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I want to analogRead() the voltage coming from a plasma cutter and serial print it.

The problem is that the serial communication between my laptop and Arduino are breaking as soon as i turn on the plasma cutter, and this is due to high nose on the ground rail (since the ground of the plasma cutter and the ground of Arduino are connected).

Is there a way to keep the grounds separated and be able to analogRead() the voltage?!

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You could use a digital isolation technique that can power a remote ADC. ADI have some chips that can do this: -

enter image description here

The ADuM5401 provides bi-directional isolated digital lines (SPI) and a little bit of isolated "juice" that can be used to power the ADC (AD7793). This is just a general idea that I have used before but if you check out what ADI have to offer on the different range of ADC chips (AD77xx) you will find other options like thermocouple inputs, RTDs etc..

You can of course use an isolation amp like this: -

enter image description here

But the slight down side is that you need to provide isolated power using an external DC/DC converter. Here's a clearer picture of the internals of one type of isolation amplifier: -

enter image description here

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  • \$\begingroup\$ I'm a little intimidated by your solution, i have an op amp LM741 and an opto isolator 4n27 on hand, can you give me a simple schematic on how to separate grounds, reading for example a simple potentiometer?! \$\endgroup\$ – Apastrix Dec 29 '15 at 11:50
  • \$\begingroup\$ @Apastrix I would never use a 741 op-amp in ANY circuit so unfortunately not. However, having a good op-amp and an opto does not give an isolated means of transferring an analogue signal to another system. Sorry dude but the method above is what I've used and I used it for the same reason - to isolate the source from the measurement system. \$\endgroup\$ – Andy aka Dec 29 '15 at 11:56
  • \$\begingroup\$ Ok i understand, but can you point me to any site to learn a bit about this method? just to clear the intimidation side : ) \$\endgroup\$ – Apastrix Dec 29 '15 at 12:00
  • \$\begingroup\$ I would google the term "isolation amplifier" or "isolating an analogue signal" and see what turns up. There should be plenty of hits, When you have something that may be of interest, leave a comment or maybe raise a new question. \$\endgroup\$ – Andy aka Dec 29 '15 at 12:05
  • \$\begingroup\$ Ok thanks for your time and for the Key word there, i will see what i can do : ) \$\endgroup\$ – Apastrix Dec 29 '15 at 12:47
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Which grounds do you want to isolate: cutter-Arduino or Arduino-laptop? It's easier to galvanically isolate digital signals than analog, so I'd suggest running the Arduino off batteries, and optocoupling the serial line Arduino-laptop.

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  • \$\begingroup\$ @How to opto isolate the serial line? i'm connecting Arduino to Laptop directly using a USB cable. \$\endgroup\$ – Apastrix Dec 29 '15 at 11:52
  • \$\begingroup\$ My apologies. I was assuming a conventional RS232 serial connection. If you feel up to it, you could try to bit-bang a one-way serial line from a spare TTL output port on the Arduino, squirting the data out (at say less than 1200 bpa). You could then use your optocoupler to drive a serial port on the laptop (which it probably won't have, so you'd need a USB/serial port convertor anyway. \$\endgroup\$ – RMW Dec 29 '15 at 12:06
  • \$\begingroup\$ This solution seems legit, i never thought of isolating the serial communication itself, i will have to try it out though, thanks for the suggestion. \$\endgroup\$ – Apastrix Dec 29 '15 at 12:49
  • \$\begingroup\$ Verily. There have been many times where I've been stymied by (in my case) computer/telephony circuitry and the conflicting requirements of 'pass analog faithfully' and 'connect it to other equipment'. The solution is sometimes to let the VCC ride atop the telephone voltage and then isolate everything in/out. \$\endgroup\$ – RMW Dec 29 '15 at 13:00
  • \$\begingroup\$ Serial isolation is accomplished through a external UART or tranceiver, then e.g. SPI or TX/RX can be wired through single-directional (simpler) optical or rf isolators. \$\endgroup\$ – crasic Dec 29 '15 at 20:15

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