# Equation to figure out the resistance to charge capacitors evenly

Discussion in 'General Electronics Discussion' started by 123456789a, Nov 13, 2013.

1. ### 123456789a

21
0
Mar 2, 2013
Hello hows everyone doing? I need help knowing the equation for figuring out the resistance need to charge capacitors (whether in series, parallel, or a hybrid of both) evenly since I heard that due to capacitance leakage some will be charged more than others which will make 1/ct or ct equations not really work

2. ### Laplace

1,252
184
Apr 4, 2010
Depends on what you mean by 'to charge evenly'. Any network of capacitors (whether in series, parallel, or a hybrid of both) can be reduced to a single equivalent capacitance that will charge evenly. Capacitors in parallel will always be charged to the same voltage. Capacitors in series will always each acquire the same electric charge (in coulombs) but the voltage across any capacitor will be in accordance with the equation farads=coulombs/volt.

3. ### 123456789a

21
0
Mar 2, 2013
Again like I posted I need to know what the the equation for the resistance needed to charge the capacitors evenly due to what I heard about capacitor leakage (especially this being more true to electroyic types). From what I heard, when capacitor leakage or symptoms of it does happen the charge would not be evenly distributed which then some would hold more than others (assuming all of them have the same amount of farads and voltage breakdowns).

I hope this make this more clear to what I am asking

http://m.electronicdesign.com/analog/whats-all-capacitor-leakage-stuff-anyhow

4. ### Laplace

1,252
184
Apr 4, 2010
The given reference speaks of leakage resistance on the order of 100 million megohms. Capacitor leakage/soakage is normally a concern in sample & hold applications using plastic film capacitors. Even so, leakage is not a concern during the charge cycle. If electrolytic capacitor leakage is a concern in your application, then perform a measurement of the leakage resistance and include that resistance with the capacitance in your circuit model for analysis. Please post your circuit model.

5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,809
Jan 21, 2010
From what I can see, you have a misunderstanding of what capacitors do, or you're trying to do something you're not explaining well.

Some comments on that page you linked to indicate the writer of it is somewhat uninformed as well (I note especially the comments with regard to paper capacitors).

(edit: and what Laplace said)

6. ### 123456789a

21
0
Mar 2, 2013
Here are the concept circuit models I made before hearing about the leakage (the voltages represented may or may not be used in the actual application since I made them in an app that didn't let me go on with the circuit models without a voltage value)

1.) http://s10.postimg.org/x5wqq36dl/image.jpg

2.) http://s10.postimg.org/m51ll2e4p/image.jpg

Basically I want to make a capacitor bank (whether in form of a series or a series-parallel hybrid if I do not feel confident with capacity of the electrodes). Yes what you said about film and electrolytic capacitors definitely applies to me since the electrode films I am going to make are going to be coated with graphene and the electrolyte is aqueous and it's all going to be hand made which concerns me

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,809
Jan 21, 2010
If your problem is that these capacitors are rated for (say) 3V and you want to make sure that the voltage on any capacitor doesn't rise above (say) 2.5V, then there are many alternatives in solving this problem.

One simple one is to place zener diodes across the capacitors. There are better active solutions which will allow you to get max capacitor form capacitors even if their capacitance varies significantly.

8. ### KrisBlueNZSadly passed away in 2015

8,393
1,270
Nov 28, 2011
Bob Pease uninformed?

Edit:

In response to the original question. The problem of uneven charging of multiple capacitors only happens if they are connected in series. It is often solved by connecting a resistor across each capacitor (resistor values are the same if capacitor values are the same, which is common). This is equivalent to connecting a single resistor, with a value equal to the sum of the resistors, in parallel with the combined capacitor, and it will cause the capacitor to discharge following the usual logarithmic curve. In some cases this is not important - for example if you're balancing the voltage across two capacitors that are used in a smoothing or decoupling application, where the charge is being continuously topped up. In other cases it will be significant - a sample-and-hold circuit is an obvious example.

An alternative for fixed DC voltage applications such as smoothing or decoupling capacitors is to use zener diodes with small series resistors. This will also ensure that individual capacitors are not exposed to overvoltage even briefly.

If you tell us what you actually want to do, we will be able to give you more applicable advice.

Last edited: Nov 15, 2013
9. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,809
Jan 21, 2010
I simply read the article with its implication that a capacitor discharged through a fixed value resistor would have a straight-line drop in voltage.

Perhaps he meant on the first day?

10. ### Fish4FunSo long, and Thanks for all the Fish!

464
105
Aug 27, 2013
lol, I suspect we are headed toward a "capacitor energy storage system " with zero losses...."obviously" a more energy efficient storage system than super conductors It is truly a shame that those physicist working on super conductors never thought of using capacitors for storage,

But maybe there is a different reason for needing 11 capacitors series connected across a 12Vdc potential or 9 Capacitors connected in Series/Parallel across a 5Vdc source? It is hard to say....

To the OP:
For a discussion about the inefficiency of capacitor stored energy: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capeng2.html#c4

Regardless of what you believe, capacitors are not an efficient means of storing energy nor shifting voltage levels. Since I cannot fathom any other reason for connecting series/parallel combinations of capacitors to a DC source, I must conclude that you were working under the assumption that a capacitor is the "perfect energy storage device"; let me assure you, this is NOT fact.

Fish

11. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,448
2,809
Jan 21, 2010
I have a paper somewhere that discusses an active circuit for levelling the charge on series capacitors. Whilst not without loss, it uses an inductor to transfer the energy from the higher voltage capacitor to its lower voltage neighbour. Since this is only used to transfer imbalance the loss in efficiency is dependant on the difference between the capacitors rather than some other simple methods where the inefficiency also rises with charge level. I'll try to look it up for you.

12. ### KrisBlueNZSadly passed away in 2015

8,393
1,270
Nov 28, 2011
Yes, I assume it was just a simplification and approximation to put the discharge into context with his other calculations.