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ok so we all know that it is normal to choose an electrolytic capacitor with maybe 15% voltage headroom or something similar, but is there a point at which a high voltage electrolytic capacitor will suffer if it is subjected to a far lower voltage than it's rating?
I don't know how a lower voltage limit could be possible, other than a reverse voltage connection, being in most circuits capacitors spend large parts of their service life in zero volt conditions.
You know, when the device is turned off.
Are you sure about that, when voltage goes down current increases. That may or may not cause flash over in the cap "repeatedly running" at the lower " could create heat and breakdown of the electrolyte ? It's not low voltage like 12vdc Higher Voltages do strange things.
Huh? Wouldn't the internal ESR tend to cause a decrease in current as the circuit voltage drops?
In many circuits electrolytic capacitors work with widely varying voltages anywhere from zero when the circuit is off and all the capacitors are bled down all the way up to the circuits maximum peak operating voltage.
My experience with electrolytic is that if they run warm from high ripple voltages using s higher rated voltage capacitor helps compensate by having a larger mass and an inherently larger ripple voltage tolerance.
For example 10% ripple on a 25 volt cap only gives me a 2.5 volt tolerance while a 10% ripple on a 100 volt cap gives me 10 volts tolerance despite the capacitance values being equal which means that if I had 3 volts of ripple current on a 12 volt circuit the 25 volt capacitor would run warmer than what a 100 volt capacitor of the same capacitance would when used in the same circuit assuming they has equal ESR numbers.
Thats my understanding anyway.
That's the part I don't understand. I figured that the load would dictate the current and that the cap would tend to carry the current through regardless. But, I guess the EM field strength would run hand in hand as the voltage dropped and so would current? Take a motor for instance it would increase in current as the voltage dropped and if our cap were in series it seem reasonable to think it would also have the current increasing as well ?
Okay I see where you are going now. Induction motors do show an increase in their running amps at a specific load if the line voltage drops, but there is a limit to how far that effect goes depending on a number of factors which are heavily influenced by the motors inherent design below that point if the frequency is kept constant the output torque and relating power drop off and the input amps follow along proportionally.
If you take a capacitor to be the electrical polar opposite of an inductor you will see that it exhibits a specific impedance at a specific frequency and thusly can be modeled like a simple resistor where if the voltage drop across it is lowered while the frequency stays constant then the current flow through it will also drop proportionally.
Mostly I think this electrolytic capacitor application theory is just getting over analyzed really.
Looks like it is not as big a problem today as it was years ago.
**broken link removed**
Here is a quote from appendix C of the 1990 Sprague electrolytic capacitor catalog.
"When aluminum electrolytic capacitors were first developed, deterioration 'on-the-shelf' was a major problem and frequent replacement of stock parts was necessary. Additionally, use of capacitors for extended periods at small percentages of rated voltage permitted the dielectric oxide film to 'deform', just as it would 'on-the-shelf'. Both problems were solved in the early 1950s with the introduction of high-purity aluminum foil. Oxide film stability was greatly enhanced and today aluminum electrolytics can be used after storage, and at any percentage of rated voltage, without loss of capacitor quality."
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This thread, like so many similar discussions, seems to be a fairly even blend of fact and folklore. It sure would be nice to be able to separate the two. Any pointers to papers on this matter from reliable sources (not Wikipedia!!!) would be much appreciated.
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This thread, like so many similar discussions, seems to be a fairly even blend of fact and folklore. It sure would be nice to be able to separate the two. Any pointers to papers on this matter from reliable sources (not Wikipedia!!!) would be much appreciated.