***BANG!***

he

ent

lytic clean off the board. I found some paper packing and eventually the c ap itself. The TV still worked and the only evidence left on the board was two very clean leads where the cap used to sit.

very carefully for a couple of leads and no part between them.

I had a HP desktop computer lose an electrolytic capacitor in the power sup ply. The can blew out through the fan, along with flames.

I've had the large (160uF, 250V input capacitor in a v0ltage doubler explo de in a TV while it was on the workbench. Confetti for about 20' radius. :)

I had just installed a new amplifier in a car dealership. They called the n ext day that t had a loud hum. Lightning had hit the early Japanese busines s telephone system's optional paging adapter. It blew the cover off the met al box, and it was another mess of confetti all over the old telephone equi pment room. They replaced it, and that one exploded a week later. Neither d esign had any surge protection. Both times, their neighborhood was hit with 100W of 60Hz hum and other noise from the PA horns in their used car lot.. ;-)

Me:

We're assuming it was this X cap as it's clearly damaged, but I admit I would expect from the bang (which I did hear *whilst* in the shower 30' feet away) I'd have expected to see more extreme damage to that cap than I did.

You're right. I need to eliminate the possibility that the x-cap damage had maybe been there since years ago and I still haven't found the true culprit. If that may be the case, then what other part from where could account for it? I've already eliminated the big electrolytics from suspicion.

Capacitors across the power line, when they fail, they fail spectacularly. And usually fast enough that they don't always take out the line fuse.

This is why they make X caps. When they fail they, in addition to failing short, they don't explode with a vengeance or set fire to themselves or the surrounding bits.

Y caps, on the other hand, go from the line, or neutral to ground. They are designed to fail open.

This one has no mains filter AFAICS; just that busted cap. I do have the

You seem to be contradicting yourself in the second paragraph. You initially say "they fail spectacularly" then you say, "they don't explode with a vengeance". It can't be both, so which is it?

Capacitors, NOT X rated.

Vs X rated.

So you're saying the x-cap that looks like the culprit can't be, then? Maybe you're right; we'll see in due course....

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They are designed to burn away the short, so there is no contradiction. The foil is very thin, so it vaporizes away from the point of failure, leaving the remaining part of the capacitor, in circuit. If you measure the capaci tance of a damaged X or Y capacitor, it will be lower than marked.

Oh for f*ck's sake. X rated capacitors are a recent development. There weren't any when HP built the VNAs. They used standard capacitors at the time. Like everyone else. Which, is why, when it failed, it failed spectacularly.

Back in the mid to late 60s, RCA used a .1uf cap right across the AC interl ock. They would let go on occasion and fail, well, spectacularly. They wou ld blast bits of metal and tons of carbon under the whole chassis. Because they were wired before the breaker, they kept going until the house fuse b lew or they spent their mass.

RCA recalled them and I remember putting in several hundred replacement cap s through the 70s. Back then, most TVs weren't registered with the manufact urer and without internet, it was difficult getting consumers aware of the issue. But whenever we'd see one (dark brown), we'd automatically replace it with the revised cap.

Hmmm. Amazing a little .1uF cap could make that much noise! I freely admit I've never been in the same room when a cap let-go, so have only ever heard them from some way off. I really should blow some up on purpose so I know for myself; that's an essential part of my education that's been missing all these years. Is the violence you get commensurate with the capacity? Say for example I blow up a 10,000uF electrolytic on purpose, is it a *much* bigger deal than say a 25uF one? What does the noise approximate to - as much as a pistol shot?

I caution others to research before you buy, some work great others have a higher failure rate and the external appearance seems to be a clue. The one I saw recommended has a white front with a lizard climbing down on the right side. But,people do like them, there are several groups running about the Nano VNA and new open source software being written. Mikek

It must have been in 1961 when I had a holiday job bewteen school and college at a local PA manufacturer. I was present when the snooty designer came to appraise the prototype of a new model. He was peering at the build detail when an electrolytic exploded. I don't remember the capacity or much noise, but the confetti of foil draped over him caused a lot of bitten lips while the audience suppressed their giggles...

Mike.

NT

Just snip it then, it's not needed.

NT

Spoken like a true hack.

are you being childish again?

The bean counters would never let an unnecessary part into any design. Therefore, leaving a part out of a design is "hack".

That is, unless one is Mad Man Muntz.

Peter Wieck Melrose Park, PA

The larger cap will likely make less noise. The metal can will have similar wall thickness, but much greater circumference, so the hoop stress will be much bigger - it will split down one side under much less internal pressure.

Small diameter, large C (meaning low voltage), low ESR, will make the biggest bang.

Clifford Heath