4 Replies Latest reply on May 14, 2020 9:10 PM by Jan Cumps

    Voltage drop


      I am using Keystudion Uno and it says on their sites that the current output is 20mA
      I have measured the Keystudion Uno supply power and Communication cable without plugging it into the Keystudion Uno yet, and gave me a reading of 5.09 V with 0.19 mA.
      Then I plugged in my Keystudion Uno cable to theKeystudion Uno and I measured the Default 5V output with my digital multi-meter(UNI-t UT50C), it gave a reading of 5.04v And Current of 0.19mA
      Last thing, I hooked a small resistor of 47 Ohm in series with the Arduino 5v Terminal, the Multi-meter shows a reading of 4.90v and Current of 0.11 mA
      What I am trying to do is to find out where the 0.14V was dropped. I understand that Multimeter has an Internal resistance(Which I am trying to calculate). Is it possible that Keystudion Uno5v output source has also an Internal Resistance, if so (which I think) how can I Calculate it ?
      Also why there was a voltage drop of 0.05 between 5.09v and 5.04v ? where that voltage has gone ? Thanks

        • Re: Voltage drop

          The readings don't make sense to me.

          If it is not plugged in the current should be zero. This sounds like the meter is not working well or is not connected properly.

          If the current doesn't change when you plug in the Arduino (still 0.19 mA), then the Arduino didn't really get connected or the meter is not connected properly. An Arduino Uno draws about 50 mA. Maybe the cable is faulty.

          Because the voltage drops when the Arduino is plugged in, it sounds like the Arduino is taking current, just not measured properly.

          The 50 mA that an Arduino draws will cause a voltage drop in all connectors and all wiring between the power supply and the Arduino.

          If you measure the voltage at the power supply and the at the Arduino end of the cable before and after plugging it in, you should see where the voltage is dropping.

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            • Re: Voltage drop

              My bad I forgot to mention that I am using Keystudion Uno and it says on their sites that the current output is 20mA


                • Re: Voltage drop

                  I assume you mean this bit from the Keystudio web page:


                  DC Current per I/O Pin 20 mA


                  This means that the maximum current you can draw from an IO pin is 20mA.

                  The data sheet for that parts says that with a 5V supply and a 20mA load current the voltage on an output pin could be as low as 4.1V, so the effective source resistance is about 45 ohms.

                  Something is wrong with your meter or technique, in your 3rd experiment you show 4.9V across a 47 ohm resistor and a current of 0.11mA, actually the current would be more than 1A. Did you measure the voltage first and then the current ? I suspect that you might not have programmed the output pin to be an output driven high but that it is being pulled up by the internal pull up resistor which is between 20k and 50k. In your third experiment the current is 0.11mA so the resistance limiting the current is 5/0.11e-3 = 45k - which looks about what you might expect from the internal pull up.



                  2 of 2 people found this helpful
              • Re: Voltage drop
                Jan Cumps

                Experiment 1b seems to shortcut the USB. The current meter directly over the power supply.