capacitor aging

[Paul Perry]

At 08:15 PM 21/10/98 -0500, Rob Brzykcy  wrote:
>When a capacitor ages, does the capacitance go up or down? I guess this
>mainly happens to electrolytics.
>
well, if it dries out, the capicatance goes down....
but, like most things, it isn't as simple as this....
you can have quite a lot of capacitance left, but with a high ESR
(equivalent series resistance), so the ckt starts to behave in really strange
ways. The good thing is that, if there is any doubt, it is easy to just hook
another cap across the suspect one and see if it helps.

The other common failing of old caps is to go leaky, that is, to appear to have 
a resistor connected across them... to make life interesting, this
resistance can be 
intermittent, voltage sensitive, and vary with the phases of the moon.
Hitting a suspect cap with a pencil can sometimes reveal problems. (I call
this the 
'dentist' approach - ouch!)

I had a 25v rated 1000mfd cap in a half wave rectifier from a 16v plugpack.
(thought I'd be ok.. 16vx 1.4 peak.... but, it shorted, boiled, and developed a 
pinhole in the base that sprayed goop on the board, dissolving one wire.
Curiously, the power supply line (after the regulator) looked perfect!
SO, if it hadn't taken out a crucial part of the ckt, I would be wondering some
day what was going wrong as it dried out & the input ripple swamped the
regulator.

paul perry melbounre australia (that 25v cap is a 35v one now)