Measure internal resistance of an apparatus

Discussion in 'The Projects Forum' started by marko2388, Mar 29, 2015.

  1. marko2388

    Thread Starter New Member

    Mar 29, 2015
    Hello all,
    I have an idea to measure apparatus's internal resistance before applying mains voltage to it. Doesn't sound so difficult at first, i'd just apply 5v and read the voltage at the other end. But I also want to make sure that my microcontroller (Arduino for prototyping) will survive if the same circuit comes in contact with the mains (220VAC). My friend draw me a schematic for that, but he was not absolutely sure and suggested to collect more information. And that's why I'm here now. Please have a look of the schematic and give me your opinion. Thank you.


    Moderators note: inserted full image
    Last edited by a moderator: Mar 29, 2015
  2. #12


    Nov 30, 2010
    Some proportion of the mains rectified to a negative voltage which is sent into an ADC port?
    I think, "Start over and explain this better".
    Why would anybody apply an out-of-bounds voltage to an ADC port and what are these, devices you want to check for survival ability?
  3. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
    Dont connect your Arduino to the mains directly it will blow up, you need to use a transformer ,

    that circuit doesn't look good do you have a better one.
  4. marko2388

    Thread Starter New Member

    Mar 29, 2015
    Maybe we should leave this schematic on the side and start from zero. Idea is to make kind of smart-plug (s-p) device which will be connected between mains outlet and apparatus. As an example if we would have a desktop lamp, s-p would measure fist if the bulb inside the lamp is ok. If controller A/D reading is zero, means that there is no bulb or it's broken. But just in case, I would make the circuit in a way that if it comes in contact with high voltage, it won't get destroyed.
    I draw a schematic to give the very basic idea and let's move on from there.


    Moderators note: inserted full image
    Last edited by a moderator: Mar 29, 2015
  5. bertus


    Apr 5, 2008

    Looking at the last drawing, the apparatus is isolated from the mains by a relays and connected to the uC.
    NEVER connect the mains to the uC without any isolation.

  6. Wendy


    Mar 24, 2008
    Good choice, that schematic as shown would result in this thread being closed.
  7. ScottWang


    Aug 23, 2012
    I saw the report and the first circuit, so I close the thread, the TS must be posted the second circuit before and during I closed the thread, so I didn't see it.
  8. wayneh


    Sep 9, 2010
    Why? This would usually be a totally useless measure. In fact I can only think of one instance where it would be useful, when you know that the "apparatus" is a resistor, with a more-or-less constant resistance. Even a lightbulb would not qualify, as its resistance changes hugely once current flows. Motors? Nope. Any apparatus with active components? Nope.

    What are you really trying to do?