# What is the advantage of using many low-farad capacitors instead of a few high-farad capacitors, when trying to maximize current? [closed]

I am working on a high current application, and I am wondering, which maximizes current (over a short period of time), using a bank of small caps, or using fewer large caps?

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We have several previous questions that discuss decoupling caps and why you should use different values. Have you looked at any of these? For example Decoupling capacitors: what size and how many? –  Kellenjb May 22 '12 at 3:21
Small caps tend to have lower series resistance, and paralleling them parallels the (smaller) series resistance. @Kell: I think Masz is asking about 10x1mF versus 1x10mf, not about 100nF + 22uF. –  Wouter van Ooijen May 22 '12 at 6:28
We're supposed to know what you think high current and short period of time mean!? We do engineering. Hand waving is off topic here and will be closed. –  Olin Lathrop May 22 '12 at 11:51
Please edit your question to better define your current and time requirements, and we can reopen this. Until then, we can only make guesses. –  Kevin Vermeer May 22 '12 at 14:08

## closed as not a real question by Kellenjb, Olin Lathrop, Kevin VermeerMay 22 '12 at 14:07

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Go for the larger one. There's lots of talk about lower ESR for the smaller ones, but I checked on Digikey, and it all seems to make very little difference. Of the 1000$\mu$F/25V electrolytics the ones with the lowest ESR listed are 38m$\Omega$, USD 1.77. A 10000$\mu$F/25V with 50m$\Omega$ ESR costs USD 3. So that's not much difference. Lower is possible at a price.

The single 10000$\mu$F will also take far less space than 10 $\times$ 1000$\mu$F.

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Does the same apply for higher voltages, like 50-100 V? –  clabacchio May 22 '12 at 9:52
@clabacchio - for the affordable ones with listed ESR: 1000$\mu$F/100V: 160m$\Omega$, USD 5.27. The 10000$\mu$F: 18m$\Omega$(!), USD 12.2. –  stevenvh May 22 '12 at 10:01
@Stevenvh: It seems you overlooked the effect of parallel resistance: Paralleling 10 caps @, say, 0,1 Ohm ESR gives an effective ESR of 0,01 Ohm, which is certainly lower than that of a single cap of 10x the capacity of the smaller ones. –  Hanno Binder May 22 '12 at 12:47
I think you're not looking at the right parts for comparisons. For your 25V example, !this 1000uf capacitor costs half as much in single quantities and has less than half the ESR you found and can handle a ripple current of 2.6A. –  W5VO May 22 '12 at 12:48
@Hanno - How about the impedance the PCB traces add when connecting those caps? –  stevenvh May 22 '12 at 12:50
show 2 more comments

One reason is the one stated by Wouter: parallelizing capacitors allows to sum their capacity decreasing the series resistance. So you can handle bigger currents with less dissipation and heating.

The other reason can merely be cost-availability: high voltage, high cap capacitors are much more expensive and difficult to find than smaller ones, and using more of them may result easier.

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Don't forget you want to decouple as near as possible to eg. the IC. Distributed decoupling is much more effective than centralized. So that's a reason to use many caps too. Especially when looking at a schematic diagram this does not always appear trivial. –  jippie May 22 '12 at 7:41
@jippie it's not about decoupling I think :) –  clabacchio May 22 '12 at 7:47

I assume that "high current over a small time" means that
you are discharging a capacitor
in the 1000 uF +++ range
as rapidly ass possible
into a load such that I actual is < Vcharged/Rload
due to capacitor discharge limitations.

An issue can be "ripple" current, the ability of capacitors to tolerate high RMS current flow. This usually relates to frequent discharge a lower rates than you are using but there are some common points.

Ripple current per microfarad is often increased by reducing capacitor sizes. eg a 2200 uF capacitor may have a 3A ripple current rating but the 1000 uF caps in the same series may have a 2.5A rating so 2 x 1000 uF caps give 5A rating compared to 3.5A for the 2200 uF.

Similarly, pulse discharge ratings may be improved with smaller caps overall. Data sheets should help. Caps can be had which are designed for pulse discharge use.

High discharge current example:

Made up figures. Specify what's actually wanted for better results.

100,000+ uF (= >= 100 mF)
30V+ Maximum permitted discharge current.

For whatever reason, say you decided to use capacitors as per this reasonably usefull detailed data sheet from EPCOS.

(1) 100,000 uF, 40VDC, 64mm dia, 100mm tall can
ESR milliohms: 4.1/ 8.2 typical/max at 20C 100 Hz,
Impedance 10 kHz, 20 C = 7 milliohms
Max AC current at 40C/85C: 45A, 19A.
Case limits current to 45A.

(2) 10 x 10,000 uF, x 10 caps used.

36mm dia x 56mm tall. 330% of area of one cap. 60% of height About double volume

ESR's 16 & 37. Divide by 10 = 1.6, 3.7 = 2+ x better than 100 mF.

Impedance 34 m.ohm. About 50% of 1 mcap for 10 IF you can get leads short. Probably worse.

Max AC current 1= 18A, 6.3A. 10 caps = 180A, 63A. 4 x better than one cap at 40C. 3+ times better at 85C.

Case limits current to 34A or 340A for 10.

The biggest gain for using 10 is ripple current or peak allowed discharge current. 1 x 100 mF is 45A AND is case limited.
10 x 10 mF = 180A and case would limit current to 340 A.

ie if discharging VERY heavily the 10 x caps are 180/45 = 4 times higher current rated.

Power out: 180A x 30V = 5400 Watt / 5.4 kw / ~= 7 horsepower.

Energy out for discharge from 30V to 20V
= 0.5 x 0.1F x (30^2 - 20^2)
= 25 Watt.second!
ie not much - until you do the discharging with a hand held piece of wire :-)

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