overcharging a ceramic cap

It will ? Is that a property of ceramic dielectrics ?

With 10uA I'd be surprised if any damage happened. The power is *very* limited.

Reverse bias leakage ? At 10uA I'd expect you'd be safe again. I've avoided using the avalanche effect in bipolars to make a 'cheap zener' though at even the mA level since no-one can typify the long term junction characteristics.

Graham

Good question. Film caps punch through because there is an avalanche effect in the plastic. Surely this is also pretty likely with ceramics too? Whether there's a reliable pre-avalalanche leakage I don't know. In my experience "leaky" 1uF ceramics hold their charge for days. Most ceramics have a very high overload capacity, 3x for example. Maybe this is to ensure that you don't use the "sloppy zener" region? However, it would be wrong to jump to any conclusion like that as Hi-K ceramics lose their Hi-K when a voltage is applied.

Unfortunately, it's not. There is a certain amount of energy in the capacitor, making the momentary power effectively unlimited. If any positive feedback mechanism exists there will be current crowding followed by destruction.

If the "sloppy zener" effect is real, I would be suspicious of what sort of conduction is occurring. Odd thermal electrons are OK, but you do not want 10uA of metallic ions moving through the lattice :)

I was taught that there's no particular failure mechanism associated with zener or avalanche breakdown as long as the temperature rise is not excessive. Unfortunately, breakdown cannot be guaranteed to be simultaneous across the whole junction, so you can't push the device to its nominal dissipation. I wouldn't guarantee long-term stability though.

Zeners (above about 7.5V) don't break down by the Zener mechanism, but by avalanching, and people have photographed interesting looking light emissions from very lcalised "microplasmas". The interesting question is why one of these localised avalanches doesn't heat up the narrow tube of silicon involved, carry all the current available and heating the silicon above its melting point.

Something like this does happen in laser diodes, which can be ruined by a microsecond or so of over-current.

Possibly because the TC of avalanche breakdown is -ve so you get automatic current sharing/ spreading?

I'm skeptical about "microplasmas". Plasma just means "fully ionised"

- there's no absolute requirement for it to be a hot gas. I seem to recall the term being applied to silicon during avalanche even though the crystal streucture is not disrupted.

You say "light emissions". Do you mean IR or can silicon emit in the visible (other than due to incandescence!)?

John, the involved 10V sound small, but considering the tiny distances between conductors, the field strength is rather big. I wouldn't be surprised if it reached MV/m. And then depending on the "arc-ing" mechanism, the stored power is rather big to created damage.

Again here the microscopic "arc-ing" mechanism defines the possible damage. Is is just a tunnel current through some potential barrier, or does it involve local ionization, oxidation, ... There may be microscopic conducting channels that overheat and change their doping.

Rene

Hello John,

Just my 2 cents. I wouldn't rely on a certain leakage behavior. If it decides to really break down the available current is going to be very high because it holds a lot of charge by the time you are up there in voltage.

Once when I had to find field failure causes on a Doppler unit I found that an unintended charge pump effect was trickling up charges into a Z5U cap. It destroyed those quite spectacularly.

Regards, Joerg

Bummer. I connected a 1 uF, 16 volt 0603 ceramic cap to a power supply, through an ammeter. Stepping the voltage up, and after waiting for annoying dielectric absorption things, current goes up sort of linearly with voltage. I've run out of power supply at 120 volts, 35 nA. I'll let that cook for a while but, after 40 minutes, the current only seems to be going down. So I don't have enough voltage, with this supply, to see anything dramatic.

John

variac + neon xfmr + string of 1N4007s.......

actually I recall seeing a paper (marcon?) that discussed how high these things can go, but alas cant recall any details, other than the actual voltage rating gives the mfg a *lot* of wiggle room.

also, dynamic would be worse than static. next I shall teach my grandmother to suck eggs....

Cheers Terry

Probably surface moisture. You need to de-salt and then dehydrate the device before testing it - preferably in dry air.

Still, 120V is quite impressive for a 16V cap :)

I imagine that it was near infra-red - the papers I saw showed photgraphs of of localised light emission. It certainly wasn't incandescence.

I tried this a while ago. I was running a bit close to the capacitor voltage rating, and also I was worried about momentary over-voltage from spikes (e.g. ESD events).

I couldn't get high enough either!

I guess I'll have to dig the old Spellman 30 kilovolt supply out of the dungeon.

John

Hello John,

Last time I had a ceramic capacitor accidentally run well above its rated voltage there was a loud bang, the circuit breaker tripped, much of the cap was gone and what remained of it had turned into a blob of streaky green glass. Almost like a piece for jewelry.

Regards, Joerg

Well, it makes it easy to find the bad part when that happens. Kind of like the custom made sheet silvered mica capacitors used in old high power TV transmitters. When they fail, they really fail. A loud bang, pieces of mica flying out of the transmitter, and the high voltage power supply tripping a lot of overload relays and circuit breakers. Its rather impressive to observe up close. I was about ten feet from a UHF PBS TV station in Oxford, Ohio one day when one exploded, and the whole engineering staff got busy. I was just visiting the station, but I got the impression that this happened three or four times a year because of the way they rushed to get back on the air without doing the required preventative maintenance to prevent the next failure. I know they had a lot of equipment problems, and most of the staff were college students. I remember that the custom made sheet silvered mica capacitors for the RCA TTU-25B 25 KW UHF transmitter were over $600 each, when they were still available.

So... *are* ceramic capacitors ESD sensitive? Would be slightly ironic if so. A bit like the ESD warnings you see on Transorb packaging.

Hello John,

Depends on the wave form and energy. A large spike into a smallish cap without adequate resistance up front can certainly fry the cap. And has :-(

Regards, Joerg

I suspect that the microplasma phenomona are at one boundary or the other of the depletion region and thus quickly suck up all the availble carriers. Incandescance is a bulk effect, but i'll bet like all plasma emmissions that they radiate from infrared through ultraviolet and would look white to the uv only protected eye.

Yes, returning to OP topic, yes they will break down, and at successively lower voltages. They will arc around the grains of the ceramic material, and the 10 uF will certainly store enough energy to see to the eventual total destruction of the capacitor. If you are (un)lucky enough the cap may explode on its first breakdown.

Do you know that as fact, or is it conjecture?

I'd be surprised if a ceramic cap could store enough energy to explode. We're talking millijoules here.

Guess I'll have to dig out a HV power supply.

John