newbie here.
If apply a slightly higher than maximum rated voltage on a ceramic chip resistor to destroy it, what will happen. The circuit is open? short ? always? What will happen to other forms of capacitor like electrolytic or tantalum?
I guess it depends on the dielectric material in between. any inputs?
Most likely you would get away with a "slightly" higher voltage.
Distinctly remember what happens when you hook up a rather large 400 Volt electrolytic in the wrong direction. It made a noise you don=92t forget. The paper cover with the polarity marked was on backwards. After that I checked to verify that the paper cover was on correctly.
The feature of a damaged Cap depends on what kinds of it. In my experiences, most of ceramic cap get short after destroyed, and SOME of Tantalum Cap get open after destroyed - although they publicize that All of the Tantalum can open after damaged.
P.S. most of my experience based on the Low voltage situation's overvoltage.(standard voltage
One major factor is the voltage source impedance.
With most capacitors, the dielectric fails in an over-voltage situation.
For a ceramic chip, the result is a fracture in the ceramic. This will likely reduce the capacitance and the voltage at which future failures occur. Depending on the source impedance, the result can be that a conductive path is formed through the fracture ("short") or the capacitor blown apart ("open"). Or that the only effect is a reduction in capacitance.
The same generally applies to other capacitors.
Some (eg air-spaced) have "self-healing" dielectrics and are virtually unaffected, as long as the amount of energy released isn't too great. Some are designed to have internal impedances such that the plate area adjacent to a failure point will vapourise and thus the capacitor "recovers".
Tantalum capacitors don't have any such measures. if the dielectric fails, the result is almost invariably a dead short between the very low impedance plates.
A lot of energy released into an electrolytic used to result in its case being fired out of the apparatus with enormous force. Certainly enough to kill an assembly line worker that had just powered up the uncased chassis in front of them. That was because the end seal was the weak point. Modern ones have weak points on the top, to prevent this.
Generally, an extremely high source impedance only results in reduction in capacitance. A high source impedance generally results in a short. A low source impedance results in an open circuit - if you can actually find the remains of the capacitor, that is.
Years ago we had a problem with tantalum caps being inserted=20 backwards. The problems included major fires. The flame from an=20 exploding cap would short the inner planes of a card. When the card=20 is supplied by 1000A ugly things happen. Many solutions were found,=20 including three and four pin caps (+-+ or +--+), fused caps, and=20 differing lead sizes (presumably with differing holes to match). =20 Once a manager of the card stuffing line called to complain that his=20 "girls" were getting sore thumbs from pushing the caps into too- tight holes. That's the amazing thing about fools; they're so=20 ingenious.
--=20 Keith
Once upon a time, I helped manage a PDP-11/45. The main memory was all core; no MOS memory was in common use yet. And the DEC core was expensive and took backplane space, so we had some third-party core in an external box, with an interface that pretended to be a Unibus jumper (so it didn't take any backplane space at all). I can't remember the manufacturer anymore; maybe Dataram?
One time when the interface was out, I noticed that a tantalum bypass capacitor had fried. I removed it and replaced it with a new one, carefully matching polarity with the board markings. The next time we powered the system, on, there was the lovely smell of a tantalum cap burning.
Took a closer look at the board, following the capacitor traces to +5 V and ground. Sure enough, the silkscreened polarity markings for the capacitor were backward! So the original cap plus my first replacement were both installed backwards, and both died. The original probably died during burnin at the factory, with nobody noticing (or they just didn't burnin any of the interface cards).
Dave
I had a /35, /34, and /45 (IIRC), built into some tektronix gear. The /35 had a full boat of core. The others were a bit newer and had semiconductor memory.
Yep!
Makes sense. My boss did a major study of tantalum capacitors and it seemed that no matter what they did they couldn't get the incidence of backwards tantalums below about .5%. They even found caps marked backwards from the factory.
Like I said, these were causing major fires. These were radial- leaded ("tombstone") caps. When they blew they tended to shoot a flame down into the board, causing an interplane short. Since the boards were backed up with 1000A supplies the short caused the boards to get hotter, causing the boards to short further, causing... ...smoke. Management got upset when their (many) Million$ system caught fire.
The old tube radios used wax impregnated foil & paper caps. After years of sitting unused, they dry up. The first time you power them up, they seem to work fine for about 10 or 15 minutes, then you get failures. I have seen capacitors explode and blow out a long coil of foil, like a slinky, from one end. Loud, too. Electrolytics seem to fail less spectacularly, but they smell bad.
Of course, there were no circuit boards to burn up. Just the wooden cabinet or whatever flammable surface the set was sitting on!
The paper capacitors used a high acid paper, like books. they were wound on paper, then dipped in wax in an attempt to keep moisture out of the paper and foil layers. It deteriorates, with age, till the capacitor develops leakage. Depending on where it is used, it will put a positive voltage on the grid of the next tube, or short out a power rail. Even the metal can Mil spec can capacitors were paper & foil, and filled with transformer oil. These also developed leakage as they age.
The caps that explode are usually electrolytic capacitors that have lost the formed layer. By turning the equipment on without reforming, it draws more and more current as it tries to reform the electrolytic layer on the aluminum foil, which generates heat. That boils the electrolytic and pressure builds to the point a vent blows, or the aluminum can ruptures. The proper way to reform old electrolytics is to use a current limited HV power supply and slowly ramp it up to the surge rating of the electrolytic capacitor.
I recently needed a variable audio source, and remembered I had an old HP Audio Oscillator, which I'd bought as surplus some fifty years ago. I found it in the garage, (HP Model 2000AB, Serial 1651). The line cord was deteriorated and had to be replaced, which took half an hour since the power transformer had to be removed to get access to the input power terminals.
I know about reforming electrolytics, and thought about raising the line voltage gradually with a Variac, but finally just plugged it in and flipped the switch. It came on and worked just fine!
I even found the "Operating and Servicing Manual" as a .djvu file. It was Copyright 1955, and for Serial 7725 and above, but the circuit in mine used the same tubes and seemed the same.
I re-set the feedback, and re-calibrated it slightly and it played like new. There was no line frequency hum in the output, so the two electrolytics in the power supply must be OK. (At least they didn't explode! I've had experience with ones that did--it makes a real mess!)
This was one of the first instruments designed by Hewlett and Packard.
They don't make them like they used to!
HP Model 2000AB? You mean the HP Model 200AB. yes, the old HP equipment was built with premium parts. Most equipment wasn't. I'll bet that if you took the time to test all the capacitors for leakage, at least a few ate under 5 Megohms and need replaced.
I have a TS-382, which is the military version of that audio generator. It also has a reed type frequency meter. It has no electrolytics, but every oil filled metal capacitor is leaky. I am restuffing the canned capacitors with HV mylar film capacitors that should last 50 to 100 years.
It is the HP 200AB. My eyes are not too good and the spell-checker didn't catch it. Just last week, one section of the original 6SN7 tube failed. I signal traced it and replaced the tube with one from my spares box, also over fifty years old. When the oscillator still didn't play, I was beginning to mis-trust my old vacuum tube diagnostic abilities. It turned out the replacement tube had the same bad section--a different one worked OK and is still working.
Mine seems to be even earlier than the one shown there. It has a "fluted" knob on the frequency dial.
This looks like the manual I have. This Linux OS doesn't have a .djvu program. I d/l'd it and will take a look using WinXP. Thanks for the reference.
I'm sure both oscillators will outlast the both of us.
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