USB power usage checker

Discussion in 'The Projects Forum' started by cowasaki, Jun 29, 2011.

  1. cowasaki

    Thread Starter New Member

    Apr 3, 2011
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    Just knocked up a quick box with a USB chopped in half and the data and GND wires re-joined inside. Then a take off for a volt meter from GND and VCC plus two connectors to the VCC in and VCC off for an Ammeter.

    Making the Ammeter sit in series with the circuit and the volt meter in parallel.

    ie:

    [​IMG]

    When I connect this with no meters attached then any connected device clearly should not work as it will not get power so a jump lead between the ammeter connections puts everything back to normal and the USB device works....

    Now if I connect a volt meter that works....

    If I disconnect the jump lead and connect an ammeter across the connections the device and ammeter don't work.

    Have I missed something really obvious?

    I just want something to allow me to check that anything USB that I build will stay in spec.
     
  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    It looks like it should work as you are only interrupting the power connections, the data needs to be clean. Did you try buzzing it out?
     
  3. studiot

    AAC Fanatic!

    Nov 9, 2007
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    Useful gadget!
     
  4. cowasaki

    Thread Starter New Member

    Apr 3, 2011
    15
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    Well exactly!

    Nope I have checked the continuity between each point and everything is as it should be !

    Very confused....
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,145
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    What is the "device"? If it's smart, it may be shutting off it's draw on USB power when it sees a fault. Or if it sees insufficient voltage when it tries to negotiate a current draw.

    Also, you've drawn the data lines on the outside pins. The outer pins are the power lines, the inner 2 are data.
     
  6. cowasaki

    Thread Starter New Member

    Apr 3, 2011
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    0
    Just ignore the positions :) I couldn't remember them when I was drawing the diagram that is why I labelled them. The device is actually something I have made and isn't shutting itself down. I really have no idea why it isn't working correctly at all :confused:
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,392
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    Well, when in doubt, drop back and PUNT!

    Sounds like you built this device, is it still in breadboard stage? Cut the power from the USB cable at the device connector, and use another supply instead of the USB power. You should be able to monitor that power any way you wish.
     
  8. wayneh

    Expert

    Sep 9, 2010
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    So, you're saying there's a voltage drop across the ammeter, and no current is flowing thru it? Equals burnt meter.
     
  9. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
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    More like some current is flowing, but the drop is too high for the device to work properly.
     
  10. wayneh

    Expert

    Sep 9, 2010
    12,145
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    What sort of ammeter is it? It's probably meant to be driven by a 4-20mA current loop, and 5v won't do it.
     
  11. CDRIVE

    Senior Member

    Jul 1, 2008
    2,223
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    It's a known prerequisite for nubes to blow the DMM's low current fuse. Prior to DMMs they weren't happy until their meter movements were bent at at 90°. :rolleyes:
     
  12. someonesdad

    Senior Member

    Jul 7, 2009
    1,585
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    Exactly. The OP can check the fuse's integrity by using a continuity tester on the ammeter. If it's a class III or IV instrument, you're probably going to scream a bit when you find out how much those fuses cost. :p
     
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