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Registered Member #347
Joined: Sat Mar 25 2006, 08:26AM
Location: Vancouver, Canada
Posts: 106
Hey guys,
Have any of you seen dry film caps change value over time? I've got a metal halide light fixture that started killing bulbs, and I found the 10uF 400VAC film cap in series with the lamp has decreased to 8uF. I'm puzzled as to why, I've never heard of this happening before.
I cut the capacitor in half, it's of rolled construction. The dielectric appears to be black, and I couldn't see any metalization, so the metal film must be very thin.
Registered Member #75
Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711
Did you measure the capacitor before it "changed value"? 20% tolerance is not unheard of, so it may just have started out with just 8uF. If it was a self-healing capacitor (likely from your description), it could have been overvolted several times, which would decrease the capacitance somewhat, but that would be rather obvious when you unroll it.
Registered Member #347
Joined: Sat Mar 25 2006, 08:26AM
Location: Vancouver, Canada
Posts: 106
These are +/- 5% caps, and I'm sure they're changing. I don't know the original capacitance of the one that's now 8uF, but after that one failed, I put in another one, same type, out of an identical fixture that was unused. It measured over 10uF originally, and just recently I checked it, it's now down to 9uF.
It may be that it's a very poor quality self-healing cap, the fixtures and caps are Grandlite brand out of China. I'll have a go at unrolling it.
Registered Member #1262
Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451
The +/- 5% rating is not likely indefinite, I know even top shelf resistors vary about 1% after a few thousand hours so I'd guess the same thing holds true for caps. I don't know why this is happening but I can say that I doubt its out of spec, a lot of datasheets seem to say "Its +/- x% for y hours, then we dont have a clue what the heck it does".
Registered Member #347
Joined: Sat Mar 25 2006, 08:26AM
Location: Vancouver, Canada
Posts: 106
You could be right about the tolerance/time spec. Clearly the spec'd caps weren't good enough. The cap killed the original bulb before the bulb died on it's own.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Perhaps get a higher voltage rated capacitor in there. The 20% loss in capacitance is *very* considerable for a film cap and is very likely NOT within spec. Id agree in the assessment that these capacitors are failing from dielectric punch-through. If the capacitance begins to wander out of the specified range, then its likely on the edge of complete failure.
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Film Caps Changing Value Over Time
