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Large, through-hole capacitors (typically electrolytic) sometimes have a foam pad on the bottom like the one in this picture:

Capacitor with foam pad

What is the pad for?

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    \$\begingroup\$ I'm pretty sure it's not foam, but plastic or rubber. \$\endgroup\$ – Connor Wolf Dec 2 '10 at 8:51
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That doesn't necessarily look like a foam pad from the picture (textured plastic?), have you poked at a few at your bench?

Having said that, the picture does show a 400 V cap with (w.a.g.) 5 mm lead spacing, so something to isolate it as you suggest might be in order.

In general, that piece under the capacitor is some rubber or soft plastic that allows the capacitor to vent the hydrogen it produces during operation. The rubber is usually covered by a plastic spacer so the bottom of the can can't sit tight against the board forming a seal.

As to why, see CDE's Aluminum Electrolytic Capacitor Application Guide (pp. 3-4)

Impregnation

[...]

Water in the electrolyte plays a big role. It increases conductivity thereby reducing the capacitor’s resistance, but it reduces the boiling point so it interferes with high temperature performance, and it reduces shelf life. A few percent of water is necessary because the electrolyte maintains the integrity of the aluminum oxide dielectric. When leakage current flows, water is broken into hydrogen and oxygen by hydrolysis, and the oxygen is bonded to the anode foil to heal leakage sites by growing more oxide. The hydrogen escapes by passing through the capacitor’s rubber seal.

Sealing

The capacitor element is sealed into a can. While most cans are aluminum, phenolic cans are often used for motor-start capacitors. In order to release the hydrogen the seal is not hermetic and it is usually a pressure closure made by rolling the can edge into a rubber gasket, a rubber end-plug or into rubber laminated to a phenolic board. In small capacitors molded phenolic resin or polyphenylene sulfide may replace the rubber. Too tight a seal causes pressure build up, and too loose a seal shortens the life by permitting drying out, loss of electrolyte.

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  • \$\begingroup\$ Well, foam is textured plastic. I used the word foam because it definitely has some give to it if you push it. Do you have a source to point to about this venting? \$\endgroup\$ – ryantm Dec 2 '10 at 8:14
  • \$\begingroup\$ @Ryan When you use the word "foam" it generally implies some level of compliance, versus plastic. \$\endgroup\$ – Nick T Dec 2 '10 at 15:45
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It is a rubber seal. It keeps the magic smoke in. Or at least the electrolyte.

From Nichicon tech data:

alt text

Vishay tech data gives a better view of the seal:

alt text

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    \$\begingroup\$ Why are people voting this up? Yes, it's humorous, no, it does nothing to answer the question. Put droll like this in comments if you insist. \$\endgroup\$ – Nick T Dec 2 '10 at 4:43
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    \$\begingroup\$ Your diagram does not show a part protruding above the "curled section," which is what I am talking about. Are you saying the rubber-bakelite can extend above the curled section? \$\endgroup\$ – ryantm Dec 2 '10 at 8:07
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    \$\begingroup\$ Yes, it looks like the pad is the rubber seal protruding a little bit. I would guess that vibration is handled better when the cap is sitting on the pad, rather than the sleeve part. \$\endgroup\$ – markrages Dec 2 '10 at 8:28
  • \$\begingroup\$ @tyblu: +1 for good answer and incorporating magic smoke! \$\endgroup\$ – PICyourBrain Dec 2 '10 at 16:55
  • \$\begingroup\$ I think I owe markrages an ebeer or something. \$\endgroup\$ – tyblu Dec 3 '10 at 4:35
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I'd say for electrical insulation. It could easily arc since it's rated for 400v.

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    \$\begingroup\$ You don't think 35v would arc? \$\endgroup\$ – Matt Williamson Dec 2 '10 at 5:28
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    \$\begingroup\$ 35 V can't arc in air (takes minimum ~300V), not sure about along some dielectric interface though. \$\endgroup\$ – Nick T Dec 2 '10 at 15:47

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