This is something new to me. I was aware that traditional electrolytic capacitors tend to fail as their wet electrolyte dries out, and while they keep the same capacitance value their ESR (Equivalent Series Resistance) rises causing ripple and timing issues. However, I've always just thought that metallised film capacitors either worked or didn't. But now I know better.
Y'see, the capacitors are made of two layers of metallised plastic film wound together in a roll, creating alternate layers of insulator and conductor. There's no wet electrolyte, so they can't really dry out as such. But they have an intriguing characteristic sometimes referred to as "self-healing" whereby if a voltage spike exceeds the insulating capability of the insulating (dielectric) film then a tiny spark can arc through, but because the metallisation layer is very thin it tends to just vaporise and clear the fault. But what I've discovered here is that over time the metal film can get eroded away to the point that the conductive area is reduced dramatically with a matching reduction in the capacitance value.
Fortunately this is easily detected simply by measuring the capacitor with a capacitance meter.
So in devices that use a capacitive dropper to derive a power supply, the steady degradation will ultimately reduce the current it passes and cause voltage issues as current is drawn by the circuitry. In both the time switch and remote switch featured in the video it was the current requirement of the relay that made the issue visible as it either failed to pull in at all or pulled in, but then dropped out again as the voltage fell on the smoothing capacitor.
Suddenly the failure of dimmer and remote wall switches all makes sense now. I really didn't suspect the capacitor as being a likely candidate as I thought it would either work or just fail outright.
This could also cause issues with interference suppression networks where the gradual failure of the foil reduces the suppression capacitors ability to shunt or absorb transients, and could lead to problems with interference crashing processors.
The snubber networks across triacs could fail making them susceptible to inductive loads causing the triac to false trigger erratically.
In the case of the capacitive power supplies in many LED lamps with the suspiciously small capacitors suggesting thinner and thinner insulation it could lead to progressive loss of lamp intensity over time simply down to the capacitor not letting as much current pass on each half cycle of the mains waveform.
And the fix? If in doubt just change the capacitor for a new one.
Y'see, the capacitors are made of two layers of metallised plastic film wound together in a roll, creating alternate layers of insulator and conductor. There's no wet electrolyte, so they can't really dry out as such. But they have an intriguing characteristic sometimes referred to as "self-healing" whereby if a voltage spike exceeds the insulating capability of the insulating (dielectric) film then a tiny spark can arc through, but because the metallisation layer is very thin it tends to just vaporise and clear the fault. But what I've discovered here is that over time the metal film can get eroded away to the point that the conductive area is reduced dramatically with a matching reduction in the capacitance value.
Fortunately this is easily detected simply by measuring the capacitor with a capacitance meter.
So in devices that use a capacitive dropper to derive a power supply, the steady degradation will ultimately reduce the current it passes and cause voltage issues as current is drawn by the circuitry. In both the time switch and remote switch featured in the video it was the current requirement of the relay that made the issue visible as it either failed to pull in at all or pulled in, but then dropped out again as the voltage fell on the smoothing capacitor.
Suddenly the failure of dimmer and remote wall switches all makes sense now. I really didn't suspect the capacitor as being a likely candidate as I thought it would either work or just fail outright.
This could also cause issues with interference suppression networks where the gradual failure of the foil reduces the suppression capacitors ability to shunt or absorb transients, and could lead to problems with interference crashing processors.
The snubber networks across triacs could fail making them susceptible to inductive loads causing the triac to false trigger erratically.
In the case of the capacitive power supplies in many LED lamps with the suspiciously small capacitors suggesting thinner and thinner insulation it could lead to progressive loss of lamp intensity over time simply down to the capacitor not letting as much current pass on each half cycle of the mains waveform.
And the fix? If in doubt just change the capacitor for a new one.
And years ago when we had a capacitor failure on our 20kVA generator, one Mr clever clogs boldly proclaimed "Capacitors do not change in value"…. Well that one did! We replaced it with a self healing motor run capacitor. Touch wood, it's been okay so far.
Thanks, Clive, for explaining how those capacitors can lose their capacitance.
Always shows passion when you look at he screen 2 seconds in and there's just parts and shite all over the place 🤣 wouldn't get that with Linus
Oh man…. I never realized this…. I’ve been told that film types last for a very long time. In this case, it seems like a ceramic would be the best. Is that correct?
Excellent video, Clive. The common thought was metal film capacitors were pretty much indestructable. You have shown otherwise and they degrade like anything else.
Most electronics failures are capacitor related. Best dielectric is vacuum, glass, teflon, etc, and it's expensive. You can reuse resistors, inductors, etc. but caps one should never. Always take fresh new ones, especially electrolytics.
Forget taking it to bits, BC took it to flakes..
Fascinating. Who would have thought of gradual plate ablation. Someone also said that transformerless power supplies constantly draw current with no load. Maybe in your high level tinkering you could confirm or deny this. Thanks
What is fun is to take a CD and put high voltage across it and watch the high-voltage arcing eat away the aluminized foil! It's just like the film on the capacitors!
This is why capacitors have a rated lifetime.
I'm a few years late to reply…….But did you measure the thickness of the metalized layer ? Looks as if its aluminum foil. Perhaps copper might not degrade in the same manner and last longer. This has been of interest in the last month or so, relative to the old style paper & wax units from the early era where the paper degrades….then the cross over in the era to the newer synthetic materials as polyester and the vintage polyester caps becoming leaky……considering some brands are notorious for becoming leaky and other vintage caps of the same material but different manufacturer last in comparison. Makes for an interesting situation to determine expected service life as such that goes along with the electrolytic as they age and dry out. I recently had a batch of new poly caps come to me…..and the leads ended up coming out of the body or just after soldering they would come loose…….brand new.
Failure analysis….most impressive.
Thanks Clive…..Just noticed this video….and I've just found arcing in the relays of a gate opening controller….the resistors in the snubber (?) circuit intended to prevent arcing are open circuit…I'm wondering if this was caused by failing x2 rated capacitors…having read this I'm going to replace all three components. Unfortunately, my multimeter can't check capacitance….