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Author Topic: Potential problems with series capacitors?  (Read 11738 times)

Offline d3x0r

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Potential problems with series capacitors?
« on: July 12, 2014, 11:46:57 AM »
I tried to do a search for information, and am hoping someone can help me figure if this is a real problem...
if I have 4 capacitors in series, is it possible to end up starving the middle capacitors of charge carries?


If I build the system with pre-charged capacitors and mate them (+ -)(- +)(+ -)(+ -)  (just so the ends are + and -) is the overall capacitance still 1/(c1+c2+c3+c4)?  Or because there's a + + junction (and a - - junction) will this affect the overall capacitance?


Or if I charge the whole network from (0 0)(0 0)(0 0)(0 0) to 12 volts so it's (0 3)(3 6)(6 9)(9 12) volts with a 3V drop across each one... if I attach a ground (accidentally or incidentally) to the middle will it affect the overall capacitance if it ends up being (0 3)(3 0)(0 6)(6 12)  as the voltages/drops in the circuit?


If it does affect the capcitance, how can I avoid having an issue?  (very large ohm parallel resistors?)

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Offline MarkE

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Re: Potential problems with series capacitors?
« Reply #1 on: July 13, 2014, 02:38:17 AM »
Your formula is not quite right.  The series capacitance is the reciprocal of the sum of the reciprocals of each capacitance.  CSERIES = 1/(1/C1 + 1/C2 + ... 1/Cn)

The main issue of connecting capacitors in series is how to avoid blowing them up due to disproportionate charge distribution and leakage.  They can be individually precharged with whatever charge you like.  The problems will only crop up when you connect the two ends of the string together through an impedance.  Balancing resistors are one solution that costs continuous power.  Zener diodes offer some protection at generally lower power.

Offline d3x0r

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Re: Potential problems with series capacitors?
« Reply #2 on: July 13, 2014, 03:34:41 AM »
Your formula is not quite right.  The series capacitance is the reciprocal of the sum of the reciprocals of each capacitance.  CSERIES = 1/(1/C1 + 1/C2 + ... 1/Cn)

The main issue of connecting capacitors in series is how to avoid blowing them up due to disproportionate charge distribution and leakage.  They can be individually precharged with whatever charge you like.  The problems will only crop up when you connect the two ends of the string together through an impedance.  Balancing resistors are one solution that costs continuous power.  Zener diodes offer some protection at generally lower power.
(ya just scribbling formula if I ran the math I would have realized)
Okay so it IS an issue.
So these are 2.7V caps, if I put a ... 2.5V zener across each one, it could maintain a equilibrium better... I keep forgetting zeners as a useful part....
hmm... and maybe a shotkey the other way? (for low voltage drop? Or are they just fast?)

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Re: Potential problems with series capacitors?
« Reply #2 on: July 13, 2014, 03:34:41 AM »
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Offline MarkE

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Re: Potential problems with series capacitors?
« Reply #3 on: July 13, 2014, 06:11:21 AM »
You want something more like this.

Offline Farmhand

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Re: Potential problems with series capacitors?
« Reply #4 on: July 13, 2014, 07:38:24 AM »
I discovered the same issue with my supercapacitors, I wanted to put two in series to get a 5.4 volt capacitor but soon realized
that I would need to limit the voltage on each in order to avoid over volting one due to unbalanced charge.
(isn't volting a word ?) Seems not but i'll use it.

I could not think of any way to correct the issue without consuming power except to use a voltage source (battery) across each one.

Which of course is just silly unless the battery is a low output battery (home made cells) and I allow the supercaps to charge from
them until maximum voltage is achieved then use the caps until the voltage is too low then repeat. A circuit could do that
automatically for me and harness the low output of the home made cells via the capacitors intermittently.

Do 5.5 v double layer super caps have some kind of internal balancing ?

..

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Re: Potential problems with series capacitors?
« Reply #4 on: July 13, 2014, 07:38:24 AM »
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Offline Pirate88179

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Re: Potential problems with series capacitors?
« Reply #5 on: July 13, 2014, 07:47:50 PM »
Farmhand:

I have read that you can only put 2 supercaps in series/parallel.  Maxwell boostcaps on the other hand, claim that you can place as many of them in your circuit as you want.  I do not know why this might be but, I read this a few years ago...for what it is worth.

Bill

Offline MarkE

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Re: Potential problems with series capacitors?
« Reply #6 on: July 13, 2014, 08:49:33 PM »
I discovered the same issue with my supercapacitors, I wanted to put two in series to get a 5.4 volt capacitor but soon realized
that I would need to limit the voltage on each in order to avoid over volting one due to unbalanced charge.
(isn't volting a word ?) Seems not but i'll use it.

I could not think of any way to correct the issue without consuming power except to use a voltage source (battery) across each one.

Which of course is just silly unless the battery is a low output battery (home made cells) and I allow the supercaps to charge from
them until maximum voltage is achieved then use the caps until the voltage is too low then repeat. A circuit could do that
automatically for me and harness the low output of the home made cells via the capacitors intermittently.

Do 5.5 v double layer super caps have some kind of internal balancing ?

..
The series situation is one of  charge distribution.  A voltage limiter as shown above will with the right value for the zener protect the capacitor from over voltage.  If the application is  a series stack of capacitors with source/load only connected across the  whole stack then the two diode per capacitor solution's only limitation is the series impedance of the zener.  That will be a factor if you apply a steep current ramp such as turning on a power supply that does not have a reasonable current limit  Otherwise the diodes will conduct only during boundary conditions and will otherwise be invisible.  This technique has been used very successfully with many capacitors in series.

Capacitors of any kind should not have any problems with parallel connections per se.  Because of their very low impedance, one can get large current transients with super caps.  Those current transients can develop voltages across wiring resistance and inductance that could become a problem.


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Re: Potential problems with series capacitors?
« Reply #6 on: July 13, 2014, 08:49:33 PM »
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Offline d3x0r

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Re: Potential problems with series capacitors?
« Reply #7 on: July 14, 2014, 12:02:43 AM »
They actually sell charge balancing IC modules...


this one is op-amp based with a mosfet... http://www.discovercircuits.com/DJ-Circuits/supcapvoltlim.htm


http://www.instructables.com/id/Lets-learn-about-Super-Capacitors-A-Practical-G/step7/Balancing-Your-Series-Banks/


comparison of some balancing methods (zener, resistance, boost-converter (mosfet)) http://hal.archives-ouvertes.fr/docs/00/41/14/82/PDF/ESSCAP2006_Venet_2.pdf


looks like getting a series balancing chip might be a better way to go....
ALD810025 4 cap series thing $2.91 last 2 digits is voltage so a 27 is 2.7....

Offline MarkE

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Re: Potential problems with series capacitors?
« Reply #8 on: July 14, 2014, 12:46:08 AM »
The ALD8100 data sheet looks very nice.  If nothing else, it will save you space.

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Re: Potential problems with series capacitors?
« Reply #8 on: July 14, 2014, 12:46:08 AM »
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Offline d3x0r

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Re: Potential problems with series capacitors?
« Reply #9 on: July 14, 2014, 01:00:54 AM »
The ALD8100 data sheet looks very nice.  If nothing else, it will save you space.
ya... space...


It is unfortunately a unique part, and only package is 16-SOIC; There is a breakout board for another couple dollars.... *sigh* I'm no good with tiny things :)


2.7V zeners are only $0.10; but are really low current (500mA)
Although these chips are only 80mA...


---
Edit: Regarding your diode thing... won't the 0.7V drop of the PN affect it?  like then should I use a 2.0V zener?

Offline d3x0r

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Re: Potential problems with series capacitors?
« Reply #10 on: July 14, 2014, 01:22:58 AM »
Can I do something like this?
It's hard to find such low voltage zeners... might be better to just use like 4 diodes in series (0.7*4 = 2.8 )
or 3 900mV  at $0.40...
or 1 2.7Vf  at $1.60 http://ixdev.ixys.com/DataSheet/DSEI12-10A.pdf  (1000V 10A)  (actually a less than 10A the voltage drop is down to 2V... )


Hmm maybe I'm reading the spec sheets wrong... for voltage less than 2.7, there's still current?  They're all curvy, so they're not really X voltage drop, just at a specific voltage you get a certain current....

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Re: Potential problems with series capacitors?
« Reply #10 on: July 14, 2014, 01:22:58 AM »
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Offline MarkE

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Re: Potential problems with series capacitors?
« Reply #11 on: July 14, 2014, 07:47:10 AM »
Your idea looks like it should work.  Those ICs you found seem ideal as long as an 80mA limit works for you. 

Digikey has Zeners down to 1.8V, but they get leakier and leakier at low voltages. 2V MMSZ4679's are cheap, and SOD-123's are reasonable to solder even with an ordinary iron.  How much current do you plan on passing through your stack?  What kind of transients do you expect to encounter?  http://www.onsemi.com/pub_link/Collateral/MMSZ4678T1-D.PDF


Ordinary diodes will not be very square.  If you go that route there will be a lot of slop in the voltage.  Four PN's in series could get you anything from about 2V to 2.8V at room temperature depending on the current.

Offline d3x0r

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Re: Potential problems with series capacitors?
« Reply #12 on: July 14, 2014, 08:13:29 AM »
Your idea looks like it should work.  Those ICs you found seem ideal as long as an 80mA limit works for you. 

Digikey has Zeners down to 1.8V, but they get leakier and leakier at low voltages. 2V MMSZ4679's are cheap, and SOD-123's are reasonable to solder even with an ordinary iron.  How much current do you plan on passing through your stack?  What kind of transients do you expect to encounter?  http://www.onsemi.com/pub_link/Collateral/MMSZ4678T1-D.PDF


Ordinary diodes will not be very square.  If you go that route there will be a lot of slop in the voltage.  Four PN's in series could get you anything from about 2V to 2.8V at room temperature depending on the current.
Ya I went searching for mosfets that might do the job; but they also aren't very square... and those that do turn on around 2.5-2.7V are at very low current themselves until they hit 4 or 5V...
Working on implementaion of lasersaber's joule looper thing... so low current is probably OK... and they only require 80mA when they are misbalanced... or get charged way over; but was going to run a separate 9V zener with an LED in series to indicate full charge.
I found an article from july 11,2014 that said 'ALC recently released...' regarding the balancing IC.  The PDF says it starts conducting within 0.4V of rating ... so effective top is only going to be 2.3 then (9.2 total instead of the 10.8 expected)
got the caps before considering that series caps can actually have problems...
Was experimenting with some other supercaps I already have, and 2A is kinda low for charging 100F caps... that is it takes quite a while.... and my 3000F cap will take like 2 hours to charge at 2A.  (at low voltage 0.280V even it only goes up only 0.001V/sec... and they get slower and slower as they get more voltage...)


So another question though relating to these... if I use a higher voltage (12V for instance) to charge the 2.7V cap so I can maintain the amps input, is that like overvoltage?  Or if I stop before it gets to 2.6 something V is that ok?  or like an impulse charger like a bedini with something like 400V pulses?

Offline MarkE

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Re: Potential problems with series capacitors?
« Reply #13 on: July 14, 2014, 08:52:42 AM »
I am assuming here that you do not have a bench supply that has a solid adjustable current limit.  That would be a very good thing to have.  At least get a hefty current limiting resistor.  I would not charge either the stack or any single capacitor with a voltage above where you want to end up.

The rate at which the capacitors charge will be inversely proportional to their individual capacitance values. 

You can make a very square 2.5V or higher regulator with an LM4040D25.  That can operate with 60uA bias.  I don't know how low an operating current you want to realize with your JT, but 60uA is probably a lot of your budget.  You can always use a switch or jumper to disconnect when you are done charging.  Or you can precharge them separately and then connect them in series.  You will want to find out first what their relative capacitances are which you can do by charging them in series with a resistor and sampling all the voltages periodically.  Use the results to limit the voltage proportionately.  For instance if you have 3 caps:  A, B, C.

Charge to:  0.5V A, 0.4V B, 0.55V C in series.
Then charge C to 2.7V, A to 2.7V * 0.5/0.55, and B to 2.7*0.4/0.55

Offline d3x0r

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Re: Potential problems with series capacitors?
« Reply #14 on: July 14, 2014, 09:57:16 AM »
I am assuming here that you do not have a bench supply that has a solid adjustable current limit.  That would be a very good thing to have.  At least get a hefty current limiting resistor.  I would not charge either the stack or any single capacitor with a voltage above where you want to end up.

The rate at which the capacitors charge will be inversely proportional to their individual capacitance values. 

You can make a very square 2.5V or higher regulator with an LM4040D25.  That can operate with 60uA bias.  I don't know how low an operating current you want to realize with your JT, but 60uA is probably a lot of your budget.  You can always use a switch or jumper to disconnect when you are done charging.  Or you can precharge them separately and then connect them in series.  You will want to find out first what their relative capacitances are which you can do by charging them in series with a resistor and sampling all the voltages periodically.  Use the results to limit the voltage proportionately.  For instance if you have 3 caps:  A, B, C.

Charge to:  0.5V A, 0.4V B, 0.55V C in series.
Then charge C to 2.7V, A to 2.7V * 0.5/0.55, and B to 2.7*0.4/0.55
I do have a bench supply... the current limit seems to work pretty well... 30V-3A max..
well once these get to 1.2V or so at 2.7V limited input they allow less than 1A charging; which is why i was hoping to increase the voltage to maintain the amp input for testing with the bench supply... but I can understand that charging with extra voltage might stress the outermost layers (the parts immediately associated with the electrodes)... the bedini idea was like using a highly inductive charging system that without a capacitor it generates I dunno 20-100V... when adding a cap, it doesn't go nearly that high... my actual JT draws a lot of current (6 windings to base, 6 to collector and a secondary that's 140V) it generates 100V easily across LEDs, but not that much across a bridge rectifier to a cap... (that's a JT not JL)

My version is actually higher current draw (10mA...well shows as 0.01A on the power supply) at 10V.  under 10V it reads 0.00; but the meter might be rounding up, (haven't actually put another meter inline to see if it's a truncate or round)  was going to do some more testing with some CSR inline with the power...
LM4040D25 looks like a good part. But, again, isn't its current rating going to only apply when they are misbalanced?  "Wide Operating Current Range…45 μA Typ to
15 mA"... not when using the power?

The precharging example was just experimental idea to demonstrate worst case... generally I expect them to be in-balance...other than I guess percents difference between them...  and charging with human power or maybe solar (small 12V solar panel says 400mA for example)

 

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