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I recently had an old LCD monitor power supply go bad and figured I would try replacing the caps, all four of these were bulged and or leaking. They are all rated to 105 degrees C, all 1000uF and 25V yet two of them are much higher diameter. In my experience with a physically larger capacitor, you either get more voltage (more plate separation?) or more capacitance (more plates?), why are these caps different diameter?

caps

Side question, the board had more than enough space for the larger caps everywhere, why not use the large (or small) caps everywhere for higher quantity discounts.

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  • \$\begingroup\$ The 'C' ratings are different? \$\endgroup\$ – Soldersmoke Sep 7 '19 at 17:37
  • \$\begingroup\$ Im pretty sure the 'C' is a model number \$\endgroup\$ – Sam Sep 7 '19 at 17:38
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    \$\begingroup\$ The two in the middle look fine. The lower one is bad (bulged.) Can't see enough of the top one to say. \$\endgroup\$ – JRE Sep 7 '19 at 18:20
  • \$\begingroup\$ @jre the middle two were leaking, that being said, the board didn't work until i replaced all 4 \$\endgroup\$ – Sam Sep 7 '19 at 18:25
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    \$\begingroup\$ If by "leaking" you mean the white stuff, then you should know that the white stuff is glue and didn't come out of those capacitors. \$\endgroup\$ – JRE Sep 7 '19 at 18:31
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The answer lies in the datasheet and the designer requirements for cost, space, reliability, cost and temperature rating. There are many choices. (Did I say cost;)

http://www.capxongroup.com/prodsearch.aspx?lc=1&siteid=&ver=&usid=&mnuid=2082&modid=16&mode=

The Part Number defines; e.g. KF102M025I200A

  • KF Family construction of foil film and dielectric, there are many others
  • xxe value 102 for C in uF with exponents 10 00 uf
  • M = 20 % tolerance on C
  • xxx voltage rating
  • A letter code for Case Dia & radial lead space
  • xxx height xx.x mm

The height and Voltage reduce ESR while the diameter affects everything.

The parameters for selection of these low ESR caps are;

  • C, Vdc, Size, max temp range, xxxx Hrs Endurance of accelerated MTBF at extreme temp & RMS ripple current

  • the electrical variables for these choices are % DF at 120Hz, Ripple current @ 100kHz, @ 10kHz, ESR

I don't know the formula, but the diameter is determined by foil area, thickness, turns, ESR,and temp rise due to ripple current rating at max temp for xxxx hours due to thermal conductance and temp rise above max ambient rating and Arrhenius effects on Endurance.

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Either they had different ESR ratings, as @hacktastical suggests, or the bigger ones are just an older design and/or the board manufacturer buys whatever is cheaper at the moment, then throws them into one bin. Cap manufacturers are getting better at making smaller caps, they're getting better at reducing manufacturing variations so that they can consistently hit ratings with not-quite-as-good caps, and some of them just plain lie.

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  • \$\begingroup\$ If one needs to make caps with a 100uF +80%/-20% specification, one might meet that specification by trying to make 130uF caps and having them all fall within spec, or trying to make 110uF caps and rejecting a small percentage whose capacitance is too low, or trying to make 90uF caps and rejecting a higher percentage. The amount of space needed for a "130ish" cap may be larger than for a "90ish" cap, while the relative prices may vary depending upon yields. \$\endgroup\$ – supercat Sep 8 '19 at 18:49
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The relatively larger caps were likely to be a low ESR type, perhaps also with a higher thermal rating. That has some influence on the size/density.

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More advanced etching processes can increase surface area of foils, allowing higher capacitance in smaller packages.

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