X2 capacitor warning

[McMurdo]

I've just been reading the datasheet for an X2 capacitor. Part way down it says : suitable for across-the-mains use. Must not be used in series with a load.

Any idea why? Apart from risk of damaging the load due to short circuit, or causing voltage spikes due to becoming a resonant circuit?

[G8HQP Dave]

Wild guess: the self-healing does not work if the current is restricted too much.

[Refugee]

It must be because if they were used as a dropper they would blow the load up as well as blowing up themselves.
They could set the load on fire while they are burning up.

[Boater Sam]

Vindication! I have been saying for years that they are not suitable for capacitor dropper circuits.

[McMurdo]

Fair enough. But, apart from the usual RIFA paper capacitors, I've never seen an across-the-mains polypropylene capacitor go short circuit. Their capacitance normally ebbs away until they go open.

[Sideband]

Originally Posted by McMurdo

Fair enough. But, apart from the usual RIFA paper capacitors, I've never seen an across-the-mains polypropylene capacitor go short circuit. Their capacitance normally ebbs away until they go open.

That has been my experience as well. It's happened twice with the central heating timer where one is used with a 12V zener diode. So far I've replaced it twice (in about 7 years).

[Sideband]

Originally Posted by Boater Sam

Vindication! I have been saying for years that they are not suitable for capacitor dropper circuits.

...but why? Can anyone give a reason as to why X caps are not suitable?

[turretslug]

That's an intriguing one- many permanently wired low current/low voltage electronic/logic thingies like the central heating timers mentioned use X2 caps as wattless droppers, making for an inexpensive and compact package. I have also had to replace various ones over the years, though, with the capacity diminution syndrome. I wonder if this is a commonplace warning in the small print with X2 caps, or just something associated with this make/type?

There is an ever-more paranoid culture of CYA with many products now- perhaps the manufacturer is worried about a device malfunction due to capacity diminution ending up in the courts. It can exercise the grey matter when developing a product to cover all the possible permutations of malfunction where there is no control over what a third party does with it. Quite often, there's an element of "nod and wink" with small print, as in "we know that you'll use it for this purpose anyway, but this is our formal shirking of responsibility". (Cue subsequent legal head-to-head....)

[trickie_dickie]

Many years ago, 80s, I worked for a UK company that made control circuits for washroom hand dryers. We used an X2 capacitor in series with a 330 ohm as a dropper. To get UL approval we had to change the resistor. Under test conditions, the resistor splattered a metallic coating across the PCB which blew a 30amp fuse. We were gobsmacked. The UL guy wasn't. He'd seen it all before. The Capacitor was fine but we had to change the resistor.

[McMurdo]

Under test conditions, the resistor splattered a metallic coating across the PCB

I've never seen a resistor commit suicide like this, what did you have to do 'under test conditions' to make that happen?

Funnily enough the last X2 capacitor I had to change was on a hand dryer for the local cricket club gents. It had halved in value, the reported fault was the dryer was blipping the fan on and off every 2 seconds. I went straight for it, never bothered to try it before fitting the new capacitor. It was of chinese manufacture. .

I was just thinking about all the hundreds of x2 capacitors used as droppers in everything from fridges to outdoor PIRs.

[McMurdo]

The capacitor I was looking at with the warning was by Kemet. A similar note is on the Vishay product sheet. It says 'for series applications, see the application notes below'.

I have the link here but it is rather less than clear.

http://www.vishay.com/doc?28153

[Maarten]

I'm quite sure the restriction of series use to certain designs of X2 capacitors is because normal types will fail open after relatively short use because of the self healing action. Especially in environments with mains spikes and harmonics which result in overcurrent as the supply is usually clamped by a zener diode.

On a Dutch forum, we even coined a term for this phenomenon: "Senseosyndroom", translated in English: "Senseo syndrome". This is because certain series of those coffeemakers used a particularly unsuitable series capacitor, causing problems when it slowly lost its capacity.

One must not forget that X2 capacitors are by design rather weak. While they are allowed to fail shorted, in practice they mostly fail open and are constructed as cheap as possible to just barely meet the requirements. Think of a self sacrificial 700VDC MKT or MKP which isn't constructed to carry any kind of current structurally.

A good capacitive dropper in my opinion is a 1000VDC MKP type from a reputable brand. 630VDC will work, especially if it feeds a higher voltage circuit but might fail in the long run, 400VDC as sometimes seen in cheap stuff will certainly fail and it might fail shorted. Of course 1250/1600/2000V MKP types are usable as well, but a bit of overkill. Also, certain X2 types are specifically allowed by the manufacturers for series use and might be the best choice for non-hobby applications as they are proven to meet all safety requirements.

[Radio Wrangler]

An X capacitor across the mains may go shorted as the result of a high-voltage transient. 'It then relies on the current available from the mains to burn back the metalisation from the wound, clearing the short. As time goes by repeated zaps reduce the area of the capacitor and the capacitance is progressively reduced.

Use a class X capacitor as a series dropper and it will tend to pass the high frequency components of the transients straight to the load by normal capacitor action. If the load has a zener or transient protection diode etc, the X capacitor might get burned-through to a short. Then serious 50Hz current could flow and the zener or whatever could get taken out before the capacitor heals.

On top of that, the suits want to be sure their donkey/horse hybrids are covered.

David

[Argus25]

Originally Posted by McMurdo

I've just been reading the datasheet for an X2 capacitor. Part way down it says : suitable for across-the-mains use. Must not be used in series with a load.

Any idea why? Apart from risk of damaging the load due to short circuit, or causing voltage spikes due to becoming a resonant circuit?

I think I know why. Sure there is the risk any series capacitor could short, that scenario could be dealt with with the correct fusing.

The real issue (as noted on another thread) these capacitors are mainly (not always) constructed from a metalized film on paper. When a high voltage transient comes along, it destroys the film and the capacitance drops. The capacitor acts like a memory for the number of over-voltage events it has experienced. As this happens its uF capacity drops.

A while ago I set up three vintage 110V American synchronous clocks with an incandescent pilot lamp (which were mechanically modified to 50Hz operation)

Rather than dropping the voltage with a step down transformer, I used series 1.5uF X2 capacitors (the 1.5uF was about right to end up with 110V across the motor coil with 230V mains). About 6 months went by and the pilot lamp started to dim off and the clocks lost time. Examination showed that the 1.5uF caps had dropped to 0.8uF.

So I replaced the horrible X2 caps with 1500v rated poly caps, no trouble since.

[PJL]

This is my understanding.
Both X and Y capacitors are designed for suppression purposes and may not be suitable for general use. X capacitors are intended to be wired across the mains, and Y between mains and earth. Y capacitors must meet more stringent tests and should never fail short circuit.
I have no idea what the regulatory test is but it may be emulating the kind of energy involved in lightening strikes...

[G8HQP Dave]

Another wild guess: the regulatory test was 'check that nothing explodes when each component in turn is shorted or removed'. I seem to recall seeing this test described in the 1980s as a BS standard for domestic equipment like tellies.

[dseymo1]

Well something's going to 'explode' if you short out a Class X cap in its normal application

[G8HQP Dave]

In the test mentioned above it was the resistor which bore the brunt.

[Herald1360]

Originally Posted by dseymo1

Well something's going to 'explode' if you short out a Class X cap in its normal application

But to pass the test, the explosion simply has to be safely contained with no hazardous results. No requirement for the kit to work afterwards.

[Maarten]

Originally Posted by Argus25

The real issue (as noted on another thread) these capacitors are mainly (not always) constructed from a metalized film on paper. When a high voltage transient comes along, it destroys the film and the capacitance drops. The capacitor acts like a memory for the number of over-voltage events it has experienced. As this happens its uF capacity drops.

Fortunately most manufacturers dropped the paper long ago. Those who still use it, do so only in a single series (Wima MP3 for example), except for Kemet/Evox/Rifa with a few series and some smallish manufacturers of (custom) mains filters.

I do agree on the faillure mode, though. I also agree that polypropylene (not polyester) is the way to go.