X2 AC capacitor failure

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

0.1uF || LED 3.3k |----||---| |--->|----| ___ | || | | | ------|___|----| |---| |--- | ___ | | LED | | ---|___|--- ----|
Reply to
Pimpom
Loading thread data ...

"Pimpom"

** Those last two words are key here.

** Then these.

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

Nice and shiny or not ?

.... Phil

Reply to
Phil Allison

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 pounding.

Jamie

Reply to
Maynard A. Philbrook Jr.

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 quickly.

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 | | ____________________________|

House

Reply to
Pimpom

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.

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.

Reply to
Pimpom

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.

Jamie

Reply to
Maynard A. Philbrook Jr.

** What was your first clue?
** 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

** 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

Reply to
Phil Allison

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.

Reply to
Pimpom

A square wave inverer does not output a simple sine wave. It outputs garbage+noise.

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.

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.

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

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.

Reply to
Cydrome Leader

"Cydrome Leader"

** 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.

** 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.

** How about nearly every AC powered smoke alarm ?

.... Phil

Reply to
Phil Allison

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.

I don't mean just those with a series capacitor input. Surely a line that dirty would also affect other loads.

Will do that when I have time.

Reply to
Pimpom

"Cydrome Leader"

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

.... Phil

Reply to
Phil Allison

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.

Reply to
Pimpom

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.