0

I am trying to create a feedback ammeter using Arduino MEGA ADC and OpAmp LM741CN. In order to perform a proof of concept, I use Arduino MEGA, LED, and Resistor 1 MΩ, then I read its current with a digital multimeter (DMM) UNI-T UT61e+ which is around 3.35 µA. Ideally, my feedback ammeter with Rf = 1 MΩ should give me the value around V=-I*Rf = -3.35 x 10^-6 * 10^6 = -3.35 V.

However, before I connect the GND of Arduino to the circuit, I can read about -0.9xxx V. But when I connect GND of Arduino to the circuit, it suddenly shows -10.xxx V. I do not understand what could happen to my circuit, or if my OpAmp or Arduino MEGA are already damaged.

As far as I know, the output of OpAmp should not exceed its voltage supply but I do not know why it shows -10 V which could destroy my analog pin on Arduino MEGA ADC.

LM741CN is powered by +-6 V from DC power supply UNI-T UTP1306 because I want to make sure that the DC power supply has nothing to do with this error.

Please see my attached diagram for more detail.

You can refer to this page about feedback ammeter http://www.kerrywong.com/2013/01/17/current-adapter-for-low-current-measurement-using-ts1001/

enter image description here

enter image description here

Peeratchai Kleebbua
  • 1
  • 2
  • 2
    Welcome! Mandatory read for you why you shouldn't be using 741: https://electronics.stackexchange.com/questions/304521/reasons-not-to-use-a-741-op-amp – winny Feb 17 '23 at 13:51
  • What output voltage do you measure with your multimeter? – winny Feb 17 '23 at 13:59
  • it is around -5.xxx Volt when I power LED with 5V pin on Arduino. It is supposed to be -3.35 V according to the formula. – Peeratchai Kleebbua Feb 17 '23 at 14:00
  • If you're really using resistors with values up in the Mega-ohm range then the current through the LED will be so low (in the micro-amp range) that it won't be dropping the voltage you expect it to. – brhans Feb 17 '23 at 14:10
  • 1
    *it suddenly shows -10.xxx V* <-- what, beyond your negative power rail? I don't think so. Your meter must be reading incorrectly or your power rails are not +/- 6 volts. – Andy aka Feb 17 '23 at 14:10
  • I am also curious about that. So, before I use, I measure its DC supply voltage and read it with a Digital multimeter. It give me the exact +- 6V. – Peeratchai Kleebbua Feb 17 '23 at 14:17
  • How do you read negative voltages with your ground referenced Arduino? Show your full schematic. Name your nodes and list what you measure in them referenced to ground with your multimeter. – winny Feb 17 '23 at 14:27
  • Just upload new schematic. I measure Negative voltage reference to Arduino ground with digital multimeter. Its Vout should be proportional to its I in accordning to the fomular V=-IRf. I try to make feedback ammeter which could measure low current down to nanoampere. Just like this http://www.kerrywong.com/2013/01/17/current-adapter-for-low-current-measurement-using-ts1001/ – Peeratchai Kleebbua Feb 17 '23 at 14:48
  • Do you think that I should not use GND from arduino? – Peeratchai Kleebbua Feb 17 '23 at 14:49
  • 1
    If you are reading -10V in a DC circuit that has +/-6V rails then your meter's black probe must be on a point that is at least +4V in reference the the center of the rails. I would suspect that your opamp supply ground and Arduino ground are not at the same potential. – GodJihyo Feb 17 '23 at 16:49

0 Answers0