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I was trying to measure the current draw of a USB-powered device (a Raspberry Pi) to determine the suitability of powering from an external battery pack. So I took a USB cable, split the +5v (red) line, placed a DMM on the line, plugged it in, and got.... nothing. The pi wouldn't boot. Replacing the DMM with a jumper worked fine, so the cable was good and I didn't screw it up with splitting the +5v line. Is it possible the internal resistance of the multimeter caused too much voltage drop on the line for the Pi to boot? What's the right way to do this?

To clear up other possibilities:

  • The meter works fine on other applications.
  • I double checked the setting of the meter, set to mA.
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  • \$\begingroup\$ check the fuse in the meter? \$\endgroup\$ – geometrikal Jun 9 '14 at 6:35
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    \$\begingroup\$ Do you have the leads plugged in the right sockets? On most meters switching from volt-ohm mode to amp-mode requires changing which socket the red lead is plugged into. \$\endgroup\$ – Dan D. Jun 9 '14 at 6:39
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    \$\begingroup\$ The burden voltage is probably too high. Try switching to the 10A range (if you have that) and ensure good connections to the meter. \$\endgroup\$ – user36129 Jun 9 '14 at 6:59
  • \$\begingroup\$ @geometrikal Fuse's fine, works for measuring current if I just put an LED, current limiting resistor, and a power supply (not the USB charger) together. \$\endgroup\$ – David Jun 9 '14 at 7:34
  • \$\begingroup\$ @DanD. Yep, hooked up correctly. \$\endgroup\$ – David Jun 9 '14 at 7:34
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Do this:

  1. Unplug the cable from the host and the device.
  2. Make S1.
  3. Adjust the power supply for 5V out as read on V.
  4. Open S1
  5. Connect the cable to the host.
  6. Connect the cable to the device.
  7. Make S1.
  8. If required, readjust the power supply for 5V across V.
  9. Read the current into the device on I.

enter image description here

S1 is a simple ON-OFF toggle, slide, or alternate-action pushbutton switch rated to carry at least the device load current and hot-switch 5VDC, and the 100µF cap is used to debounce it.

The device's input voltage is measured on the load side of the ammeter, and by setting it to 5V using the voltmeter, the burden imposed by the ammeter will be lifted.

The circuit works by having the host and the device ready to talk to each other when the cable is connecting them, and then allowing them start when S1 is made.

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This is why you need two multimeters. Then you could measure the voltage to see if it has sagged.

What device are you using to supply power? A computer or a USB charger? Computers can be funny about this sort of thing because they limit current to 100mA by default and rely on there being a current spike to detect when a device is plugged in, which the burden resistor might be preventing. A USB charger will probably be more forgiving.

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  • \$\begingroup\$ No, the problem is that a Raspberry pi draws drastically more current during startup. It should not be powered by a USB port, but by a USB power supply. \$\endgroup\$ – Chris Stratton Jun 9 '14 at 13:55

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