Re: gEDA-user: ESR of 2.2u ceramic capacitor

[John Doty <jpd@xxxxxxxxxxxxx>]

On Feb 25, 2006, at 5:23 PM, Bill Sloman wrote:

> At 23:01 25-2-2006, you wrote:
>
> > On Feb 25, 2006, at 1:39 PM, Karel Kulhavy wrote:
> >
> > > What does it mean? That the capacitor loses capacity when switching
> > > between charge and discharge?
> >
> > Yes. If you start with your 2.2 µf capacitor discharged, charge it up
> > to a volt, it might take the full 2.2 µC it's supposed to. But then
> > discharge it to zero volts, you might only get 2.1 µC back! The
> > charge doesn't disappear (averaged over many cycles the net DC
> > current will be zero), but in general when you push charge in with
> > voltage, the capacitor pushes back with less voltage when it's
> > discharging. That's a net energy loss to the circuit, dissipated as
> > heat. A "Q meter" will register this as resistance, but it's not
> > ohmic, and may not have the same effect as ohmic resistance on
> > circuit operation.
>
> Put on pedant hat.
>
> Charge is conserved

Sure. I said that.

Charge is *always* conserved. Charge in one wire equals displacement  
charge in the dielectric equals charge out the other wire. But this  
is a little trickier. If it was ohmic leakage you could keep pushing  
charge through forever with a fixed voltage, but in dielectric  
hysteresis you can't.

> - if you could convert electrons to heat, you could fry your  
> circuit, and eliminate our dependence on oil.
>
> Remove pedant hat.
>
> What you are talking about here is "charge soak" which is often  
> modelled as extra capacitance-plus-series-resistance in parallel  
> with the ideal capacitor you thought you were buying. I've seen  
> time constants varying from a few seconds to a number of minutes,  
> depending on what I was doing. The charge is still in the  
> capacitor, but it's coming out slowly.

It can take years. That's how "electrets" work. In the vacuum tube  
era (yes, I'm that old, barely) I remember reading about folks using   
this effect to make a rechargeable "bias cell" for grid bias!

John Doty              Noqsi Aerospace, Ltd.
jpd@xxxxxxxxxxxxx