X2 capacitors

[Goldieoldie]

Hi
What is the best dielectric for a voltage drop x2 cap please ?
It’s a ch timer .22 uf and .033 uf 275 vac
Thanks
Pete

[Station X]

Class X2 capacitor used as a voltage dropper in a central heating timer?

[Goldieoldie]

Yes it’s the programmer

[Station X]

A Class X2 capacitor is a Class X2 capacitor full stop. The type of dielectric doesn't come into it.

[Goldieoldie]

I noticed some in the data sheet say not to be used as series device so I thought some dielectric was more suitable for that application

[turretslug]

Don't be tempted to use any "bargain" Rifa 271 (or similar paper types) though! Otherwise, there should be a good choice of types to provide one that will fit the PCB layout.

I'm guessing that the original has gone low value over time, and has caused the timer to malfunction- it's a widely-encountered syndrome.

[Nuvistor]

According to this article ceramic X types tend to fail short circuit, metallised paper and film tend to fail open circuit.
http://www.vicorpower.com/resource-l...e-y-capacitors

Interesting thought, how many X caps in service across live and neutral are O/C before complete failure and go S/C?

[Station X]

Does it matter which way a Class X capacitor fails? Failing short-circuit will activate any protective device or destroy the capacitor itself. Failing open circuit will result in no interference suppression.

I'm not clear as to whether the X2 caps in the central heating timer are actually interference suppression devices connected across the mains supply or whether they're being used for some other purpose, in which case Class X2 caps might not be needed. I'd be inclined to stick with what the manufacturer used though.

[turretslug]

In this application, there's a near-standard topology where they serve as wattless droppers, feeding a bridge rectifier followed by Zeners as shunt constant-current loads furnishing 24 or 48V for relays and 3.3 or 5V for the clever bit. EMI ablation steadily degrades the metallising, reducing the capacitance to a low value. Eventually, the higher-voltage Zener ducks out and insufficient voltage is left available to operate the relays, though the timer chip soldiers on until the mains capacitor is really low.

The 220 ohm composition resistor visible is likely to be used in conjunction with the blue MOV in the corner to reduce the hits on the series capacitor- an acknowledgement of the weak-point of this cheap, compact arrangement, the 1 meg is a safety discharge across the X capacitor. I've churned through quite a few of these as a favour for folk....

[Silicon]

I was examining a circuit board recently that has been in continuous use for about 26 years and still works OK.

The 24V relays are powered from the 240V mains by the circuit described in post 9.

It uses Philips HQ MKT-P X2 capacitors which have a combined metallized polyester and paper dielectric.
They are relatively bulky and expensive.

[radioman]

Certain X2 capacitors can be specified for use as series droppers, but you need to check with the manufacturer.
For example, this Epcos data mentions applications and suitable types.

[Maarten]

Originally Posted by Station X

A Class X2 capacitor is a Class X2 capacitor full stop. The type of dielectric doesn't come into it.

It does, though. Polyester will generally see more self healing action than polypropylene, while paper will get leaky over time. Ceramics are also pretty reliable for the lower capacity values.

For series use, a polypropylene (MKP) capacitor from any reputable brand is most suited, but preferrably one should be used that is indicated for series dropper usage in its datasheet (closer tolerance, less prone to deterioration).

I do agree with your later statement that an X2 might not actually be needed but the caveat is that if it is for series dropper usage, an equally or better suitable non-X2 type will be physically larger and might get you into trouble if anything happens since it doesn't have the required approval marks. So as you say, probably stick with the original solution.

[Maarten]

Originally Posted by Silicon

I was examining a circuit board recently that has been in continuous use for about 26 years and still works OK.

The 24V relays are powered from the 240V mains by the circuit described in post 9.

It uses Philips HQ MKT-P X2 capacitors which have a combined metallized polyester and paper dielectric.
They are relatively bulky and expensive.

Successors of the infamous booster caps that used to fail open in TV sets... And very likely not manufactured anymore since Vishay did phase out overlapping and obsolete ranges. I didn't know they still made them 26 years ago. HQ is likely not the series or a quality indicator but rather is the factory designator for Roeselare, Belgium though that factory generally made high quality capacitors so it's not a lie either way. I'd be interested in which series, which is usually indicated by a 3 digit number (or the full 12NC). Axial 276/278 or some newer radial type? EDIT: found them, the 330 series I assume. Never knew it existed.

That said, continuous use (in a dry environment with a stable temperature preferrably) is the most favourable scenario for paper capacitors and the hybrid dielectric and general manufacturing quality does help somewhat but if I encounter such a situation I would give 'preventive maintenance' a good hard thought.

[Goldieoldie]

Thanks everybody for your help
After looking at various caps on Rs website I cannot find anything which is suitable .
I have decided to go with Iskra as they make caps specifically for series Mains droppers .
I found a supplier on eBay who stocks them ( good feedback ) for those who are also looking for this cap.
I found a lot of caps say in their specs not to be used as series droppers .I do not know why that is ?
Cheers
Pete

[Radio Wrangler]

A class X2 capacitor is intended as an RF filter, connected directly across the mains. It is expected to get the full brunto of mains transients which will puncture its dielectric from time to time. It performs self-healing by melting its foils back from the puncture by means of a small arc for a short time. It relies on there being enough current to perform this melt-back.

Put one of these capacitors in as a series dropper element and you open the door to trouble. Whatever it is doing the dropping for won't pass the current necessary for the melt-back, so transient induced shorts don't get cleared. Boom!

For a dropper you need a capacitor which survives the transients intact. That means a very high rated voltage and a physically large capacitor as it has to have a thick dielectric foil. Bulky and expensive as Silicon describes above.

David

[govjohn]

Thank you Radio Wrangler that's the best succinct reply I've seen about dropper capacitor dielectrics. I shall remember it.
John.

[Maarten]

Originally Posted by Radio Wrangler

A class X2 capacitor is intended as an RF filter, connected directly across the mains. It is expected to get the full brunto of mains transients which will puncture its dielectric from time to time. It performs self-healing by melting its foils back from the puncture by means of a small arc for a short time. It relies on there being enough current to perform this melt-back.

Put one of these capacitors in as a series dropper element and you open the door to trouble. Whatever it is doing the dropping for won't pass the current necessary for the melt-back, so transient induced shorts don't get cleared. Boom!

For a dropper you need a capacitor which survives the transients intact. That means a very high rated voltage and a physically large capacitor as it has to have a thick dielectric foil. Bulky and expensive as Silicon describes above.

David

In practice, the self healing seems to work perfectly well, and that's the problem. The usual failure mode for a non-series rated X2 capacitor is gradual loss of capacity to the point it doesn't pass enough current anymore. The term "Senseo Syndrome" was re-coined for that, since that is probably the most popular appliance failing in this way.

The X2 capacitors that are approved for series usage, are indeed constructed more like an impulse capacitor. I think Vishay/BC uses an internal series connection to increase the survival chance on transients.

[Radio Wrangler]

With progressive punctures over time, the melt-back reduces the area of the capacitor still in play and so the capacitance decreases.

Jokingly, you could measure the reduction of their C as a sort of dosimeter showing how bad your mains has been and for how long

They are a very specialised part, and not much good outside of their intended application.

Capacitive droppers are frequency selective and are far better at passing the high frequency components of the mains than they are at passing the 50Hz part. So whatever they are powering needs to have a pretty robust shunt element to absorb transients.

David

David