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previous: ken gilbert q{In a voltage doubler, to pick a s... -- 4/27/2000 10:41 PM View Thread

Re: Power interrogation

4/28/2000 6:39 PM
Re: Power interrogation
I know you're going to think this is chicken sh*t, but I disagree with your description of a voltage doubler stack as one capacitor. It acts like one cap in discharge, but I think it is more accurate to model it as two caps in charging.  
Let's see if I can convince you.  
A few weeks ago there was a strand on what the performance of a cap between the two plates of a diff amp would be. the specific question was would the 1/2 pi r c equation describe its behavior. You wrote a well reasoned answer that it would behave as if the cap were twice as big.  
I went on spring break and never saw if someone else answered why there was the disparity of value, but I wondered what the physical explanation was. The answer bears on this situation.  
A value of capacitance for an object is defined as its ability to store a certain amount of charge, WHEN CHARGE IS APPLIED TO ONE PLATE.  
Since the diff cap was driven by both plates, it was acting like a value twice as big.  
A cap driven from two sides is like having two caps in series. The junction between them can be modeled as being at ground in an ideal circuit, assuming no external or internal resistance which would tend to act like a voltage divider.  
Now to the point at hand. I don't think of the stacked caps as one cap being recharged at one end or the other at 120 hz, but as a two cap stack which recieves charge at its two different points at 60 hz.  
The doubler does not act as a single cap because the VA being delivered at 120 hz is half what a single cap would recieve because the voltage difference between the cap end and the diode end is half as much as it would be if feeding only one terminal.  
Buy it?