Why Does a Capacitor Block DC Voltage in a Circuit?

mmmboh · Mar 12, 2010

Hi, I have this circuit:

Circuit schematic of a CR circuit: a sinusoidal source v_i = V_i sin(ωt+φ) with a DC offset battery in series, feeding a capacitor C in series with a resistor R; probe taps the voltage v_R across R, with leads labeled to Ch1 and to Ch2, and current i = I sin ωt shown.

And when I change the DC offset voltage the wave in channel one moves up and down and the wave in channel 2 doesn't do anything except for flatten out a bit in certain areas (although I'm not sure that's relevant), my question is why? I know channel 2 is the voltage across the resistor, so is the change in DC voltage appearing solely across the capacitor? It appears the capacitor blocks voltage, but why?

Help would be great! :)

ehild · Mar 12, 2010

The capacitor blocks the DC current. (Its impedance is 1/(wC) so zero frequency means infinite impedance.) No DC current, no DC voltage across the series resistor, the signal in CH2 has no DC component. The DC voltage drops solely across the capacitor, as you said.

ehild

Why Does a Capacitor Block DC Voltage in a Circuit?

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