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Thread: Concerns about connection between 3V3 ports between two Teensys

  1. #1

    Concerns about connection between 3V3 ports between two Teensys

    Hi, I am using two Teensys that communicate with each other to handle separate parts of a project. I am currently designing the wiring for my system and need to power some 3.3V devices. So far, I am planning on using the 3V3 ports on the Teensys to provide this power, however this would mean that a connection between the 3V3 ports on the Teensys would be established. Some initial research suggests that voltage regulators should not be wired in parallel, which is what I assume happens when the 3V3 ports are connected across two Teensys.

    Also, I am worried about using an external voltage regulator to power just the 3.3V devices in case the regulator provides a slightly different voltage to the analog devices (such as some potentiometers). For similar reasons, I want to equalize the 3.3V power rails on the Teensys because a couple of the more critical sensors connect to both (so that I am not limited by communication speed).

    If it helps, I can be flexible with the power source of my Teensys but prefer for it to be one of the pins (rather than the USB). Additionally, my power source provides 5V (regulated internally by the power source).

    Does anyone have insight or suggestions? Thank you!
    Last edited by markmckenzie; 12-08-2019 at 11:16 PM.

  2. #2
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    Look into using ballast resistors.

  3. #3
    Thank you for the response. Unless I am misunderstanding, it seems that using ballast resistors (presuming there are two -- one after each Teensy's 3.3V output) would cause a sizeable voltage drop at larger currents. I am expecting to draw about 300mA from my 3.3V devices and it is important that their input voltage be at 3.3V.

  4. #4
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    It depends on the resistor values. They can have a negligible effect.

    You may be able to create your own power supply and not depend on the teensys for power.

  5. #5
    Senior Member+ MichaelMeissner's Avatar
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    Note, you can power the Teensy through the 3.3v ports. I might think feeding the power into a decent 3.3v step-down voltage regulator, and then powering both Teensys and the 3.3v devices from that power. That way everybody gets the same power (unless you have a long distance between Teensys that you have to worry about the voltage drop in the power lines).

    Note, in general the Teensys are rated to produce at max 250ma through the 3.3v port, so you probably don't want to go with powering one Teensy with external power, and then powering the second one from the first one along with your devices.

    Note, I'm a software guy, so go to a hardware guy for the nuances of the regulators. I tend to like Pololu voltage regulators, such as:


    From the specs, you could use a single cell lipo battery (3.7v nominal to 4.2v freshly charged) to feed into the voltage regulator. But note, the first two regulators are not protected against reversed input wires, while the later 3 are protected, but the later 3 have higher input voltages which would rule out using a single cell lipo battery (a 5v usb battery would be fine).

    The voltage regulators can get toasty, so make sure you take that into account before enclosing them in a tight space.

    I would suggest thinking about cutting the VUSB -> VIN input, so you can program the Teensys without trying to power them from USB. Obviously if you do so, you would need to power the Teensy with the normal external power.

    If your power is more variable, there are also step-up/step-down voltage regulators. They aren't as efficient as the step-down regulators, but they can handle a lot of different voltages. The one I tend to think about is S9V11F3S5, which can produce either 3.3v or 5v, and cuts off if the input is less than 3v to prevent batteries from over-discharging:
    Last edited by MichaelMeissner; 12-09-2019 at 04:36 AM.

  6. #6
    Thanks! I didn't realize you could power the Teensy straight off the 3.3V pins. I thought it had to be via USB or the Vin pin. That should gracefully solve my issue.

    I do have a follow-up question. I noticed you mentioned and linked step-down regulators. Is there any benefit to using these over linear regulators outside of reducing power consumption? My setup is limited in space but has plenty of power.

  7. #7
    Senior Member+ MichaelMeissner's Avatar
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    Quote Originally Posted by markmckenzie View Post
    Thanks! I didn't realize you could power the Teensy straight off the 3.3V pins. I thought it had to be via USB or the Vin pin. That should gracefully solve my issue.

    I do have a follow-up question. I noticed you mentioned and linked step-down regulators. Is there any benefit to using these over linear regulators outside of reducing power consumption? My setup is limited in space but has plenty of power.
    That will need an answer by somebody who is more hardware oriented. But I would imagine it might depend on what your input voltage/amps are. I.e. linear regulators might generate more heat particularly at higher voltages because they are less efficient. But if you are talking 300mA for the devices, and 2 teensys, it probably isn't as much of a concern.

    A quick google search shows:

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