# Simple question about capacitor power banks

Discussion in 'General Electronics Chat' started by Natron999, Feb 1, 2016.

1. ### Natron999 Thread Starter New Member

Feb 1, 2016
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Can a useful capacitor bank be built either parallel or series, using a variety of electrolytic capacitors that differ in both voltage and capacitance? Is there is an online calculator that will calculate this?

2. ### cmartinez AAC Fanatic!

Jan 17, 2007
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Calculating the capacitance of caps in series or parallel is a very simple operation.

If in parallel, just add their capacitance.
If in in series, total capacitance is given by the equation 1/Ct = 1/C1 + 1/C2 + .... + 1/Cn, in which n is the total number of capacitors.

If you use them in parallel, keep in mind that the cap that has the lowest voltage rating will be the first to blow if that voltage is exceeded. And, it is usually NOT recommended (though it can be done safely, if you know what you're doing) that you use capacitors in series, because voltage between them can vary significantly during charge/discharge cycles, mainly because different caps can have different properties, such as their rate of charge.

3. ### spinnaker AAC Fanatic!

Oct 29, 2009
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4. ### cmartinez AAC Fanatic!

Jan 17, 2007
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The key to his question lies in the part in which he asks "using a variety of electrolytic capacitors that differ in both voltage and capacitance?" ... that is not so easy to find by just googling it... it takes a basic amount of experience to answer that, I think

5. ### spinnaker AAC Fanatic!

Oct 29, 2009
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OK I will give it to the OP then. Capacitors in parallel is fairly straight forward as far as voltage rating. But I am guessing either way you would want the same rating.

6. ### cmartinez AAC Fanatic!

Jan 17, 2007
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Not in the case of caps in series .... caps normally are manufactured with a huge tolerance compared to other components, such as resistors. A tolerance rating for a cap can be as high as 20%, and that would affect their charge/discharge rate, which can have a significant effect in the voltage between them when cycling.

Somewhere in this place there's a more detailed explanation as to why that happens, but I can't seem to find it... @#12, can you help me out here? I have a feeling you can point us in the right direction.

7. ### #12 Expert

Nov 30, 2010
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I don't remember someplace to point to, but I can do the job.

4 example: 2 capacitors in series, both rated at 10 uf, 250 V and you want to hit them with 450 VDC.
(This is an old problem from the Fender guitar amplifiers.)
Theoretically, you have 5 uf at 500 volts.
In reality, if one capacitor is 20% over its labeled size and the other is 20% below its labeled size, you have an 8 uf in series with a 12 uf.
Pick a frequency, any frequency, and you will see that the impedance of each capacitor is the inverse of its capacitance. The 8 uf resembles 12.5 ohms and the 12 uf resembles 8 and 1/3 ohm. Now we're in a basic Ohm's Law situation. The voltage is distributed in a 60/40 proportion. Flip the standby switch to, "on" and the 8.333 ohm resistor charges to 180 volts in a quick pulse. The 12.5 ohm resistor charges to [450V x (12.5/20.83)] and that is 270 volts. Bang. It's outside its voltage rating.

This is the kind of worst case calculation you have to do.
I've seen ratings as bad as +80%/-20%. If you want to play mix&match with that kind of sloppy tolerances, you better do the math or wear a full face shield, a leather apron, and dragon hide gauntlets up to your shoulders.

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8. ### cmartinez AAC Fanatic!

Jan 17, 2007
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And also have very little regard for money and finances, the way you've described things...
and thanks!, btw

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9. ### Natron999 Thread Starter New Member

Feb 1, 2016
14
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I think you have given me the answer, "The one with the lowest voltage will be the first to blow". I believe I understand the parallel connection as far as capacitance but the voltage is the one I've had trouble with. If connecting a 250 volt, a 100 volt and a 25 volt and using a 12 volt power supply would the 25 volt one blow? Also is the 12 volt power supply enough to charge to any degree the other higher voltage ones?

Thank You.

10. ### Natron999 Thread Starter New Member

Feb 1, 2016
14
2

I spent a large enough amount of time looking before I created this thread, I am greatful for people who spend any time at all to answer a questions. It may be my lack of proper electric description in my queries but I assure you it's not my lack of intelligence or consideration of other peoples time. I'll do my best to present a more challenging question next time like

Which Mica Capacitor would work well with a 4' X 4' polished aluminum panel 1/8" thick completely insulated with urethane and mounted at 20' with a 400 watt transformer, full wave bridge rectifier, and a spark gap that is magnetically quenched?

11. ### Natron999 Thread Starter New Member

Feb 1, 2016
14
2
Thank you for your prediction / opinion, I will keep it in mind.

12. ### Natron999 Thread Starter New Member

Feb 1, 2016
14
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I do follow you calculations and I gather there is no simple formula to solve the mixing and matching of capacitors. I can get the properly matched ones, I was hoping to make use of a lot of varied capacitors in possibly some sort of voltage booster. Thank you for your answer and explaining with math the complexity of the problem.

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13. ### Natron999 Thread Starter New Member

Feb 1, 2016
14
2

Thank you for your comment, it has been difficult googling, dogpile, duckduckgo, etc... It doesn't seem to be practiced therefore not addressed.

14. ### Natron999 Thread Starter New Member

Feb 1, 2016
14
2
I couldn't find either, thank you.

The best info I could find is that like water, the voltage seeks it's own level and is divided by the capacitance of each one and would also concur with the lowest voltage one will be the one to blow.

15. ### Natron999 Thread Starter New Member

Feb 1, 2016
14
2
When people are free to pursue goals unfettered by presumed limitations on what they can accomplish, they just may manage some extraordinary feats through the combined application of native talent and hard work.

That is a great quote and it is exactly the way some things have to be approached, I would add, the building on the knowledge already discovered through the past research and experience both gained by oneself and from others.

16. ### cmartinez AAC Fanatic!

Jan 17, 2007
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Thank you, but that quote is not originally mine. I found it in this place, and liked it so much that I decided to use it as my signature here.

17. ### alfacliff Well-Known Member

Dec 13, 2013
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series and paralelling up a bunch of electrolytics to store power would not be eficient or practical. especially if they were of different values in capacitance and voltages. even series connecting electrolytics of the same ratings requires balancing resistors to ensure that both charge the same.

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18. ### cmartinez AAC Fanatic!

Jan 17, 2007
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Now you've got me thinking about the difference between impedance and reactance... I'll more carefully study this page, and then get back here if I have further questions.

19. ### alfacliff Well-Known Member

Dec 13, 2013
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and now pick a frequency that you can store power with in capacitors. the origional post was about making a power bank.

20. ### #12 Expert

Nov 30, 2010
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Don't fry your brain over it. I am famous for not being precise all the time. Witness the, "Gold Standard" for blood pressure. A million physicians have believed it for over 100 years, but WBahn thinks they used the wrong word.
I might say a million chemists know how dense mercury is (and a few dozen other elements) at any particular temperature, but I don't want to continue that conversation.

Impedance, reactance, I don't see much difference for the first surge of current into a DC filter....but I'm sure somebody could educate me.
Unfortunately, I don't care.

When there are no other components involved, like inductors or resistors, the combined or differential reactance with the other parts that don't exist (or are not significant) isn't important. You could study this down to the inductance in the power transformer, the inductance of the wires, the resistance of the rectifier at every level of current flow, the Equivalent Series resistance and inductance of the capacitors, but it won't make a percent of difference in the example I provided.

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