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Thread: Backup Power for Teensy 3.2 for Power Failure and Brownout

  1. #1
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    Oct 2017
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    Backup Power for Teensy 3.2 for Power Failure and Brownout

    I needed a very simple way to back up Teensy 3.2 during power loss or brownout. My application uses a simple 5V wall wart and has very little additional current draw from it—Teensy, plus a 16x2 LCD display is all I have.

    Here are two approaches that worked. The simplest is a 4F super-capacitor connected in parallel with the 5V power source. In my testing, the super capacitor will run Teensy for about a minute when power is lost. It’s good for a short power interruption. But… (and a key point!) after a minute, or so, the brownout is not graceful. You may lose program memory and need to reload.

    If one minute is not enough, the second circuit may suit some applications. It uses three AA alkaline cells connected to the power supply through a Schottky diode. By my estimates, fresh batteries should power Teensy for a day or more. The same proviso applies—a longer power loss could corrupt program memory and require a reload.

    Click image for larger version. 

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    Some technical notes on these solutions: These techniques rely on the typical low dropout behavior of Teensy’s on-board 3.3V LDO regulator. Furthermore, as Teensy’s voltage falls below 3.3V, my units have continued to run merrily along, even below 3.0V. But there are no guarantees. Truly critical applications should use a more sophisticated approach. If you are building one, or just a few units, you can check for yourself how they behave. Higher quantity production would require a more thorough investigation. Most wall warts have a relatively high output impedance without AC power (not pulling current from the super-cap or batteries). But this, too, should be checked with your specific power supply.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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