i don't know....
I think caps go bad as a combination of both lack of use, and age. It would be my guess that any cap would eventually go out over time, as the di-electric material breaks down. (especially in oil-soaked paper dielectrics, like in my case). Since I'm here, and I have nothing better to do, lemme share what I have learned this week about capacitors.
All capacitors are, is two conducting plates, with a di-electric material in between them (whether that non-conducting di-electric material is air, water, paper, oil-soaked paper, porcelain, wax, you name it.)
When a capacitor is neutral, there's equal amounts of positive and negative charges on both conducting plates. Just like you and I, just like a doorknob, just like anything that has no electrical current going through it.
But, when an electrical current is applied to it, nameley, a negative current to one plate, and a positive current to the other plate (we're talking DC here,) something magical happens.
The electrons (negativeley charged particles) on the positiveley charged plate, all travel AROUND the circuit, to the other plate, to the negativeley charged plate. Keep in mind that NO ELECTRONS travel through the dielectric material in between. If electrons DO start leaking through the dielectric, the capacitor is said to have broken down, and it can blow up, like mine did. Loads of fun ensue.
All right, so we got electrons moving around, from one plate to the other. Essentially what the cap is doing during this process is CHARGING UP. Once all electrons have moved, the negative plate will have ALL negative charge, and the positive plate will have ALL positive charge.
But what the crap is the dielectric for, you ask? Well, here we go.
The dielectric material is made of atoms, all of which have negativeley-charged electrons rotating around a nucleus. When the cap charges up completeley, this causes the negativeley-charged electrons of the di-electric to be ATTRACTED HEAVILY to the positiveley-charged plate. (opposites attract, remember?) ANd they more or less LEAN in that direction. Now, sometimes, as I said, if TOO MUCH CURRENT goes through the cap, then these electrons in the dielectric are pulled away from their nuclei, and are strained through the positive plate, thus allowing LOTS AND LOTS of electrons to continuously flow through the dielectric, causing leakage of current, and di-electric breakdown, and explosion.............. you know the drill.
ALL RIGHT. PERFECT WORLD. Our functioning, stable cap is charged up completeley. So if you disconnect the cap from the current source, and if the positive and negative sides of the cap are not touching, the cap HOLDS IT'S CHARGE, much like a miniature battery. Until you short the two ends together - until the positive and negative somehow form a circuit to each other, the capacitor does not DISCHARGE. But once you DO connect the positive and negative ends together, it DOES discharge. --- and this is how that works:
The electrons in the di-electric material are kind of spring-loaded, like a pulled-back string on a drawn bow, waiting to shoot an arrow. When the positive and negative sides of the cap form a circuit somehow, these electrons begin rotating their nucleus normally again, and in the process, SHOOT the accumulated electrons on the negative plate, back to the positive plate. (so the accumulated electrons on the negative plate can be thought of as the ARROWS which are shot.) And so, the cap discharges, and these electrons travel back through the circuit, to the positive plate, until both plates have a balanced number of negative and positiveley charged particles. The cap has now fully DISCHARGED.
After this little schpeal, it's pretty easy for me to believe that since this reel to reel of mine has been in storage for the past 15-20 years, the oil inside that cap probably leaked its way down to the bottom of the cap, causing the paper in the top part of the cap to be nearly completeley dry, making it MUCH easier for electrons to flow through the paper when the cap charged up, causing a continuous feed-back of charge, that never ends, and keeps getting bigger, and bigger, until it blows up. (See? feedback is bad no matter WHERE it's at!)
I would imagine that if the reel-to-reel HAD been used a bit more frequently, the electric field put on the oil and paper electrons would have helped HOLD the oil up, preventing the oil from leaking down to the bottom parts of the paper. Makes sense, anyway. If I'm completeley wrong in this, somebody let me know!
-callie-
