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I am kind of lost because of this problem.

When my board is connected to my power supply and up and running the resistance between VCC and GND is at about 18M-Ohms.

As soon as I power the board off, and it's no longer running, the resistance between VCC and GND drops slowly to about 3k-Ohms, then stays there.

What could be causing this?

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    \$\begingroup\$ How do you measure the resistance? You know, you can't do it with multimeter in a powered circuit... \$\endgroup\$
    – Eugene Sh.
    Jun 6, 2019 at 14:24
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    \$\begingroup\$ Also, why is this a problem? Does your board not work? Or is this just something you've noticed and are assuming to be a problem on a board that works perfectly fine? \$\endgroup\$
    – MCG
    Jun 6, 2019 at 14:33
  • \$\begingroup\$ Most likely, your multimeter is confused. A multimeter passes a current through the unknown resistance, and measures the voltage produced. If the device is already powered up, then the voltage measured by the multimeter is meaningless. \$\endgroup\$
    – Simon B
    Jun 6, 2019 at 14:52
  • \$\begingroup\$ 18M ohms input resistance? you must be able to supply millions of amps, else a measurement is wrong... \$\endgroup\$
    – dandavis
    Jun 6, 2019 at 20:42
  • \$\begingroup\$ @EugeneSh. Yeah, I just noticed that as well.. my bad \$\endgroup\$
    – user18596
    Jun 7, 2019 at 6:41

3 Answers 3

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What could be causing this?

When the board is off this is a little unusual because most often the leakage current through transistors or loads when the power is off is in the 1-100kΩ range for most of the boards I have with voltage regulators on them. If you had something with a high side switch or a relay this could explain why the numbers are so high.

When the board is on, it really matters how your meter is measuring current, and often doesn't make sense. If you really want to find the equivalent load, measure both voltage and current going into the board with a nice digital power supply or a power supply and a multimeter to measure current. Then use this equation.

\$ R_{eq}=V/I\$

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You can't measure resistance between VCC and GND when power supply drives voltage between VCC and GND. The multimeter is confused because there is an external voltage source. Also, measuring resistance of anything that is not a resistor will give weird readings, as they can have nonlinear behaviour like LEDs or semiconductors. At most you can measure if there is a short circuit or not.

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Multimeters measure resistance by applying a known voltage across and measuring the current, or vice versa, a known current being applied and measuring the voltage. You can't measure devices that are on this way because the voltage of the device is almost always higher than the voltage being applied, overriding it.

On top of that semiconductors can have different resistances depending on how they're biased, which could also cause it.

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