Quote:
Originally Posted by Piper J3
Can someone with elec knowledge say what the likely cause of capacitor failure is? Age of component, excess current, heat, temp cycles, humidity, mfgr defect, etc, etc....
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TL;DR
All capacitors are two metal plates with an insulator (dielectric) between them. Capacitance is proportional to the area of the plates and the distance between them. Bigger plates... more capacitance. Closer together... more capacitance.
Capacitors have several roles but when used for noise filtering bigger is better. The two ways to have more capacitance is bigger plates or put them closer together. Making them bigger has its limitations so there are several techniques for getting the plates close together and, very importantly, without touching each other.
Most large DC filtering capacitors are electrolytic capacitors. These are made with two aluminum foil sheets with an electrolyte gel between them. The insulator between the plates is a very thing coat of oxide on one of the foil sheets. The gel conducts electricity. So in effect what you have is two capacitor plates that are separated in distance by only that thin oxide coat.
So what kind of failure modes does an electrolytic capacitor exhibit? The primary type of failure is always leakage current between the plates. Remember, they are supposed to be insulated from each other.
- Mechanical damage can cause the plates to touch. Go easy on that zip tie holding the capacitor in place!
- The oxide layer is pretty thin so too much voltage can cause current to tunnel through the insulating oxide.
- Something like reverse polarity can cause the oxide layer to break down.
- Heat is always hard on electrical components. Most electrolytic caps that I have used seem to be pretty heat tolerant, though. It would be worth looking at the data sheet for the cap to see what it says about heat range.
When a capacitor starts leaking current it may get hot if there is enough current. One amp of leakage current at 12 volts is 12 watts of heat. That is more than enough to make a cap get hot. When the cap gets hot enough it explodes.
It is hard to say why your capacitor failed. I mostly worry about over-voltage because I've blown a few that way. If you have a 12 volt system don't put in a 15 volt capacitor. If you have noise spikes you may regularly have higher voltages spikes that punch holes in the dielectric oxide. Over time the damage accumulates. In a high impulsive noise environment I wonder if it would be prudent to have TVS diode in parallel to limit noise spikes to something the filter capacitor can tolerate.
One final word. I have an HVAC guy that regularly comes to the house. He pulls the compressor starter cap and "tests" it by measuring the capacitance. That never seemed like a terribly effective way to test a capacitor to me. The capacitance is set by the physics. I.e plate size and oxide thickness. Maybe if the measured capacitance goes up significantly you could say the oxide is getting generally thinner but I've never seen that happen. Instead a much better way to judge the health of a filter capacitor is to measure the leakage current and especially at what voltage you start to get significant leakage current. But that take more than just a hand held multi-meter.
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