Why do capacitors leak?
- Thread starter bigmessowires
- Start date
Apologies for resurrecting this thread, but it appeared in my recommended 'similar threads' list. I'm currently waiting for a shipment of caps to fix the PSU in a Power Mac G4 MDD, and I speculate that the biggest contributor to failure in PSUs is heat. The motherboard caps still seem fine, but they'll need replacing eventually. I've taken the opportunity to buy replacement fans for the "wind tunnel" PSU, but I've gone with ones with an even higher CFM rating than the stock fans, so I suspect noise reduction will be minimal. It's always a bit of a let down when you have to replace electrolytic caps with electrolytic caps knowing full well that leakage will eventually be an issue again. I guess, in death, I won't care about cap leakage.
Take care with replacing fans based on just CFM - CFM is usually quoted for the fan operating in an open environment with no back pressure. It doesn't account for how the fan reacts to back pressure and Mac fans do more work than regular PC case fans as they usually are pushing through the PSU, the case, and out again, and there is only one or two.
Think of it like a sports car and a tractor - both might be 300hp, but one uses it to go fast, the other uses it to drag a plough through a rocky field. CFM is sort of like quoting straight line speed.
My first thought about such motherboards is whether there is a battery in situ, and, if so, has it shed its load?
There are a few common failure modes to aluminum electrolytic capacitors. Heat buildup, over current, age breaking down seals, and poor quality are some of the modes most of us have experienced. That said, there is one failure mode that's not so obvious, and keeping a capacitor in a cool dry place can extend the amount of time it will survive, but all AEC's will break down over time.
Here's where we get into the complexity of how aluminum electrolytic capacitors work. I'll keep it over-simple. AEC's are made by wrapping conductive aluminum layers with an insulator and saturating them with an electrolyte. Once they are made, a dielectric aluminum oxide layer is formed via a dc current flowing through a limiting resistor. In order for this layer to not break down, the capacitor has to be used. If not, and over time, the oxide layer will break down and the cap will fail. That's why capacitors often list a shelf life in the data sheet. When the oxide layer breaks down, it can sometimes be reformed the same way it originally was created, though you would need to remove the capacitor from the circuit it's in before it could be done. In the end, and other complexities aside, it's far easier to just get a new cap.
So, will a sealed board last longer than one that's in a computer? Answer: Depends. Assuming all capacitors are the same, if your board is in a climate controlled place and sealed in a dry bag, then it will probably last longer than one that is exposed to humidity but otherwise stored in the same temperature. Breakdown times are also influenced by the quality, chemical and physical properties of the cap, so it's near impossible to accurately predict how long one will last.
Though in the end... entropy *always* wins.
Here's where we get into the complexity of how aluminum electrolytic capacitors work. I'll keep it over-simple. AEC's are made by wrapping conductive aluminum layers with an insulator and saturating them with an electrolyte. Once they are made, a dielectric aluminum oxide layer is formed via a dc current flowing through a limiting resistor. In order for this layer to not break down, the capacitor has to be used. If not, and over time, the oxide layer will break down and the cap will fail. That's why capacitors often list a shelf life in the data sheet. When the oxide layer breaks down, it can sometimes be reformed the same way it originally was created, though you would need to remove the capacitor from the circuit it's in before it could be done. In the end, and other complexities aside, it's far easier to just get a new cap.
So, will a sealed board last longer than one that's in a computer? Answer: Depends. Assuming all capacitors are the same, if your board is in a climate controlled place and sealed in a dry bag, then it will probably last longer than one that is exposed to humidity but otherwise stored in the same temperature. Breakdown times are also influenced by the quality, chemical and physical properties of the cap, so it's near impossible to accurately predict how long one will last.
Though in the end... entropy *always* wins.
Remember that some capacitors (I appreciate you mention "Poor Quality" above but this is even beyond that) were victim of the "Capacitor Plague" which will mean certain machines not only fail quickly, but we can pretty much guarantee any affected have already failed: https://en.wikipedia.org/wiki/Capacitor_plague
