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EMI issue with USB connection

Discussion in 'General Electronics Discussion' started by Istvan, Jun 8, 2014.

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  1. Istvan

    Istvan

    20
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    Jun 8, 2014
    Hello there! I'm a hobbyist and I have a major and urgent issue.
    I've made a USB device by using the PIC18F4550 and on the same board I have a power transformer (220 V -> 2x12V, 2A).
    The problem is, whenever I turn the power transformer on/off (entering it into the mains outlet or decoupling it), it disconnects my USB device from the PC for 1 second and then it automatically reconnects. As I've read on the internet, it's an issue caused by EMI, but I don't know how to prevent it. The USB wire is a shielded B-Type...
    And I'm sure the issue is caused by the transformer, cause even if it's not on the board, unconnected to anything, not even a rectifier bridge, at 20cm from the device, it disconnects my usb device...

    Help please, I need to implement a solution very fast!

    P.S. I'd rather not place the transformer in a separate box...
     
  2. OLIVE2222

    OLIVE2222

    690
    25
    Oct 2, 2011
    Hi Istvan,

    Please posting sharp picture(s) of your board as your schematic will help us to help you.
    Olivier
     
  3. Arouse1973

    Arouse1973 Adam

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    1,087
    Dec 18, 2013
    Try powering it from a d.c source like a battery or power supply. If it still does it then it's not the transformer or it's connections, lets rule that out first. Post any schematic and a PCB layout diagram also so we can see what we are dealing with.
    Adam
     
  4. Istvan

    Istvan

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    1
    Jun 8, 2014
    Here are the images of the breadboard... I don't have a schematic yet for it, sorry... You can see that the transformer is unconnected, still, when I plug it in the mains, or pull it out of there, it briefly disconnects my USB connection for a second. Even if my laptop is running on battery mode, it does the same thing (There's no issue with the wall outlet).
     

    Attached Files:

  5. Istvan

    Istvan

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    Jun 8, 2014
    I need to power it from a transformer, as I'll need it to power a solenoid later... DC power from a battery would waste fast.
     
  6. OLIVE2222

    OLIVE2222

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    25
    Oct 2, 2011
    Don't see the USB connection on your picture. If not yet the case unshielded wires must be very short between the PIC pins and the connector, around 1 inch maximum.
     
  7. Istvan

    Istvan

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    Jun 8, 2014
    Should I connect the shielding from the usb to the device's gnd?
     
  8. OLIVE2222

    OLIVE2222

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    Oct 2, 2011
    The cable shielding is to be connected on the device GND.
    The connector frame is to be connected to device GND via a ferrite bead or a 1-10nF capacitor, to be experimented
     
  9. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Yes I know I am not talking about replacing the transformer with a battery. I am talking about trying it to take out any problems that might be attributed to the transformer.
     
  10. Istvan

    Istvan

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    Jun 8, 2014
    Tried it, there's no problem at all when on batteries...
     
  11. Istvan

    Istvan

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    Jun 8, 2014
    Connected it to GND via 15nf cap.... same results IMG_20140608_194455.jpg IMG_20140608_194505.jpg
     
  12. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Ok Good. So we now know it's the transformer. It this a switching power supply or a normal mains transformer.
     
  13. Arouse1973

    Arouse1973 Adam

    5,165
    1,087
    Dec 18, 2013
    Sorry replied before pictures. It's a standard transformer then.
     
  14. Istvan

    Istvan

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    Jun 8, 2014
    Yep, standard...from mains
     
  15. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    When it fails do you get a warning message from the PC or does it just disconnect and then re-connect?
     
  16. Istvan

    Istvan

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    Jun 8, 2014
    That typical disconnected BAdumm sound (win 7), and reconnect sound (baDUMM) :) Nothing more. (Besides losing my dataconnection in my application, which is sensing atm the analog value of the phototransistors)
     
  17. OLIVE2222

    OLIVE2222

    690
    25
    Oct 2, 2011
    OK regarding the USB connection, problem seems more located on the Power supply side.
     
  18. Arouse1973

    Arouse1973 Adam

    5,165
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    Dec 18, 2013
    I have to go now. I'll give this issue some thought. If it was a switching regulator then I could understand but a linear transformer is not normally an issue. But it could be your wiring, trying to run a USB connection like this on bread board is challenging at the best of times. There are strict guidelines for USB and bread board is not one of them. :)
     
  19. Istvan

    Istvan

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    Jun 8, 2014
    Probbly you're right, though I wouldn't want to make a pcb without the guarantee that it'll work...
     
  20. Istvan

    Istvan

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    1
    Jun 8, 2014
    Is there a way to make the induction-suction process at the transformer to slow down (resistors, caps?) when disconnected from powersource, so it won't make that much of a sudden noise?
     
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