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enter image description hereIs it possible to measure current that is flowing for 2 or 3 milliseconds inside a circuit through oscilloscope by measuring the voltage drop across a resistor ?

The supply voltage is generated by triboelectricity and it has a peak to peak of around 5 volts, it lasts only for about 3 to 7 milliseconds. I have connected this to two resistors in series, (10 Ohm, 10 Ohm) and i measure the open circuit voltage that is generated by triboelectric impact and the voltage drop across the first resistor to find the current in the circuit. So they are connected to two channels in oscilloscope (Voltage probe, 1x attenuation, probes are compensated)

But, it turns out that i see nothing in the oscilloscope, i cannot measure the the tribo electric voltage as well the voltage drop across first resistor.

When the two resistances are very high, say 100 Kilo Ohms, i see that both the channels in oscilloscope show the same open circuit voltage that is generated by triboelectricity during impact. How can i measure current ?

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2 Answers 2

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It sounds like you are inadvertently shorting out the resistor you hope to measure current through because the scope probes have a common earth point. If you have one scope probe's earth crocodile clip on the bottom of the lower resistor you can't arbitrarily connect the other probe's earth clip to a different point in the circuit because it shorts the two nodes together.

Take a step back and think what you are trying to do. If you connect one resistor (10 ohm) to your triboelectric generator and measure the voltage across this one resistor you can perfectly assume that the current through that resistor look exactly like the voltage across it.

This is basic ohms law i.e. I = V/R - if R is a constant then I is proportional to V and will be identical to it other than scale.

If it so happens that you see no voltage across the ten ohm resistor this is because your scope isn't sensitive enough. In which case, increase the resistor value or use a scope preamp.

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  • \$\begingroup\$ Thanks but how could i measure both the triboelectric voltage and current simultaneousy in two channels of an osciloscope otherwise ? or should i use two oscilloscopes ? I dont have a current probe. \$\endgroup\$
    – user3396084
    Jun 6, 2015 at 18:01
  • \$\begingroup\$ As Andy said, there is no need to measure both simulateously. I = V / R, so knowing one (voltage being the easier one to measure), you know the other. In fact, measuring the voltage across a series resistor is THE common way to emasure a current. \$\endgroup\$
    – Wouter van Ooijen
    Jun 6, 2015 at 18:02
  • \$\begingroup\$ I you realy want to measure the same thing using 2 oscilloscopes, make sure they have (are measuring using..) the same ground. \$\endgroup\$
    – Wouter van Ooijen
    Jun 6, 2015 at 18:03
  • \$\begingroup\$ @Wouter van Ooijen: Thanks, i tried it with one resistor, but the case now is i get the same voltage signal in the oscilloscope, the probe is at the beginning of the resistor and the crocodile clip at the end of the resistor. Whether or not i connect the crocodile clip to the end of the resistor, i get the same voltage signal. How is this possible ? \$\endgroup\$
    – user3396084
    Jun 6, 2015 at 18:05
  • \$\begingroup\$ @user3396084 It's possible because the tribo generator is also earthed and the scope earth point is adding another earth. If the tribo output is earthed then the scope measures (reasonably) correctly because the scope input ground point is also earthed thru the AC power wiring. Quite common to see. \$\endgroup\$
    – Andy aka
    Jun 6, 2015 at 18:07
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Shown below is basically what you have, where Rx is the resistor you want to use to measure the voltage across so you can determine the current through it by using Ohm's law.

You only need the one resistor, Rx, but bear in mind that your scope probe's resistance, Rp, and capacitance, Cp, are in parallel with Rx and will affect the measurement.

The values shown, 10 megohms and 15 picofarads, are typically what the circuit being probed sees when a times 10 probe is plugged into the scope, I believe, but you need to check your probe's data sheet to be sure.

enter image description here

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  • \$\begingroup\$ I did the same as in your sketch, but what is confusing me is this : the resistor will result in a voltage drop. Say the actual voltage generated by the generator is 9.2 volts, the voltage that i measure across the Rx in the diagram is around 9.1. I need a way to measure both \$\endgroup\$
    – user3396084
    Jun 7, 2015 at 10:58
  • \$\begingroup\$ How do you know the "actual voltage" is 9.2 volts? \$\endgroup\$
    – EM Fields
    Jun 7, 2015 at 11:51
  • \$\begingroup\$ i measured it without the resistor in place \$\endgroup\$
    – user3396084
    Jun 7, 2015 at 12:36
  • \$\begingroup\$ And what value is the resistor? \$\endgroup\$
    – EM Fields
    Jun 7, 2015 at 12:39
  • \$\begingroup\$ value of the resistor is 10 kilo ohms, case 1:without resistor, voltage is 9.220, case 2: with 10 kilo ohms, voltage is 9.063, case3: with 20 kilo ohms, voltage is 9.125 \$\endgroup\$
    – user3396084
    Jun 7, 2015 at 12:41

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