X2 AC capacitor failure

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I devised the circuit below for use as a continuous mains  
presence/absence indicator on my front porch.

                           ||           LED
              3.3k    |----||---|   |--->|----|
              ___     |    ||   |   |         |
       ------|___|----|         |---|         |---
                      |   ___   |   |   LED   |   |
                      ---|___|---   ----|<-----   |
   230VAC                 1M                      |

           C = 275VAC X2,  LEDs = Blue 5mm

It worked well for some time (a few months maybe) but the LEDs  
then grew dimmer and dimmer until they were barely visible in  
daytime. Turned out that the capacitor had greatly decreased in  
value - down below 1nF.

I replaced the cap and the LEDs glowed brightly once more - for a  
few weeks, and then began to dim again. The replacement cap has  
already gone down to about a tenth of its nominal value.

These caps were not quite new. I pulled ten of them from another  
project after testing it for a day. Each cap initially served as  
a snubber in series with a 100-ohm resistor across a triac. But  
the load for each triac was light (<5W) and essentially  
non-inductive, so I took the caps out after the test, during  
which they were switched on and off some dozens of times.

The test was run in a place where there's no public power supply  
and I used a square wave UPS. Could that have weakened the caps  
during the test? But then why would the effect show up only after  
they had been subjected to normal sine wave AC for some months?

Re: X2 AC capacitor failure


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** Those last two words are key here.

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** Then these.

How do the leads on the failed X caps look ??

Nice and shiny or not ?

....   Phil

Re: X2 AC capacitor failure
Phil Allison wrote:
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Do I understand correctly that you suspect environmental effects?  
Moisture seeping in around the leads? I can't dismiss that  
offhand. And the leads do look like they're lightly oxidised. But  
then that's quite common here as this is a tropical area, even in  
gadgets and spare parts kept indoors.

The indicator is on the porch, but there's a five-foot overhang  
above and beyond it. I also have a cheap Chinese 4-ft flourescent  
tube light much closer to the edge of the overhang and it's kept  
on throughout the night. Neither the tube nor the simple  
electronic choke is covered and are often subjected to spray and  
fog, but they're quite durable, the current set being at least  
five years old. The tube just needs to be rotated in the socket  
perhaps once a year. The choke doesn't use an X2 capacitor but I  
would have expected some deleterious effect from the weather if  
that's what's causing the caps in the indicator to fail so  

But if you're saying that X2 capacitors are particularly prone to  
such degradation, then I'll accept your experienced judgment.

               Roof edge
                           Tube light
         Verandah               |
            Mains indicator->  X|
                                |   House


Re: X2 AC capacitor failure

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 ** What was your first clue?

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** Causing corrosion where the lead is crimped to the metallised film.

Had a whole bunch of 100nF and 220nF 400V caps go open from the same  
ause  -  and they were subjected to only low DC voltage and kept indoors or  
inside equipment all the  time.

 I can't dismiss that
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** That is just how the ones above looked too.

You have a 3.3kohm resistor in series with your X caps so spike voltages on  
the AC supply are not causing any damage.

By their "two in one" design, class X1 and X2 caps are immune from internal  
corona so lead corrosion is the only possibility left.

....    Phil

Re: X2 AC capacitor failure
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 Modified sine wave UPS/inverters do play havoc on things like that  
however, normally if you have other things on the line the wave isn't so
bad but if that is all you have operating at the time I really don't  
think the X caps were intended for constant pulsing.  
 from my understanding of them, X caps are design to slowly degrade
by self healing when excessive pulses are seen but all they do is kind
of chip away at the metalized foil to remove the short. This makes the  
cap weaker over time until it no longer works.

 Difference manufactures may do their own thing but that is how I  
understand it.

 Have you thought about using a passive xformer or Common mode choke?
 Maybe even a Y cap going to common before the X cap may capture the  
fast raise.  

 Or you could try and use a mica cap which I am sure will handle the  


Re: X2 AC capacitor failure
Maynard A. Philbrook Jr. wrote:
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That would seem to be a logical explanation. I've tested the rest  
of the caps that were removed from the other project and they  
measure around 0.07-0.08 uF instead of the nominal 0.1uF. But why  
did they continue to degrade while being used in a simple sine  
wave circuit? As you can see, the present application even has a  
3.3k resistor in series to limit the worst-case inrush current.  
I'd love to understand the mechanism that continues to cause more  
and more of the metal film to vapourize.

The whole circuit is fitted into a standard 2-pin AC plug which  
is plugged into a 230V outlet on my verandah. If my house AC  
supply were repeatedly subjected to fast rising spikes and  
surges, I'm sure they would have had a noticeable effect on at  
least some of the other gadgets around the house.

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Well, I'm not using these caps as mains filters. And they're no  
longer run from a UPS/inverter. That was only while testing the  
other - and much more complex - project for a few hours.  

Re: X2 AC capacitor failure
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 If you can put a o-scope across the capacitor maybe you can see some
noise that should be there. Like high freq ripples caused from  
generators and things like switching devices on the same line.
 Better yet, if you have low level clamp on current probe for a scrope  
you can see this happening.


Re: X2 AC capacitor failure
Maynard A. Philbrook Jr. wrote:
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I dug up a batch of caps that had not previously been used in a  
square-wave circuit, all weighing in at about 0.11 uF and  
replaced the failing one today. That should settle the question  
once and for all. Now I just have to wait for a few weeks.  

Re: X2 AC capacitor failure
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A square wave inverer does not output a simple sine wave. It outputs  

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dv/dt. I am a bit surprised line rated caps would degrade so fast though.  
They are mass produced items so they are "just good enough" though, at  
least for plain sine wave use.

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there aren't too many items that just use a capacitor to drop voltage. The  
only thing I have that does that that I can think off off hand is a P3  
Kill-a-watt meter. See if you can blow one those up somehow.

Inductive loads do weird stuff with square waves, sometimes. Switching  
power supplies don't seem to care, but they tend to have lots of filering  
on the input anyways. Resisitve loads should not givea hoot as long as the  
RMS voltage you feed them is correct.

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Mica is good for abusive conditions. It's still interesting the other caps  
are getting destroyed so fast.

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a check with a scope on the porch might be good to see if you're getting  
some crazy voltages on your caps from the inductance of your wiring.

Re: X2 AC capacitor failure

"Cydrome Leader"
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** Huh??

Metallisation will only vaporise if there is an arc inside the cap and that  
series resistor really slugs the peak dv/dt compared to normal use.

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** Hang on a mo, class X1/2 caps are designed to be wired continuously  
across the AC supply with all its hazards.

The vast majority are polyester or polypropylene film types with a special  
wind that makes two in series with a floating electrode in the middle. They  
are tested at over 2kV DC and must not arc.

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** How about nearly every AC powered smoke alarm ?

....   Phil

Re: X2 AC capacitor failure

"Cydrome Leader"

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** There is no inverter  -  the LEDs are being powered from 230V AC on the  
OP's porch.

....   Phil

Re: X2 AC capacitor failure
Cydrome Leader wrote:

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I meant that they continued to degrade while being used with  
normal domestic AC supply - after they were used for a trial run  
with square wave in that other project.
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I don't mean just those with a series capacitor input. Surely a  
line that dirty would also affect other loads.

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Will do that when I have time.  

Re: X2 AC capacitor failure
Pimpom wrote:
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This is a full quote of the opening post of a thread I started  
about 2.5 months ago. Some people were convinced that the  
recurrent failures of the X2 caps must have been caused by  
environmental factors. Others were inclined to think that the  
caps had already started to degenerate from being run from square  
wave power while they were fitted in the previous project.

Well, I replaced the cap once more while the discussion was going  
on, but this time with a *new* one with a quickly measured value  
of about 0.11uF. Some 10 weeks later, it's still going strong and  
the LEDs are still glowing brightly. Those 10 weeks were among  
the wettest of the year (late monsoon season here) but the cap  
value has not changed significantly - still ~0.11uF.

If this were a crucial scientific experiment, I'd have to run the  
test with several other capacitor samples before reaching a  
conclusion. But I think it's fairly safe to say that the first  
two caps must have been damaged by being subjected to square wave  
earlier. What still puzzles me is why they continued to degrade  
further and further while being used in a sine wave circuit.  

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