1
\$\begingroup\$

I had an issue using an Arduino Nano to investigate simple RC circuits. The capacitor would not fully charge to the supply voltage. I assumed it was something to do with the digital pin circuit on the Arduino so I set up the RC circuit with a 1.5V battery. Same issue.

Here's what I found. Caps are ceramic non-polarised.

  • 100 ohm + 10nf = charged to 1.2 Volts

  • 100 ohm + 100nf = charged to 1.2 Volts

  • 110K ohm + 100nf = charged to 900 mV

  • 1M ohm + 100nf = charged to 650 mV (this was the original circuit analysed with a 1-second pulse from the Arduino) 650 mV charge after 400ms, which is about right for the time period.

I am using a standard breadboard and a 6022 Hantek PC oscilloscope. My voltmeter confirms the same charged voltage results.

Why does the capacitor not charge to supply voltage with high resistance values? Any clues?

\$\endgroup\$
1
  • 3
    \$\begingroup\$ You should include a circuit to show how everything is connected. There's a circuit drawing tool available when you edit your question. \$\endgroup\$
    – Bimpelrekkie
    Commented Aug 3, 2018 at 15:31

1 Answer 1

Reset to default
3
\$\begingroup\$

I suspect that the input resistance of your meter and scope is about 1 M\$\Omega\$ and this resistance, connected in parallel with the capacitor, is changing the circuit behavior.

You can check this with your meter by measuring the voltage across the capacitor and then the voltage across the resistor. If the sum of these does not equal the voltage across the battery then you have a measurement problem.

\$\endgroup\$
1
  • \$\begingroup\$ Thanks Elliot. You were spot on. The Hantek has dual settings on the probe 1X and 10X. At 1X the impedance is 1M ohm. Changed the setting to 10X (10M ohm) and it's giving a sensible reading. \$\endgroup\$
    – Graham287
    Commented Aug 3, 2018 at 21:16

Not the answer you're looking for? Browse other questions tagged or ask your own question.