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I need to connect an AC mains referenced measurement circuit with a notebook and I wanted to know if its safe to do that without an isolator. The measurement circuit primarily steps down AC voltage and current and then runs it through a set of instrumentation amplifiers (powered by a 5V wall-wart) to give a single-ended output which is then connected to the ADC pins of a MCU (which is a TI Launchpad). Now can I safely connect this LP to my Notebook USB without any isolator between them?

I saw an eevblog video where Dave shows that in a notebook, the charging pin 'ground' and the earth pin on the mains plug have very low resistance between them (essentially shorted). On my Dell notebook thats exactly the case. In that case, would the shown connection arrangement work and be safe? The first USB port on the extreme left is the notebook's USB port.

Measuring mains referenced power output

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  • \$\begingroup\$ Since people generally just do a design that works in every country, I doubt that the average circuit connects GND to anything else than the power plugs safety earth. \$\endgroup\$ – PlasmaHH Jun 14 '16 at 9:35
  • \$\begingroup\$ You should draw circuits from left to right, just as you do with writing. At a first glance, the circuit you've drawn seems to somehow generate mains voltage from a USB port. \$\endgroup\$ – Dmitry Grigoryev Jun 14 '16 at 10:15
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I need to connect an AC mains referenced measurement circuit with a notebook and I wanted to know if its safe to do that without an isolator.

No, of course not. Live and neutral could become swapped anywhere in your house wiring and what you might have thought was a "groundy" neutral reference will in fact be live. Don't do it.

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