Fused Capacitor lead

[Gaz]

I have an amp I built using radial caps in the power supply, and have a lead that appears to have fused open (It looks burnt and was arc'ing). I would have chalked it up to a broken lead, but have had this happen once before in another amp. I attached a schematic of where the fused lead was.

No fuses blew, and I'm wondering how this could have happened Everything else checks out (tubes), and the amp is working fine now, but I would really like to understand how this may have happened. I attached a schematic of the power supply and where the broken/fused lead was.

The only other bit of info that may be useful is that the caps are stuck down to the board using GE Silicone II, which is 100% silicone and contains no acetic acid (which I know can lead to to corrosion in enclosed spaces when not allowed to cure). I am paranoid the silicone had something to do with it, but I was under the impression it is not only not conductive in a wet or cured state, but also a good insulator as well

Thank you for any guidance!

[Tom Phillips]

There is nothing inherent in that power supply design that would stress the capacitor lead you indicated. I also don't think the GE silicon II would cause the problem.
Clear photos of the failures would be helpful. Otherwise I can only speculate that your two failures were coincidence or had something to do with the parts you are using or the handling/installation of the parts.

[Gaz]

There is nothing inherent in that power supply design that would stress the capacitor lead you indicated. I also don't think the GE silicon II would cause the problem.
Clear photos of the failures would be helpful. Otherwise I can only speculate that your two failures were coincidence or had something to do with the parts you are using or the handling/installation of the parts.

Thanks for the reply, Tom. I agree, and there's nothing I can find either that would cause that (at least without blowing the fuse first!), which is what brought me here. I wish I would have taken a photo of the cap, but I already threw it and took out the trash! Imagine a radial cap with the lead broken right near the base and some burned marks right around the lead.

I typically bend them like this gently with very small screw as to not stress the lead where it actually joins internally.



I think the burned area might be from the arc'ing because I can't figure how so much current could have actually fused the lead, but at the same time this has happened before, which what really started me scratching my head.

[Tom Phillips]

Well this remains a mystery. Hopefully, it won't happen again.
Tom

[Gaz]

Well this remains a mystery. Hopefully, it won't happen again.
Tom

I suppose so. Even if the leads were 20AWG it would take at least 11A to fuse it, and I doubt that because the lead is so short. I would think a lot more would also be going wrong if something were pulling that much current. Still, no explanation for the breakage in the first place or why I've seen it happen in two separate amps... grrr.

[gbono]

Maybe a defect in the capacitor where the lead meets the foil? This would cause a high resistance path that would fail the way you describe in your post. Did you mention a particular supplier and what side of the capacitor failed?

[Gaz]

Maybe a defect in the capacitor where the lead meets the foil? This would cause a high resistance path that would fail the way you describe in your post. Did you mention a particular supplier and what side of the capacitor failed?

The caps were Nichicon from Mouser. In both amps they failed on the positive side. You can see exactly where in the schematic I posted above. So that type of situation could cause the lead to just fuse open? Thanks.

[Enzo]

probably the arc cut them in two. If solder cracks or any other tiny gap forms in the circuit, an arc can strike. An arc is plasma and so is VERY hot, and will surely burn circuit boards and such. It acts like a welding torch you might cut with. It wouldn't be because so much current flowed through the wire as to melt it like a fuse.

Why the arc? I have no idea. ANy chance your bent lead came too close to the cap can edge and the + end arced to the can?

[Gaz]

probably the arc cut them in two. If solder cracks or any other tiny gap forms in the circuit, an arc can strike. An arc is plasma and so is VERY hot, and will surely burn circuit boards and such. It acts like a welding torch you might cut with. It wouldn't be because so much current flowed through the wire as to melt it like a fuse.

Why the arc? I have no idea. ANy chance your bent lead came too close to the cap can edge and the + end arced to the can?

Enzo,

This is definitely the best explanation I have heard so far (that's why they pay you the big bucks, of course). The arc severing the wire seems very reasonable, and the lead arc'ing to the can is definitely possible the way I bend the leads and plop them down with silicone. I wish I still had the failed caps or some photos! If I use the same cap mounting method I'm going to be certain there is plent of space between the lead and can insulation. And speaking of, what's that insulation rated for?

[Mick Bailey]

I worked on an amp a few weeks ago where a PCB track running close to a cap had broken down and arced. It cut right through the wrap and up the side of the can. It also burnt through the board and formed a conductive path to a track running underneath.

[Enzo]

Arcs almost have a life of their own, don't they?

[gbono]

From Nichicon application guidelines for aluminium electrolytic capacitors:
2.1 Application Guidelines
2-1-1 Circuit design
---(8) 1 Outer sleeve of the capacitor is not guaranteed as an
electrical insulator. Do not use a standard sleeve on a
capacitor in applications that require the electrical
insulation. When the application requires special
insulation, please contact our sales office for details.

Did you find evidence of arcing on the PCB?

[Gaz]

From Nichicon application guidelines for aluminium electrolytic capacitors:
2.1 Application Guidelines
2-1-1 Circuit design
---(8) 1 Outer sleeve of the capacitor is not guaranteed as an
electrical insulator. Do not use a standard sleeve on a
capacitor in applications that require the electrical
insulation. When the application requires special
insulation, please contact our sales office for details.

Did you find evidence of arcing on the PCB?

Thank you, I really appreciate you checking that out. I am clearly being lazy ;/ I definitely do not treat the sleeve as an insulator, BUT admittedly I have been very thoughtless about it when using this cap mounting method. I'm definitely going to leave a bigger gap where I bend the leads at a right angle. I'm using a turret board, and I suspect when mounted normally in a PCB this would not have happened (since the leads would just be going straight down into the pads.

[Gaz]

Ok, bumping my own thread because this happened again in a different amp! I have a couple of photos this time, and there is no evidence of shorting the cap's can.

It was the positive lead again, but this time on the screens and the cap connected directly to them.

Any ideas what would cause this? It really worries me!

Thanks.

[JoeK]

When you said you were using silicone on the caps I didn't realize it was across the terminals?. Maybe it is conducting? Another possibility is lower rated caps sealed in a larger rated shell?