capacitor

[kowalskil@alpha.montclair.edu]

Kyle Forinash wrote:
> I had students charge up a capacitor (10 microF) in series with about a 
> 1 megOhm R with a 9V transistor battery. They measured the voltage on the 
> cap with a CBL at about 2 samples per sec and get a very nice 1-exp curve 
> which levels off nicely after a few seconds. But it levels of at a voltage 
> much lower than 9V (around 5V)! Am I missing something? Shouldn't the final 
> voltage be 9V? Is there something about the CBL which would prohibit doing 
> this? It seems to be about the same for each setup.

I do not know what the input imperdance of a new CBL probe should be but your 
result may be explained by assuming it is nearly as low as 1 mega-ohm. You 
have a resistive voltage divider with C connected to one of its components. 
Test this by lowering your R=1 megaOhm to 0.5; this should move you closer to 
the expected 9 volts. You can measure the probe impedance by discharging a 
known capacitor through it (from the rate of discharing). Please share your 
conclusion; we can all learn from it. 
                                                 Ludwik Kowalski