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Thread: using multimeter with teensy

  1. #1
    Junior Member
    Join Date
    Feb 2021

    using multimeter with teensy

    I have a teensy 3.2 plugged into a laptop via usb hub, and the teensy 3.2 has a sensor attached. The same teensy 3.2 is also sending analog input to a teensy 4.1 plugged into the same laptop via a different USB port. The ground of the teensy 3.2 is plugged into the ground of the teensy 4.1, although I may change it so that the ANalog ground of the teensy 3.2 is connected to the teensy 4.1

    How do I measure the total current used by the teensy 3.2 with a multimeter?

  2. #2
    Senior Member PaulStoffregen's Avatar
    Join Date
    Nov 2012
    Measuring current requires cutting the wire carrying the current **, so you can connect your multimeter probes to the 2nd ends, causing all the current to flow through your meter.

    The 2 options for doing this are to cut apart the VIN-VUSB pads on the bottom side of Teensy, or to splice into a USB cable and cut the red power wire. If you have extra (hopefully cheap) cables and you're not comfortable soldering, the spliced cable option might be the better way to go.

    Most multimeters have a separate place to plug in the positive lead when measuring current. The normal place for voltage measurement is a very high impedance (usually 10M ohm) between the leads. The current measurement has a fuse and low-value resistor connected between the leads. Internally, the meter measures the small voltage.

    If you've ever had the leads plugged into the current inputs and then you tried to measure a voltage on a power supply or battery, the fuse is probably blown. It's an easy mistake and one that's very common on multimeters. The problem is the meter can't tell you whether the fuse is ok. So if you don't normally use your meter this way, I recommend first connecting a resistor (like 1K) and power supply or battery, so you can check if your meter is good by measuring a that current. If it doesn't work, you'll know to open up your meter and replace the fuse... before so much frustration trying to make a measurement which matters.

    ** - There are some non-contact measurements which sense the magnetic field around the wire, but they involve expensive equipment and the accuracy is poor for DC current.

  3. #3
    Senior Member+ defragster's Avatar
    Join Date
    Feb 2015
    There are also usb 'dongles' that go inline from the USB Host and give an idea of the current passing to the device.

    They can be cheap or elaborate and measurement accuracy or resolution will be depend on which one is chosen.

    Amazon, Tindie, Ebay will give an idea of what is available and what they claim as features.

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