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I bought these big yellow capacitors from ebay, just to try them

yellow capacitor

(only information present there: "10pcs Yellow feedthrough capacitors axial capacitors 100V 15UF").

Obviously, they are simple axial capacitors, that have nothing of a "feedthrough". They are not electrolytic as well, since they are not polarised. I suspect they are "film" capacitors but I'm not sure.

Anyway, I found them particularly effective for killing medium to high frequency noise of PSU: Even for a switching PSU whose noise (near 2.5MHz in my case) is particularly difficult to filter, such a single bypass capacitor at PSU output kills half the noise.

Someone knows what are really these yellow capacitors, and for what they are used?

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    \$\begingroup\$ General rule of thumb with components: If it doesn't have a datasheet, don't buy it. \$\endgroup\$ – Tom Carpenter May 26 '17 at 13:13
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    \$\begingroup\$ And you are right, they are very definitely not "Feed-through capacitors". \$\endgroup\$ – Tom Carpenter May 26 '17 at 13:14
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    \$\begingroup\$ @OlinLathrop: You are way off base on this one. The advertised specs are completely believable. Just search for "15uF 100V film" and you'll see dozens of images of capacitors that look just like this one. Commonly used in low-impedance audio applications, such as speaker crossovers. Higher-voltage units are used with certain kinds of induction motors and in fluorescent lamp ballasts. \$\endgroup\$ – Dave Tweed May 26 '17 at 14:10
  • \$\begingroup\$ Dearborn makes caps like that. \$\endgroup\$ – analogsystemsrf May 26 '17 at 14:39
  • \$\begingroup\$ Could be polypropylene \$\endgroup\$ – Dirk Bruere May 26 '17 at 14:40
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Yellow axial caps like that are (almost) invariably polyester film (for which Mylar is a trade name). They have very low leakage and fairly mediocre dielectric absorption performance.

The part number seems more like a 4uF/100V than a 15uF. Unless the OPs fingers are huge the size would better fit as well. The other types of film cap that are relatively common (PP and PC and PS) would be much bigger again. Stick in on a capacitance meter and check it. Put 100V across it and verify it doesn't fail. There's not much that can go wrong with a cap like that.

PP = Polypropylene (good DA performance)

PC = Polycarbonate

PS = Polystyrene (would be huge even in 4uF)

There are more modern films such as PPS but they are uncommon in that style of capacitor.

As to where you would use such a part- it might be used for an analog filter or an analog controller. Much of this kind of thing is done in the digital domain these days, but there are still applications.

Your bypass application would probably be better served by a ceramic capacitor (perhaps in parallel with a low-ESR aluminum electrolytic)- lower inductance, smaller and cheaper most likely (depending on voltage). The film caps do have longer lifetime than electrolytics.

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