Two caps or One cap with equivalent capacitance - Page 1
Forum section: electronics / projects

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 27, 2017, 03:22:09 pm
content: Hi All,What would be the best way to go to keeps costs down ? i have two electrolytic caps one is 10v 100uf (Dia 5.00mm)and the other 1000uf (Dia 12.50mm).To cut down the cost of two size while manufacture should i go with two 560uf Dia 8.00mm or one 1000uf Dia 12.5mm

author: SKHILLBILLYSlures
author_rank: Regular Contributor
timestamp: August 27, 2017, 05:57:14 pm
content: You'll raise the ESR of the device this is due to parasitic inductances caused by not only the wire but also the way the capacitor is constructed which it is rolled into a cylinder with a dielectric between the conductor plates best go with one to reduce ESR ratings

author: Jidis
author_rank: Regular Contributor
timestamp: August 27, 2017, 06:08:13 pm
content: Don't know the application, but I had a suggestion once, which I followed, to use a pair of half value caps on some power supply rails rather than one large one. I was told it would filter some of the ripple (see post #22 fromhere).Take Care

author: Siwastaja
author_rank: Super Contributor
timestamp: August 27, 2017, 06:28:49 pm
content: Quite often, paralleling several smaller elcaps instead of one biggie is:- cheaper,- lower inductance (trace inductance to the nearest small cap vs. trace inductance to a further away big cap)- possibly lower ESL & ESR on the cap combination itself (but not necessarily)- more flexible, since you can fit it in different shapes- has more cooling surface area to get rid of ESR heating- offers BOM flexibility in some cases: for example, if you need 500 uF in one place, 1000 uF in another and 1500 uF in the third place, you can use 560uF caps only instead of three separate BOM items.Dave has a video on this IIRC.Placement of the extra cap is something like $0.0001, so the actual capacitor price dominates.But, the most optimum design cannot be achieved by using rules-of-thumbs. For extreme optimization, you need to run actual calculations. Usually it doesn't matter unless you produce it in millions; so you can just use rules-of-thumb based on design experience. Since 1000uF 10V is notthatbig of a cap, whether it's 1000uF 10V or 2x560uF 10V doesn't matter much from technical or cost viewpoint, unless you are constrained by a special shape (for example, component height).

author: David Hess
author_rank: Super Contributor
timestamp: August 27, 2017, 07:00:25 pm
content: Quote from: Siwastaja on August 27, 2017, 06:28:49 pmThis also increases the ripple current rating because of the increase in surface area. In some cases this is the only way to achieve the ESR and ripple current rating requirements.

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 27, 2017, 07:16:38 pm
content: "increases ripple current " you mean reduces the ripple voltage ?

author: David Hess
author_rank: Super Contributor
timestamp: August 27, 2017, 07:32:20 pm
content: Quote from: anishkgt on August 27, 2017, 07:16:38 pmNo, I mean increases the ripple currentrating.Capacitors have a maximum ripple current rating based on ESR, surface area, temperature, and operating life. The ripple current through the capacitor creates heat via the ESR just like any other resistor which must be dissipated by raising the operating temperature which shortens the capacitor's operating life. Multiple smaller capacitors have more surface area versus one large one so all other things being equal, they can dissipate more power at a lower operating temperature yielding a higher ripple current rating. Think of it as a power rating in watts but more convenient because it takes ESR into account.Usually worn out capacitors in a switching power supply indicate an insufficient ripple current rating for that period of operating time.

author: DaJMasta
author_rank: Super Contributor
timestamp: August 27, 2017, 07:34:38 pm
content: Increase ripple currentrating, allowing the same capacitance to better handle ripple currents and reducing ripple voltage for the same current changes.While it's not really an issue for bulk capacitance so often, you can run into issues with the changing impedance characteristics of the caps - two 560uFs would have their low impedance point at the same place and it would be different than the combined 1000uF low impedance point and the low 100uF impedance point, and of course you can run into other issues by picking a different construction or series of cap that responds differently to your load conditions..... but outside of high speed bypassing and sensitive filter work, it's unlikely swapping caps like this will cause a big problem. Just something to be careful of if you suspect your circuit's function relies on the low impedance points in the original cap's characteristics.Otherwise, as said, there's a whole host of potential benefits if the caps you're replacing them with are about the same construction and type.

author: anishkgt
author_rank: Frequent Contributor
timestamp: August 27, 2017, 07:37:26 pm
content: basically my circuit would be regulating a 9v AC to an LM7805 and these caps go in between with a 0.1uf of course.

author: DaJMasta
author_rank: Super Contributor
timestamp: August 27, 2017, 09:35:37 pm
content: Then swap away as needed, that's probably a circuit very tolerant of swaps of parts, if they're just post-rectifier you probably do want a good reservoir, but if they're after the regulator, unless you have some really hard digital switching to gulp down the current in little spurts, you probably could even swap for lower values.

author: David Hess
author_rank: Super Contributor
timestamp: August 27, 2017, 10:40:00 pm
content: Quote from: anishkgt on August 27, 2017, 07:37:26 pmThe 7805 and similar linear regulators are pretty tame.If the bulk input capacitor is close to the regulator, then that is all that is required on the input side. Its large capacitance is required to hold up the input voltage between AC line cycles (1) so because of its size, other parameters like ESR, ESL, and ripple current rating may be ignored. Parallel input capacitors might be used simply for economic or space constraint reasons. If there is a large distance between the bulk input capacitor(s) and the regulator input, then a small value decoupling capacitor should be added at the regulator.A 7805 does not strictly require an output capacitor but it is a good idea to include one anyway and I usually use a 1 to 10 microfarad solid tantalum or a 10 to 100 microfarad aluminum electrolytic. Low value ceramic decoupling capacitors should be placed across the loads instead of at the output of the regulator where they are too far away from the load to do anything useful. Unlike the 7805, the 7905 negative regulator *must* have an output capacitor for stability.(1) 8333 microfarads per volt of ripple per amp of load when driven by a 60 Hz full wave rectifier.

author: wraper
author_rank: Unknown
timestamp: August 27, 2017, 11:03:25 pm
content: I'd say just use 1000uF. Don't bother adding 100uF. Also 560uF is not that common value, so likely will increase total cost. 470uF is much more common. Also higher diameter capacitors often have significantly longer life rating within the same series. In any case, as it's just for 7805, does not matter what you will use. If it was some SMPS then there might be some point of choosing particular solution depending on price, size, ESR, life rating.

author: ThunderCat
author_rank: Contributor
timestamp: August 27, 2017, 11:24:36 pm
content: What about phase issues with multiple caps? IIRC this was brought up in another forum as a reason not to bypass caps.

author: Dave
author_rank: Super Contributor
timestamp: August 28, 2017, 02:00:35 pm
content: Quote from: ThunderCat on August 27, 2017, 11:24:36 pmWhat do you mean by phase issues?