# Charging a capacitor

1. Jan 8, 2008

### Joza

I was doing an experiment to determine the capacitance of a capacitor in a circuit with a particular resistor R. This was also set up to an oscillator and oscillscope.

Now t=RC, where t is the time constant. At a time of t, the value of V across the capacitor is .63(V_0), says my guide. So if I know V_0, I could workout out the time constant t, and hence the capacitance.

But what is V_0? The guide does not state. I am confused, and think I may have done the experiment wrong. I have an averaged value of the max V across the capacitor and the time it took to reachhis value if that's any help....

Is this value of V_0?

2. Jan 8, 2008

### pam

V_0 is the initial voltage on the capacitor before it was connected to the resistor.
It usually equals the voltage of the charging battery.

3. Jan 13, 2008

### harshu

v_0 is the maximum value of potential difference which can which can occur between theplates of the capacitor and is equal to the EMF of the battery

4. Apr 15, 2008

### jame1970

5. Apr 15, 2008

### jame1970

6. Apr 16, 2008

### indr0008

have u measured the resistance of the resistor used?

7. Apr 16, 2008

### jame1970

resistance 5 ohm.

8. Apr 16, 2008

### indr0008

then u're good to go, u knew all the things needed, the time, the resistance, what made it difficult to get C?

9. Apr 16, 2008

### jame1970

I known to calculated from t=rc, but I want what capacitance to capture 3dB at 1 MHZ.

10. Apr 16, 2008

### jame1970

Thanks,
I have plans to filter the ARINC 429 data lines.
Note: the ARINC 429 lines can have a pulse rise time as short at 1us. This translates to an effective signal on the order of 1MHz. The pi filter 3dB frequency has to be above the ARINC 429 frequency (ARINC 429 have frequency is 1Mhz).

11. Apr 17, 2008

### Redbelly98

Staff Emeritus
V_0 is the final voltage. You'll need to set the time scale on the oscilloscope so that you can see what the value is.

The capacitor voltage charges from 0V initially to V_0 eventually. The time to go from 0V to 0.63V is the RC time constant.