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Old 4th Mar 2018, 10:33 am   #3
Argus25
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Capacitor problems

Faulty wax capacitors can make equivalent circuits of 4 components:

THe problem with the paper capacitor (and you can totally ignore the wax) is that paper contains salts. When it is dry, it is a pretty good insulator. Add H20 and you get a weak electrolyte.

This causes the value of the original capacitor to increase by a factor or 10 approx. however it acts like a higher value capacitor with about a 1 meg or more series resistor, that is placed in parallel with the original value.

Then because it has parallel leakage, this adds a parallel resistance. So what you end up with is:

The original capacitor value, a series capacitor of about 10 times that value with a large resistor in series with it placed across the original capacitor.. And a pararallel resistor across the capacitor's terminals representing the leakage from the electrical equivalent circuit perspective.

So, if you have a paper capacitor that has absorbed H20, it can only be modelled as a new network of 4 components, with varying proportions for three of the values, except for the original value partially masked by the other 3 added R & C values

Of course, in some circuits, no issue would be noticed, especially say
If in an audio circuit where the grid resistors were 100k or less.

But it really shows up in timing circuits, like vertical or horizontal oscillators in TV's where the impedance in the oscillator circuit might be 1meg and as a result the frequency goes way low due to the larger value created, despite its large effective series resistance.

Some years ago I recorded the effects of this in wax paper capacitor's displaying two time constants in high impedance circuits, It actually represents an extreme case of dielectric absorption when a paper capacitor absorbs water, which is prevented if it is immersed in oil and becomes a Pio capacitor. This is why PIO types give a lot less trouble than wax paper types.
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