old caps change

[birt]

i just measured 2 50uF 350V caps i found and noticed one was 75uF and the other one 80uF.
then 4 50uF 370V caps and they were all around 70uF.

this got me thinking... do cap values allways drift to a higher value when they age or is this just a coincedence? and can i still use those caps in amps?

[R.G.]

The problem with electrolytic caps is getting them to be a large enough capacitance. So the tolerance is poor. Also, the insulation is an oxide layer grown on the surface of the aluminum foil that make up the two terminal plates. As the thickness of the oxide layer in creases, the voltage rating goes up, and the capacitance per unit area goes down. As capacitors age, the oxide gets thinner by chemical degradation, so the capacitance tends to drift up with age.

As a result of all that, the makers of electrolytic caps tend to put big tolerances on their parts. Most electros are -20%, +80% when new.

Other capacitors, notably film capacitors, have closer tolerances (+/-10% to +/-1%) and don't drift like electros do.

[Peter Snowberg]

I recycle lots of things, but electrolytic caps are one I won't recycle. If there is any qestion about the cap's life, I would replace.

[markm]

I recycle lots of things, but electrolytic caps are one I won't recycle. If there is any qestion about the cap's life, I would replace.

Same here.
Electros only have about a 10 year lifespan and they're fairly cheap.
Just change 'em 

[birt]

tnx for the great (as allways) explanation

[idlechatterbox]

Is the general rule against recycling electrolytics applicable regardless of the voltage of the circuit they were originally used in? E.g., would a cap that came out of something that was running, say, 50 volts, be happy and relaxed in a much "weaker" circuit running off a 9v?

[RedHouse]

The problem with electrolytic caps is getting them to be a large enough capacitance. So the tolerance is poor. Also, the insulation is an oxide layer grown on the surface of the aluminum foil that make up the two terminal plates. As the thickness of the oxide layer in creases, the voltage rating goes up, and the capacitance per unit area goes down. As capacitors age, the oxide gets thinner by chemical degradation, so the capacitance tends to drift up with age.

As a result of all that, the makers of electrolytic caps tend to put big tolerances on their parts. Most electros are -20%, +80% when new.

Other capacitors, notably film capacitors, have closer tolerances (+/-10% to +/-1%) and don't drift like electros do.

Hey RG, why aren't Electro's sealed in epoxy like most film caps?
(specially since they rot)