Capacitor life

[Ken9876]

What is the expected life of a dry film capacitor? I?ve been testing these in some metal halide fixtures we have that have failed, and I am finding that the capacitor value has dropped from a15uf down to 8 and 11uf. What would cause a capacitor to lose value?

 

[mike7330]

http://market-ticker.org/akcs-www?singlepost=2139475

 

[K8MHZ]

http://market-ticker.org/akcs-www?singlepost=2139475

Interesting. I had a Dell monitor fail and when I did a search I found out there was a problem with the caps and hundreds, if not thousands of these monitors were failing.

One of these days I will re-cap it, but 'till then I just bought another (not Dell) monitor.

 

[Speedskater]

I think that the capacitor failures referred to in posts #2 & #3 were electrolytic capacitors rather than film capacitors. However I did read about some AC power film capacitors (X & Y rated) having their value decrees as the aged.

 

[broadgage]

Line voltage capacitors generally consist of a thin plastic film coated with a very thin layer of aluminium, the whole being coiled up.
In time the plastic film breaks down and causes a local short circuit, this does not render the capacitor useless as the aluminium is vapourised from the shorted area. This feature is known as "self healing" In time though the capacity is reduced as more areas suffer thus.

In critical applications, the lifetime of a capacitor can be greatly extended by use of one intended for a higher voltage than the actual supply. For example on a 277 volt circuit, use a capacitor intended for a 480 volt circuit.

If a capacitor is part of a lamp operating or motor operating circuit, then the correct value must be used.
If however the capacitor is simply connected accross the supply to improve the power factor, then no harm will come of useing one 25% or 50% larger than the original. This gives more margin for age related loss of capacity.