy 3.2 external power 9Volt problem

    Hello all,
    I've been working on a teensy audio project, and everything has been going swimmingly for the most part.
    Code is good, Inputs and controls are good, but my problem is I can't figure out how to power my my teensy 3.2 with an external 9v power supply.
    This is going in a guitar pedal enclosure, and will be running with a bunch of over 9v DC guitar pedals.
    I would prefer to have the teensy be powered by either USB or 9v, So I have not cut the VUSB 5v pad on the back.
    I'm hooking up a 9V power plug through a voltage divider, and which should just be any value of resistor, as long as both are the same, I'll get 4.5v in the middle.
    I've tested my divider on a bread board, and I consistently output 4.5v.
    My problem, is when I connect my 4.5 v to the vin pin, nothing works, and when I try to power on with USB after disconnecting the power, I can't upload code, and previous code is not running. 3.3v pins read as 0v, and I'm pretty sure I fried my teensy? I've done this 3 times now and I cannot figure out what I'm doing wrong.
    Should I be using a different value resistor for my voltage divider? On my breadboard they all seem to yield the same result , but I've been using 5% 470 Ohm.
    I've been working on this project since summer and I really just want to get it finished and working.
    I'm using a 3.2, teensy audio shield, and teensy breakout board.
    Thanks!

  2. #2
    Senior Member
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    Torrance CA
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    You can't really use resistor voltage dividers to supply current to anything that takes a nontrivial load. From https://learn.sparkfun.com/tutorials/voltage-dividers

    Any current that the load requires is also going to have to run through R1. The current and voltage across R1 produce power, which is dissipated in the form of heat. If that power exceeds the rating of the resistor (usually between ⅛W and 1W), the heat begins to become a major problem, potentially melting the poor resistor.

    That doesn’t even mention how inefficient a voltage-divider-power-supply would be. Basically, don’t use a voltage divider as a voltage supply for anything that requires even a modest amount of power. If you need to drop down a voltage to use it as a power supply, look into voltage regulators or switching supplies.
    This link from StackExchange goes over some alternatives:

    https://electronics.stackexchange.co...to-3-3v-and-5v

    Here's a sample circuit using a very common LM317:

    http://www.circuitstoday.com/few-lm3...lator-circuits

  3. #3
    Junior Member
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    Dec 2017
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    Thanks for the quick reply!
    I guess I have to figure out a way to stuff a voltage regulator into my enclosure with everything else. Heh...
    Is the resistor value being to low what burned out the teensy? I'd like to be able to salvage as much as possible out of this project.

  4. #4
    Senior Member
    Join Date
    Feb 2017
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    I wouldn't expect just using a voltage divider would burn out the Teensy by itself. As the load draws current, more voltage is dropped across R1 and the voltage delivered to the load actually drops. So, just that shouldn't burn it out. Perhaps some secondary effect?

  5. #5
    Senior Member
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    Jun 2013
    Location
    Torrance CA
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    Try disconnecting the Teensy from all other wires and connections with only the USB cable from the computer to the board, then try uploading something basic like the Blink sketch. If it doesn't work try pressing the Program button. If none of that works then yes my guess is the board is fried.

  6. #6
    Just to offer moral support... I recently burned out a Teensy 3.2 in a very similar configuration!

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