bobJandal

Hi Zeplin. Thanks for your reply. Theory says that there should be no oscillation in the second setup, because the capacitor only sees the positive voltage from the source because of the diode, which blocks the negative voltage, making the only source of discharge for the capacitor being the leakage current in the resistor itself. I think I may have come up with an explanation for why two diodes causes oscillating voltage across the cap: It is to do with which part of the circuit the sampling instrument (eg: voltmeter, oscilloscope) sees as ground. If the circuit ground is used, then the voltage can only be the voltage coming in from the source, hence the oscillations that were confusing me, because there is no path from the top of the cap to the bottom thru the sampling instrument, because of the diode on the bottom. However, if the sampling instrument samples the voltage between the two pins of the capacitor, there is no oscillation (save for the very small ones due to the discharging from the internal cap leakage). :D ?

Oh yeah, And here are the multisim files too, in case someone wants to play, but doesn't want to click. This is the problem, including the solution that I found to it. why_two_diodes_no_charge_cap.zip

The weirdest thing. I drew a picture (well, multisim did) to describe this thing better. In words, I have found that using an AC source, with a diode between it and a grounded cap (half wave rectifier) will charge a cap, but if another diode is put between the cap negative and ground (naturally, pointing the right way, as to allow charge to flow), the cap will not stay charged, but rather its voltage will oscillate with the source. How very strange, methinks. Any suggestions ? Anyone ever heard of this before, even ? I have built the circuit depicted (hence the strange values of the caps - thats what I physically had) and its results are concurrent with the simulation results. I tried bigger capacitances, and still the same thing occurs. The only solution I found so far is to have a very small cap attached after the first diode, and then going straight to ground, which makes the cap in between the diodes charge as I would have expected (no oscillation) ??? !!!!