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Timothy Garott
Timothy Garott
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I'm using an oscilloscope with probes that are specified to have to have an input resistance of 10Mohms, I apply 5 volts (using power supply) in parallel to my capacitor and use the scope in parallel as well. Using the scope I get, at 2.5 Volts the time is 90 ms. My capacitor is 100 nF. Using the 90 ms I know that the time constant is just 90ms/ln(2) = 0.129s.

so using time constant = R*C I get that the R = 1.29 Mohms, but shouldn't I have obtained 10 Mohms? Can anyone please help, is the scope input reactance negligible (Watching discharge after disconnecting power supply) ?

I'm using an oscilloscope with probes that are specified to have to have an input resistance of 10Mohms, I apply 5 volts in parallel to my capacitor and use the scope in parallel as well. Using the scope I get, at 2.5 Volts the time is 90 ms. My capacitor is 100 nF. Using the 90 ms I know that the time constant is just 90ms/ln(2) = 0.129s.

so using time constant = R*C I get that the R = 1.29 Mohms, but shouldn't I have obtained 10 Mohms? Can anyone please help, is the scope input reactance negligible?

I'm using an oscilloscope with probes that are specified to have to have an input resistance of 10Mohms, I apply 5 volts (using power supply) in parallel to my capacitor and use the scope in parallel as well. Using the scope I get, at 2.5 Volts the time is 90 ms. My capacitor is 100 nF. Using the 90 ms I know that the time constant is just 90ms/ln(2) = 0.129s.

so using time constant = R*C I get that the R = 1.29 Mohms, but shouldn't I have obtained 10 Mohms? Can anyone please help, is the scope input reactance negligible (Watching discharge after disconnecting power supply) ?

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Timothy Garott
Timothy Garott
  • 13
  • 4

Find R value from capcitor discharge plot obtained using scope

I'm using an oscilloscope with probes that are specified to have to have an input resistance of 10Mohms, I apply 5 volts in parallel to my capacitor and use the scope in parallel as well. Using the scope I get, at 2.5 Volts the time is 90 ms. My capacitor is 100 nF. Using the 90 ms I know that the time constant is just 90ms/ln(2) = 0.129s.

so using time constant = R*C I get that the R = 1.29 Mohms, but shouldn't I have obtained 10 Mohms? Can anyone please help, is the scope input reactance negligible?