# Super Cap Question



## Bwells (Mar 30, 2014)

Super caps have a low voltage rating so I was wondering if putting them in series would allow them to be used on higher voltage. For example, I see a super cap that is rated at 2.7 volts so I would assume putting 5 in series would allow me to use them on a 12 volt source as the combined voltage would be 13.5, 1.5 volt headroom. You do loose a lot of capacitance this way but to get to the right voltage, that's what it takes, I guess. I found these but wanted to check what you think.
http://www.digikey.com/product-detail/en/illinois-capacitor/304DCN2R7SCBB/1572-1250-ND/5964374
It states they are 300MF which is huge and even five in series would be big. I calculated it quickly and think I got 1 million F, need to recheck that!
Just looked again and the expanded description shows 300mF, I thought they were a good price. Question still remains.


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## daschnoz (Dec 12, 2016)

The short answer is yes. However, putting them in series will change their overall capacitance value.

Capacitors in parallel - ADD values together, like resistors in series.

Capacitors in series - treat like resistors in parallel, use the same formula to determine resulting value

If you're going to series capacitors, make sure that you have balancing resistors - 1 in parallel with each capacitor.









The balancing resistors make sure that each capacitor drops whatever portion of the voltage that the voltage divider produces. If you don't have them, it is possible that 1 cap will drop more than the rated voltage and it will eventually let its smoke out.

SO,
for your application, you're looking to use 5 caps in series. The resulting capacitance of 5x 300mF caps in series is
(0.300^-1 * 5) ^ -1 = 60mF . I would use 470ohm balancing resistors for a total of 2.35k which yields a balancing current of 5.1mA @ 12V.


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## Bwells (Mar 30, 2014)

Hmm, never saw adding the resistors.


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## daschnoz (Dec 12, 2016)

You're also going to need to charge the cap bank. You will need to charge it through a series resistor or you're going to bring your power supply to its knees.

Just out of curiosity, what is your application?


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## Bwells (Mar 30, 2014)

Actually, a cap bank! I currently have two 1F 5 volts in series fed by 9 volts and when the power is removed, the circuit continues to operate for maybe 30-45 seconds. Total capacity would be .5F. I was thinking of a larger cap bank to power all my building lights to give them a slow dim feature. The 1F 5v were $5.50 apiece and not good for 12volts. When I saw the add for the ones I linked to, I thought a 10 pack would give me a start but being that they are 300mF, they are worthless.


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## Bwells (Mar 30, 2014)

Let me see if I understand the resistors. I assume they are to insure that each cap receives only the alotted 2.7 volts and is sized according to the voltage rating of each cap, correct?


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## daschnoz (Dec 12, 2016)

Bwells said:


> Let me see if I understand the resistors. I assume they are to insure that each cap receives only the alotted 2.7 volts and is sized according to the voltage rating of each cap, correct?


Pretty much. Let's assume that you are using 2 caps. If you size the resistors equally, each resistor will drop 1/2 of the supply voltage. 3 resistors, each one drops 1/3 of the supply voltage. etc.

I like to size these resistors so they pass 4-6mA. The actual current is not super critical - I aim for 5mA, do the math, then grab the next closest resistor up or down from the shelf. I get what I get for current. If I'm feeding it with a component that needs a minimum current draw to maintain stability (voltage regulator), I will use that minimum draw requirement as my target current. That kills 2 birds with 1 stone.



The voltage rating on the cap is the maximum that the cap can handle without blowing up. You can put less voltage on the cap with no ill effects.

You have 5 caps rated for 2.7V each. You have a 12V input.
12 / 5 = 2.4

If you use 5 equal resistors, each cap will have 2.4V across it - well within the 2.7V rating.

Balancing resistors are a common practice in tube amplifiers where you have anything between 500 and 700 volts hitting the power tubes and caps that are only rated for 400V.


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## daschnoz (Dec 12, 2016)

It's not exactly $10 cheap, but what about something like this?
http://www.ebay.com/itm/LANZAR-Lanz...or-/252637913324?_trksid=p2141725.m3641.l6368

I've come to the point in my life where it's just easier and faster to buy my way out of some problems. Sure, soldering your own mini Chernobyl would be a cool project, but you can use that time for other things - like building something that you CAN'T get off of the shelf at your local toy store (hobby shop, gun shop, etc).


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## Bwells (Mar 30, 2014)

Geez, 5 lbs! 3F would make Tim "the tool man" Taylor go AR<AR<AR. I was going to do it on a breadboard as I have a few 35v large caps like a 10000µF, a few 3300µF and some 2200's that I could parallel down the sides and use the center for the super caps. Maybe best to enclose it in a metal box of some kind. The 10,000 is a shade smaller than a C cell but looks really cool. It doesn't hold a candle to my two 1F's in series though but they are only good for 10v. Thanks for the tip on the resistors, didn't see that anywhere but I think I see it is a precautionary measure.


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## Tom_C (Jan 8, 2016)

Bwells, I haven't really thought about this more than the time it took to read the thread, but might it not be a more elegant solution to use a small r/c circuit on the front end of an op-amp to provide the slow dim but without the huge caps?


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## daschnoz (Dec 12, 2016)

Tom_C said:


> Bwells, I haven't really thought about this more than the time it took to read the thread, but might it not be a more elegant solution to use a small r/c circuit on the front end of an op-amp to provide the slow dim but without the huge caps?


That would be a better solution unless the goal is a slow dim when power is removed.


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## Tom_C (Jan 8, 2016)

daschnoz said:


> That would be a better solution unless the goal is a slow dim when power is removed.


Then put a big cap bank to power the op amp! 

I kid.

I suppose there could be a timer circuit to power down the op amp after 5 minutes or what ever, but it may be getting more complicated


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## Bwells (Mar 30, 2014)

This could be a good location for a bunch of super caps, provided that is what it contains!
http://www.ebay.com/itm/68800mAh-Po...2d19afd&pid=100005&rk=6&rkt=6&sd=112097236948


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## highvoltage (Apr 6, 2014)

Bwells said:


> This could be a good location for a bunch of super caps, provided that is what it contains!...


I'm guessing it's a lithium ion battery.


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## gunrunnerjohn (Nov 10, 2010)

Don't use resistors, consider Zener diodes instead. They provide charge balancing, but they don't discharge the caps. No reason to reduce the effective storage capacity with parasitic loads.


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