Live voltage on a capacitor can - why?

[Gyro]

I thought the sand dampen microphonics?

I imagine the ultimate capacitor would have a damper of some kind around it, like a rubber shell.

In the faker situation it is there to add convincing weight and make the can sound solid with the tiny cap that they put in there. In this case it would just be something easy to settle into the narrow gaps and pour out if the OP needs to replace the cap again in future. It would probably add some degree of thermal conductivity [better than air anyway], that he mentioned, too.

[lasseo]

It's being used increasingly in outdoor mains jointing boxes. You can get them pre-filled so that the connector sinks into the top and bottom halves when clamped closed, but here's an example of using the raw fluid compound (scroll forward to 13:10 if you want to save time)...   

Cheers! Encouraged by this video, I ordered a 300 ml kit of the Magic Gel. While opening the box, I was a little disappointed to see it didn't come with its own beaker or a mixing spoon like shown on the brochure pictures. Presumably the accessories only come with the 1000 ml kit, the smaller is all BYO. Oh well.

I decided to do a little test before putting the gel to use, so I mixed a small amount in a paper cup and stuck in a plastic sleeve I had lying around. I let it set for a bit longer than the instructed 10 minutes - et voila! The consistency of the cured product is reminiscent of Aeroplane Jelly - perhaps made with a little less water than usual - and the colour is close to the Blue Heaven flavour. There is no shrinkage, no odour, and it stuck to the sleeve well enough to keep it in check while I was poking around, but not so well as to be impossible to pull it out. Just like a spoon stuck in Aeroplane Jelly - perfect!

[lasseo]

I gutted the old capacitor and cleaned the can and drilled holes through the connectors on the existing base. I pushed the legs of the replacement Rubycon replacement capacitor through the old base, and glued it down to the base using neutral cure RTV silicone. I let it sit overnight just to be sure, and in the meantime figured out the volumes of the old and replacement capacitor cans. This was so I knew how much to pour in. I wasn't keen for the gel to be squeezed out while I'd be pushing the new cap and the base back into the can. I filled the old can to some 60% then quickly inserting the replacement and clamped down the base to the can. Once the Magic Gel is mixed its open time is only one minute, so no time for stuffing around - literally and figuratively.

[lasseo]

Some more photos. Once the gel had set I placed the mounting ring back on the base, carefully observing the orientation so that the capacitor legs would fit through the hole pattern on PCB - and that the cap had the correct polarity before soldering the negative leg onto the ring. I then rolled (well mangled more like) the lip of the canister back around the base, before fitting what looks like PP sleeve and cap back onto the canister. I had to resort to a heat gun while getting the guts of the old capacitor out, and the sleeve expanded just enough to slip off but luckily no damage. I then used a soldering iron set to low temperature to fold the edge of the sleeve back over the lip. The craftmanship here is not my best work, but the end result looks pretty reasonable, and if the can is going to do anything for the EMI protection I think this will be fine.

[coppercone2]

I was scared of it not venting properly if it was seated in the old cap like sealed in. What I did with mine is drill a hole in the base of the old cap,solder the new cap in place by wrapping wire the through hole leads on the solder posts on the bottom, then I put epoxy putty around the perimeter from the bottom, and poured liquid epoxy in maybe 1cm deep from the top (so a segment of the top was flooded with epoxy). This way the new cap was firmly  anchored in the base of the old cap (with glue on the swaged area) but the top was completely unrestricted so it can vent properly.

Granted you lose most of the can if you do this and it looks weird. If I had to put the old cap in the new cap and have it look original, I would cut a big hole in the top of the old cap and glue in a stainless mesh from the bottom, so if it vents it can vent nearly freely.

but mine had a solid metal top (it was a solid metal can). IDK if you have some kind of plastic baffle there?

I know for a fact that potting capacitors totally is a recpie for shockwave during failure, you are supposed to put a tube over it so the vent can breach freely. The burst disk is designed to vent it gracefully, if you obstruct that it goes boom



I am curious how well a tinned copper mesh would solder to the capacitor body (mine was not shielded like yours) if you actually wanted EMI shield. It should be good to the GHZ with a fine mesh that is soldered or conductive epoxy on.

[lasseo]

but mine had a solid metal top (it was a solid metal can). IDK if you have some kind of plastic baffle there?

I know for a fact that potting capacitors totally is a recpie for shockwave during failure, you are supposed to put a tube over it so the vent can breach freely. The burst disk is designed to vent it gracefully, if you obstruct that it goes boom

The cap is solid, the plastic baffle is part of the PP sleeve that goes over the aluminium cap. The new cap is potted over where it's supposed to vent, so yes there probably is some risk of it going boom after the rated 12000 hours of use. I'm rather counting on (hoping?) the jelly-like consistency to give up and the bottom plate to be fitted loosely enough so it'll go PFFfffftttt... rather than BANG!!!, but it is not guaranteed. I didn't really want to drill holes because I had no way of knowing whether that would change the EMI protection.

I am curious how well a tinned copper mesh would solder to the capacitor body (mine was not shielded like yours) if you actually wanted EMI shield. It should be good to the GHZ with a fine mesh that is soldered or conductive epoxy on.

I had the same exact thought - pull a mesh sock over it like you have the shielding in a coax cable. However, I was worried that the copper mesh sleeve would end up unruly. There's a bridge rectifier right next to the capacitor so the fit had to be pretty tight.

[DavidAlfa]

The multiple legs are just to provide mechanical support.

[lasseo]

The multiple legs are just to provide mechanical support.

One would think so, yes. However, it is a double-sided board, and each of the ground ring legs also had a trace coming off them leading elsewhere on the board. Perhaps ground planes weren't a thing back then.

[DavidAlfa]

Yes, I've seen a lot of that on 80s equipment, using the cap legs as part of the trace. It's a free jumper after all.