# Natural discharging duration of an unpluged capacitor

Can natural discharging period of a charged capacitor be detected by a certain formula? If it is related to initial voltage and the internal parasite resistance, what are the factors affecting this quite high resistance that is measurable through a simple circuit? May it be related to capacitance or something else? If there is, can you give a certain expression in terms of V and C to find the total time needed for a charged capacitor whose initial voltage is V, capacitance is C to completely be discharged or remarkably get closer to zero while not being connected to any circuit?

• Do you know anything about the dielectric? – Ignacio Vazquez-Abrams Dec 6 '14 at 18:18
• I am very noob in this field and at 2nd grade in college yet(Electrical and Electronics Engineering). – newbie_developer93 Dec 6 '14 at 18:35
• I believe what Ignacia means is that the self-discharge rate of the capacitor will depend on the properties of the dielectric material used to build that capacitor. For example, is it an aluminum electrolytic? Is it a multi-layer ceramic, and if so, what is the dielectric type or temperature coefficient? Is it a tantalum capacitor? – mkeith Dec 6 '14 at 18:42
• Apologies for reading dielectric as electric, yet my answer won't change. – newbie_developer93 Dec 6 '14 at 18:42
• I know that capacitance fomula is something like C=A/d*€*€0. So there is an effect of the matter squeezed between plates. I think, it is more logical to explain my actual intention in asking the question. Let's assume that I want to make a simple capacitor of a pair of aluminium foil layer and a piece of paper inbetween to use as power supply.What can I do to maximize the time interval it can keep its voltage value when turned off? – newbie_developer93 Dec 6 '14 at 19:00

Use an adjustable bench supply and connect its outputs to the capacitor through a microammeter as shown here: In some cases it may be necessary to use an even more sensitive meter.

Adjust the voltage supply to various voltage levels from near zero up to the upper limit of interest while keeping in mind the upper voltage limit of the capacitor. At each stable voltage step measure the voltage and current and make a table of data. For each data point you can calculate the DC leakage current of the capacitor using Ohms law. The resulting resistance can then be plotted in a graph to see how constant it is with respect to voltage.

Once you know the effective leakage resistance of the capacitor you can calculate the net self discharge time using the standard RC formula from the starting voltage level. If the leakage resistance is constant this technique would be quite accurate. If it is not linear then extra work would be needed to integrate the changing resistance versus voltage into the calculation.

• Did you mean this? postimg.org/image/lpuim0e9r – newbie_developer93 Dec 6 '14 at 19:47
• And also in lab experiment of 2 week ago involving finding parasite resistence, teacher said that we can find the internal resistance by taking voltage and current value when current is very small. Is representation of any non-ideal capacitor exactly C//R? If so, is the reason why the proportion of voltage(of capacitor) to current(going through main branch) gives the parasite resistance when current value is very close to zero because KVL require the external resistence in serial with the non-ideal capacitor should have smaller voltage for capacitor voltage to be closer to of power supply? – newbie_developer93 Dec 6 '14 at 20:38
• Thus, we can get an approximate value for the parasite resistance by directly dividing voltage of power supply by the small current read on the ampermeter in your( and also of the second part of the old experiment) configuration of V-R1-(C//R2). – newbie_developer93 Dec 6 '14 at 20:44
• I mean resistance of the cable or any physically connected one(of relatively high value compared to the cable) by R1 of course(you probably got what I have tried to mention :) , sorry for my slow process while writing.) – newbie_developer93 Dec 6 '14 at 20:50
• Can you verify what I said yesterday? Am I correct? If there is any mistake, can you say at which parts I am wrong? Lastly, what do you suggest as material that can be used to built a capacitor suitable for using as power source? (And thanks for the answer by the way. I couldn't upvote because I am new here) – newbie_developer93 Dec 7 '14 at 10:03