Capacitors leaks current.
So a real world capacitor is like an ideal capacitor with a parallel resistor (of course, series resistor and inductor, too.).
The parallel resistor value seems to be high in some capacitor (10000 MOhm) and low in some electrolytics (less than one MOhm).
Anyway, it that parallel resistor value stable with time (many years) and temperature as long as the capacitor is used well within the temperature and voltage spec?
So a real world capacitor is like an ideal capacitor with a parallel resistor (of course, series resistor and inductor, too.).
The parallel resistor value seems to be high in some capacitor (10000 MOhm) and low in some electrolytics (less than one MOhm).
Anyway, it that parallel resistor value stable with time (many years) and temperature as long as the capacitor is used well within the temperature and voltage spec?
Often, the leakage current is given by 0.03*C*V or 3uA whichever is greater.
So the parrallel resistor will be 1/(0.03*C).
For example, an electrolytic capacitor with the spec of 470uF, 25VDC, will have a parallel resistor larger than 0.071 MOhm?
Is this the right calculation?
Anyway my question is whether that resistance value is constant over time, temperature variation (but far below 85 degrees C), etc.
So the parrallel resistor will be 1/(0.03*C).
For example, an electrolytic capacitor with the spec of 470uF, 25VDC, will have a parallel resistor larger than 0.071 MOhm?
Is this the right calculation?
Anyway my question is whether that resistance value is constant over time, temperature variation (but far below 85 degrees C), etc.
No, that resistance varies all over the place, with age, temperature, on-time and anything else you can think of. It's an interesting exercise to set up a power supply and measure leakage current with a series resistor and DVM. Do it. Vary everything and see what happens. You'll learn a lot.
BTW, I don't think you have this problem, but don't confuse the simple parallel loss model with the DC leakage resistance term.
BTW, I don't think you have this problem, but don't confuse the simple parallel loss model with the DC leakage resistance term.
If the parallel resistance changes (with age, temperature, etc.) +/- 60% from 1000 MOhm, it would not matter much for me. If it changes +/- 60% from 0.2 MOhm, it matters.
Agree.
Or to put this theoretical discussion in more practical or useful terms: are you designing a timer where the time constant comes from an electrolytic capacitor and its own internal resistance and nothing else?
Because if so, it will be an excercise in frustration and you will come out with the most unreliable and unpredictable timer in the World.
Now if you plan to use in a power supply, the load it feeds will be hundreds or thousand times more important than the load presented by its own leakage resistance, which then fades into insignificance.
EDIT:
back to the original question:
Plus it´s not linear and varies heavily with instantaneous applied voltage ...... and temperature, of course ... and thermal history ...
Or to put this theoretical discussion in more practical or useful terms: are you designing a timer where the time constant comes from an electrolytic capacitor and its own internal resistance and nothing else?
Because if so, it will be an excercise in frustration and you will come out with the most unreliable and unpredictable timer in the World.
Now if you plan to use in a power supply, the load it feeds will be hundreds or thousand times more important than the load presented by its own leakage resistance, which then fades into insignificance.
EDIT:
back to the original question:
No, it´s not stable *at all*.
Plus it´s not linear and varies heavily with instantaneous applied voltage ...... and temperature, of course ... and thermal history ...
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