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Request test jig suggestion for microUSB phone charging current

Discussion in 'Electronic Repair' started by Danny D'Amico, Jan 3, 2014.

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  1. How would you make a test jig out of a spare USB cable?

    For $1.99, I bought at Frys today, this 6-foot USB-A to microB USB
    male-to-male cable.

    If I cut the cable in half, and isolate the wires, how would you
    recommend I set it up so that it could become a test jig
    (to see how much charging current a device actually draws)?

    Have you done this before and have advice for how to make that jig?

    Specifically, how would you fasten the bare wires, which I presume
    are very very thin, and therefore fragile?

    Also, we'd need a way to insert the ammeter inline to measure

    Any test jig ideas I can benefit from?

    NOTE: This is an offshoot of the USB charger thread, where we
    determined that a 3.1 Amp dual-USB charger that is 10 Watts
    is very different than the same spec at 15 Watts.
  2. sms

    sms Guest

  3. Hi Colin,
    That works, but I was just going to slice a cable in half, and then attach
    the inner wires to a series of screws.

    What I'm thinking is to screw two row of (five?) brass screws into a
    piece of wood, and then attaching the cable wires to each row.

    Then I can either jump the distance with a copper wire, or with the
    meter leads.

    But before I build the test jig, someone might suggest a better platform
    out of parts commonly found in the garage or shop.
  4. mike

    mike Guest

    Cool...I was ready to order one until I saw that the data doesn't pass thru.
    WTF? The connectors are almost touching, why no data?
    Kinda hard to measure the drain of a disk drive in action when it won't
    come out of sleep.

    Why would you not connect the data unless there were problems?

    Anybody bridged the data? Results?
  5. mike

    mike Guest

    That works if you never want any data thru it.
    Better would be to cut the power wire and run that thru a meter.
    Get out your ohms law calculator and verify that your meter resistance
    won't defeat you.

    Leave all the high speed data wires and ground alone.
  6. Lab Lover

    Lab Lover Guest

    Interesting looking device. I wonder how accurate it is? Kind of expensive if
    you ask me.
  7. mike

    mike Guest

    Other than the numbers on the first page of the link?
    Kind of expensive if
  8. ....or here:

    About 5 US$ with free shipping to USA.

  9. Juan Wei

    Juan Wei Guest

    Alexander Y. Sure has written on 1/4/2014 11:21 AM:
    Per piece. They sell lots at about $5/piece!
  10. Guest

    Followups set to .

    With a sharp knife, remove 2 inches (50 mm) or so of the outer
    insulation of the cable, in the middle of the cable. Try hard not
    to nick the insulation on the four individual wires inside the cable,
    but if you do, it's not a total disaster.

    Next, try to identify the two data wires and the two power wires. Often
    the data wires will be twisted; the power wires may or may not be. The
    power wires will also often be one or two wire gauges thicker than the
    data wires, but not always, and this is kind of hard to tell. The power
    wires *might* be red and black, but don't count on this.

    Pick what you think is one of the power wires and cut it, right in the
    middle of the stripped part. Strip the cut wire ends 1/4" (6 mm) or so.
    Get your multimeter and test between all four contacts on the big (PC)
    end of the cable, and the stripped wire that comes from that connector.
    It may help to use a paper clip or other small piece of solid wire to
    touch the contacts in the USB connector if the multimeter probe won't

    If the stripped wire is continuous with only one of the two outside
    contacts in the USB plug, you did indeed cut one of the power wires.

    If the stripped wire is continuous with only one of the two inside
    contacts (it doesn't matter which one), you picked a data wire; solder
    the stripped ends back together, insulate with a small amount of tape
    (electrical tape is ideal, Scotch tape will work fine for this), and
    pick another wire.

    If the stripped wire is continuous with more than one contact, then
    you probably nicked the insulation on more than one wire when you were
    initially stripping the cable - find the places that are touching and
    separate them, and maybe insulate the bare spots individually with

    Once you have figured out that you did indeed cut the power wire, you
    can figure out how to hook the power wire up to your multimeter. Your
    meter may have come with alligator clips that go on the probe ends; if
    you have those, use them. What I use for things like this, because I
    already own some, are clip leads - short stranded wires with alligator
    clips on each end. Rat Shock 278-1157 or 278-1156 are typical; Fry's
    probably has a better price on similar products.

    If you have alligator clips that fit your meter probes, put them on the
    probes. If you have a digital meter, clip one probe to one stripped
    wire and the other probe to the other one - it does not matter which
    way around the red and black wires are. If you get the probes
    "backwards", you will get exactly the same current reading, just with
    a minus sign in front of it. If you have an analog meter, you have to
    get the probes the right way around, but IIRC you have a digital one.

    If you have clip leads, clip a red clip lead to the red probe of your
    meter and to one of the stripped wire ends. Clip a black clip lead
    to the black probe of your meter and to the other stripped wire again.
    As above, if you have a digital meter, it does not matter which way
    around the red and black wires are.

    Next, figure out how to make your multimeter measure current. With the
    meters most people have, you need to rotate a dial to an "A DC" range,
    and move the red meter lead over to a different jack on the meter. Do
    whatever you need to for your particular meter.

    Plug the mini-USB end into your peripheral (external hard drive or
    whatever). Put the peripheral somewhere close to the meter, so you can
    watch it and the meter at the same time. While watching both the
    peripheral and the meter, plug the PC end of the USB cable into the PC.

    If all is well, the peripheral should come on (hard drive spins up /
    LED comes on / whatever), and the meter should show some amount of
    current. If nothing is happening, or if you smell smoke, unplug the
    USB cable from the PC immediately and investigate. If the peripheral
    draws "too much" current, the PC may shut down that particular USB port;
    this isn't permanent, but sometimes it takes a reboot to re-enable the

    Don't be surprised if the current jumps around some as the device
    operates. Even "simple" stuff like keyboards will draw slightly
    different amounts of current when you are typing vs not, or when (say)
    the Caps Lock LED is on, or whatever. If your meter has a "max hold"
    or "peak hold" function, that can be useful to capture the highest
    current reading the meter sees. Some peripherals will have a current
    rating printed on them, but this is usually the maximum current it
    will ever draw - it won't usually draw that much continuously.

    When you are done measuring current with your multimeter, disconnect
    the leads from the circuit, and *IMMEDIATELY* move the red lead back
    over to its regular socket - don't wait until later to move it back.
    The reason is that in the amps range, the meter is nearly a dead short.
    It's *easy* to measure current and then try to measure voltage without
    moving the lead back, and blow the fuse in the meter. I've watched it
    being done and I've even done it myself.

    Standard disclaimers apply: I don't get money or other consideration
    from any companies mentioned.

    Matt Roberds
  11. mike

    mike Guest

    If I understand you, you're saying that it's ok to measure in the ground
    In a perfect world, that's no problem...Assuming there isn't a shield
    wire that bridges it.
    Given all the things that can go wrong, I'd suggest you stick to
    measuring in the +5 wire.
    Breaking the ground never breaks anything...until it does.
  12. Guest

    That is what I was saying, yes.
    The few USB devices I've taken apart seem to do a decent job of not
    returning the power supply current on the shield, but I am aware that
    there are millions of devices out there, and they probably don't all do
    it the same way. (Hey, if we run the power supply ground on the shield,
    we can use 3-wire cable instead of 4-wire and save 0.00013 cents per
    I agree that that's generally a good plan. I figured that not being
    picky about which power wire was opened would be easier to implement
    for the original poster.

    Matt Roberds
  13. Tony Hwang

    Tony Hwang Guest

    Jig? Don'have a bread board?
  14. josephkk

    josephkk Guest

    Oh lordy. For more than nominal charge currents (or standard supported
    currents) the thing is "negotiated" with the source device. Get yourself
    a copy of the Standard, 3.1 is current, older versions can be found.

    Start here:

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