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evilissimo The diagrams show you how you _can_ connect the voltmeter if your tested circuit is in the range of 4-30V then they can share the negative rail (-). But in case of a separate power supply for the device itself, so that it is able to test the full supported voltage range, the negative rail (-) cannot be common otherwise the circuit wouldn't be complete and no current can flow. That is, because the device itself needs at least 4v to operate and cannot tolerate more than 30V. The latter would burn the voltage regulator and and that would end up frying the devices circuitry.

2020-02-24 11:45:18 (4)
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