Capacitor discharge

[Hero999]

Constant current loads only exist in theory Hero, at least ones of sufficiantly low series resistance to truly completly discharge a capacitor. Then again you're talking about nano volts in a practical circuit if even that. Thermal noise is probably higher than the voltage left over in a real world capacitor when it's discharged through a low ohm load.

It's all theory. In practice capacitors do fully charge and discharge (as close as f*ck is to swearing anyway).

 

[Sceadwian]

Nigel, there IS no existing context, the original poster hasn't provided a circuit surrounding this current source and it's capacitor or how it's switched (if it's charged and discharged switching is a requirement which makes the boot strapping question VERY valid) Especially in a discharge application like through a filament as G8RPI was trying to mention.... Mind you an inductor can be described as a current source. Upon a little more research on Wikipedia they're apparently used in Class E amplifier design as switched current sources. But once again we're digressing further and further from the insufficient information the OP provided.

 

[Ambient]

We are hungry for a schematic. Feed us!

Edit: You could also be forcing too much current through the cap. At any rate, a schematic or more info is needed. But at least use a higher voltage and beefier cap and that may fix it.

 

[mneary]

The OP appears to be asking a hypothetical question, and has NOT cited an example where the "problem" occurs.

Therefore, the simple answer is, a small amount of the charge may be retained upon discharging, depending on the dielectric and the discharge time. When charging, the voltage on the second charge may be higher. But on the second and subsequent discharges the slightly higher voltage will cause a greater discharge current. This is self-limiting, generally at a total voltage near the original charge voltage (in this case 10V).

 

[Nigel Goodwin]

(if it's charged and discharged switching is a requirement which makes the boot strapping question VERY valid).

You've added the word 'switching', and deleted the word 'constant' from 'constant current' which was in the meagre information we do have - those are important changes which still only give a tiny possibility that it's anything to do with switch mode. By sticking simply to the information that's been given, 'constant current source' and no mention of 'switching', it reduces that tiny possibility to zero.

 

[Sceadwian]

I'm sorry Nigel did you actually read any of the posts that prasannan made? Not one of them has the word constant in it anywhere and he explicitly stated he was charging and discharging it using two seperate time constants and that the capacitor was burning out from charge accumulation which REQUIRES switching to occur... Why on earth are you still arguing about this?

 

[Nigel Goodwin]

I'm sorry Nigel did you actually read any of the posts that prasannan made? Not one of them has the word constant in it anywhere and he explicitly stated he was charging and discharging it using two seperate time constants and that the capacitor was burning out from charge accumulation which REQUIRES switching to occur... Why on earth are you still arguing about this?

Sorry about that, I obviously 'invented' a 'constant' that wasn't there (but may well be anyway ).

But what he DOES'NT say is that he has ever burnt out a capacitor! (not that 'burnt' is a word I'd use), and I've always considered his fears were (and are) purely theoretical without any actual understanding of the theory involved. But, as always, we need to see what he's doing (or thinking of doing), however with a question based on such little knowledge it's HIGHLY unlikely he's making charge pumps or switchmode devices.

My answers were (and are) as accurate as possible with the meagre information supplied, whilst making the smallest amount of extrapolation on the original posts. Imagining charge pumps and switchmode designs is a MASSIVE leap from what we've been told.

 

[Sceadwian]

In one of my application i am charging a capacitor using a current source for some time x sec and discharging the capacitor for y sec.But some voltage is present after the discharge also.
When this cycle repeats the charge accumulates in the cap and voltage increases in each cycle and the cap burns.
I want to discharge the cap fully to absolute zero value.

Nigel.. it seems strange that you forgot he said that considering you quoted it yourself in a reply to him further stateing that it was impossible to have a capacitor charge above VCC...

 

[Nigel Goodwin]

Nigel.. it seems strange that you forgot he said that considering you quoted it yourself in a reply to him further stateing that it was impossible to have a capacitor charge above VCC...

It is with the information given, unless you make a wild extrapolation on the meagre information we do have.

 

[palol]

try to discharge it with negative current

 

[Roff]

That's nonsense, no where in capacitor theory does it say that you can't fully charge or discharge a capacitor. It just says it is impossible with a series resistor.

A capacitor will charge linearly if given a constant current source and discharge linearly if given a constant current load.

A 1F capacitor charged to 1V will discharge through a 1A constant current load in 1s.

The formula is:
dv/dt = CI

Correction: dv/dt=I/C