Unreliable measurements from the Scopy M2K DC voltmeter when using the DC voltage source

I set up a simple experiment to show students the DC properties of resistors, inductors and capacitors. I connected a 0.1uF capacitor in series with a 10mH inductor and a 10 Ohm resistor to drive a white LED with the DC power supply. (See figure) My goal was to show that the capacitor is an open circuit and the inductor has DC resistance. When I eliminated the capacitor, the LED lights up nicely with a DC voltage source set at 3-3.3V. I thought that students would be able to leave the circuit as is and measure the DC voltages across the inductor and resistor to estimate the value of the inductor DC resistance. Unfortunately, when I did this (with the LED still in the circuit), the relative voltage values did not reflect the resistance of the inductor. They values fluctuated some but generally predicted a much smaller inductor resistance. I could get the resistance to come out properly If I eliminated the LED and made a voltage divider out of the inductor and resistor. It appears that at least one of the wires for each measurement channel has to be connected to ground or a voltage source to get a reliable measurement. If this is the case, this limits the applicability of the M2K for determining voltages and currents in a circuit. 

  • I was going to try this experiment again but now I cannot get Scopy to run on one of my laptops. It turns on and recognizes the M2K that is connected, but when I click on connect, it crashes while calibrating. I tried it with my other laptop and it worked fine. The voltage measurements also made more sense because the voltage across the inductor was close to 3X the voltage across the resistor (30 Ohms vs 10 Ohms), which is what it should be. I uninstalled and reinstalled Scopy but the same thing happened. This is quite confusing because the laptop Scopy does not work on is much newer, faster and has a lot more memory. Both are Lenovo X1 Carbons. 

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    •  Analog Employees 
    on Oct 26, 2020 4:06 PM 29 days ago in reply to KenC

    Hi, 

    What Scopy version are you using ? After installation, usually a restart is required. 

    -Adrian

  • 0
    •  Analog Employees 
    on Oct 26, 2020 4:53 PM 28 days ago

    Hello,

    Have you considered the voltage drop on the LED?

    If the voltage drop on the led is close to the supply voltage, then you possibly measure a very low voltage (tens of mV maybe) on both ch1 and ch2. This value can be affected by the gain setting of the m2k, because in low gain  (in Scopy >= 1.0v/div)  resolution step is around 15mV .

    Try to measure these voltages with a multimeter, if possible, and see if the values correspond with what you measure in Scopy.

    I tried a similar experiment with 3 resistors in series and the voltage measured differential across each resistor was as expected.

    Regards,

    Andreea

  • 1.1.2 and I did restart after installing. 

  • As I noted above, I tried the experiment with a different computer and it seemed to work reliably. I did that because I now cannot get Scopy to work on my newer computer since it keeps crashing. Everything worked fine, except for the DC measurements, for a few days while I was running a series of experiments. No idea why I should be having problems now. As for source voltages near the LED voltage, I tried many different source voltages because I wondered about the same thing. All results had problems. There may be something going on with my newer laptop that is the source of problems, but everything else seems to be working. I am stumped.