Forum Rule: Always post complete source code & details to reproduce any issue!
Results 1 to 7 of 7

Thread: Powering Teensy and peripherals

  1. #1
    Junior Member
    Join Date
    Feb 2013
    Posts
    7

    Powering Teensy and peripherals

    Hi everyone,

    I am planning a project where a Teensy 3.0 should read data from a SPI Imu and write it onto a micro-sd card (I recently purchased the teensy card reader).

    Now I realized that are two difficult issues in this project:

    First, the whole project won't be powered most of the time via USB, but via an external 5V power source. If I make shure that eiter USB or this external supply power the Teensy, do I need to cut anything on the board, or can I use it as is? (I refer to the Teensy manual)

    Second, the SD card and the IMU consume more than 100 mA. So, both devices have to be powered externally, is that correct? Can I connect the SD-card to external 3,3 V and connect both teensy and SD to a common ground?

    Sorry, I am still a beginner in electronics... maybe somebody can give some hints on this?

    Regards

    TP

  2. #2
    Senior Member Constantin's Avatar
    Join Date
    Nov 2012
    Location
    In the yard with a 17' Dia. Ferris Wheel
    Posts
    1,408
    Quote Originally Posted by ToPi View Post
    First, the whole project won't be powered most of the time via USB, but via an external 5V power source. If I make shure that eiter USB or this external supply power the Teensy, do I need to cut anything on the board, or can I use it as is? (I refer to the Teensy manual)
    Yes, refer to Paul's documentation re: VIN and VUSB. There is a small trace between those two pads that you should cut in order to disable the on-board voltage regulator. From then on (or until you solder the two pads together) you have to supply external 3.3V to the VCC pins on the Teensy 3.

    For my 3.3V-only boards, I use a dual Schottky diode array to source power from a external source and/or the VUSB. However, you can also do this with individual diodes.

    The diode array can handle up to 1A. Being Schottky diodes, they feature less voltage drop, leaving plenty of headroom for my current voltage regulator of choice, the MCP1702. That can handle up to 250mA of current and if you buy the TO-92 version, it can even be fitted with a very large external heat sink.

    I'd like to think that with the right decoupling capacitors for the SD card and the IMU that the single power supply could handle the load. However, that's easy to test, simply breadboard it all up and see if the Teensy runs reliably. FWIW, I write to an SD card, run a Teensy 3, and a MCP3911 without any issues off one MCP1702.HTH.
    Last edited by Constantin; 10-10-2013 at 04:31 PM.

  3. #3
    How about a simple voltage regulator to deliver 3.3V?
    I suppose you want to read datas from a IMU (with 9 or 10 DOF) and since you are not using the USB, you use (I suppose) a battery which supplies 4.8-6V. Ok, I suggest you to supply the Teensy with the battery and and than (on another track on your circuit) to supply Card and sensor with 3.3 V (through a voltage regulator). Pay attention that in this case you should connect AREF of the Teensy to the 3.3 V, in order to read analog values correctly.
    I think, it should be pretty easy to you.

    Regards

  4. #4
    Senior Member PaulStoffregen's Avatar
    Join Date
    Nov 2012
    Posts
    23,383
    PJRC is working on a WIZ820 + SD adaptor board, to be released soon. It will have a MCP1825 regulator.

    If you can hold out a couple weeks, that little board will make the SD socket easy, and you can tap its 3.3V power for much more current, since the MCP1825 is good for a lot more.

  5. #5
    Junior Member
    Join Date
    Feb 2013
    Posts
    7
    Dear Constantin, Dave and Paul,

    first of all thank you very much for your advice! I am really greatfull for all your hints!

    @Paul: That's great to hear. I am impressed about your creativity to develop this unique tiny but powerful ARM Cortex form factor with the easy of use of an Arduino system! One thing I wanted to ask: Why not develop a whole system of teensy-compatible micro components, e.g. BLE or nRF24 modules which could even be stackable? However, unfortunately I have already ordered a number of PRJC micro-SD-modules from a retailer, so I will need to deal with what I already have...

    But now my main question: So I have the micro-SD, which should not exceed 60 mA, and my IMU (YEI 3-space embedded SPI module) which also consumes about 60 mA.
    All data lines are connected with the respective teensy pins. Now I want to power the teensy board via USB and at the same time the SD-card and the IMU should be powered externally at 3.3V.
    Can I set up this design without any modifications of the teensy board (No cutting)?
    Should Gnd of external power and USB power be connected?
    Do I really need to add external capacitors to the peripherals? How to calculate the capacity? I thought that the PRJC Micro SD adaptor has a capcitor...

    Again, many many questions, I am sorry for asking stuff which is most likely very obvious. Thanks for all the explanations given above!

  6. #6
    Quote Originally Posted by ToPi View Post
    All data lines are connected with the respective teensy pins. Now I want to power the teensy board via USB and at the same time the SD-card and the IMU should be powered externally at 3.3V.
    Can I set up this design without any modifications of the teensy board (No cutting)?
    According to my experience for simple debugging and programming porpouses doesn t happen anything. But please use at least an USB Hub like this one for example.

    Quote Originally Posted by ToPi View Post
    Should Gnd of external power and USB power be connected?
    Of course
    Quote Originally Posted by ToPi View Post
    Do I really need to add external capacitors to the peripherals? How to calculate the capacity? I thought that the PRJC Micro SD adaptor has a capcitor...
    I think you need not a capacitor for the IMU but for the Teensy. For USB applications should be about 100 uF

    Regards

  7. #7
    Senior Member Constantin's Avatar
    Join Date
    Nov 2012
    Location
    In the yard with a 17' Dia. Ferris Wheel
    Posts
    1,408
    Given that SDHC cards can pull up to 3W (at the high end - see Wikipedia article on power consumption), an external power supply seems like a really good idea. Granted, our SD cards are unlikely to pull more than 0.5W thanks to the limitations on the SPI bus that the Teensy currently enjoys, but as much as I'd like to use the power supply inside the Teensy, I see no good reason to do so for my application. I prefer a external voltage regulator like the MCP1702 that produces a nice stable voltage and which has up to 250mA of capacity (properly heat sinked and all that). The MCP1825 is a step up from the MCP1702, comes in a larger package and delivers up to 500mA of output. Overkill for my application but a good choice for what Paul is working on.

    Another reason, IMO, why an external power supply is a really good idea is to keep the heat out of the K20 chip. If you use the ADC, you get less thermal drift, if the power supply is overtaxed, you don't potentially damage the K20, and so on. I can buy a lot of through-hole MCP1702's for the cost of one K20, never mind the trouble of SMD-reworking a K20 vs. a 3-pin TO-92 voltage regulator. Fore the digital power supply, I use 100uF upstream and downstream of the MCP1702 to deal with nasty transients. Locally, all VCC connections are also de-coupled with a 0.1uF ceramic capacitor. For the analog power supply, I use just 10uF on the downstream side since the power needs are so low.
    Last edited by Constantin; 10-10-2013 at 04:32 PM.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •