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Thread: Power Sharing Between Two Sources

  1. #1
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    Power Sharing Between Two Sources

    Hi all,

    I am working on a project with two Teensys. In regular operation either or both of them will have power coming through their USB ports. If only one is powered, regardless of which one it is, the other needs to use the same power.

    My current plan is to sever the jumper on the back of the boards, connect both Vins together, then use Vusb from each board through an selector/combiner. I could do this with a diode or, but I don't really want that voltage drop. I have found the TI TPS2115A (https://www.digikey.ca/en/products/d...115APWR/652785 ) which I believe would also work.

    I'm curious if there's another solution using discrete MOSFETs that might be an option.

    Thanks!

  2. #2
    Member MorganS's Avatar
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    So you want either USB cable to power the pair but if there are two USB cables plugged in (maybe from different computers on different power supplies) then you don't want power sharing? Is that right?

    What else is powered from 5V? Do you have any high-current motors or anything there? Any analog electronics on the 5V? What else is powered from 3.3V?

    The Teensy runs on 3.3V with a "low dropout" regulator so you can afford a lot of losses on the 5V line before the Teensy can't function. The 0.6V voltage drop from a diode is insignificant unless you're on limited battery power. So just put in 2 diodes in parallel, facing opposite ways. One unit powered by 5.0V USB passes 4.4V to the other unit, which is just fine for powering the Teensy. It's not fine if you have 5V analog devices. If the second one gets powered by another USB then no current flows in either diode, so long as the second one is +/-0.6V from the first.

    Depending on how far "apart" (electrically) the power supplies are, you may have issues with different grounds. We would need to know more about the application to tell.

  3. #3
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    Hi Morgan, thanks for the reply.

    Yep, you've got the scenario right. The 5V side has a seven segment display and some indicator LEDs, but no analog. Any analog is on the audio board and thus run from 3.3V.

    Interesting idea with antiparallel diodes. I hadn't thought of that. Does that give any advantage to using two diodes as a more traditional OR?

    The two power supplies hopefully won't be too far apart electrically. These are computers, so both will be switching PSUs. They will be powered off the same source, and physically next to one another.

  4. #4
    Member MorganS's Avatar
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    A 7-segment display will probably be happy on 4.4V. It will change brightness, which may be noticeable unless you're using a constant-current driver. Beware of LCD displays with backlights - the backlight uses a lot of current, so size your diodes for that.

    Two diodes in a T shape would allow two sources to power a single thing. You're powering two things and they're both relatively self-contained or self-powered. Neither is the 'master' or 'controller' of the other.

    Two computer PSUs could give trouble. That's not enough information to say for sure. It is also dependent on anything else plugged in. Like maybe you have powered speakers on one and a powered USB hub on the other and one of those devices has a mis-wired mains plug. They would work perfectly well on their own and might appear to work with USB grounds connected but there could be damaging currents flowing in that USB ground which cause a problem 2 years from now when your chair crushes a USB cable and sets the house on fire. Unlikely, but that's the worst case.

    Best case is one of the computers is a laptop running on battery power and you always remember to unplug the USB before plugging in the laptop charger. But you can't rely on your memory working that well.

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