X2 Capacitors - loss of capacity

[G4XWDJim]

Originally Posted by ukcol

I had the same problem with a door chime [/URL]

Thanks for the tip. My door chime has been sounding like wow on a record. I checked the capacitor dropper which should have been 560 nF and it had dropped to 105 nF. I replaced it with a pair of 220 nF strapped together and it sounds good again and considerably louder.

The original cap was a yellow 'SUNTAN'. The new ones were significantly bigger despite being electrically of smaller value. I wonder if I can interpret that to mean the new ones will last longer. Wishful thinking I know.

The door chime appeared to have no fuse in it. It was wired straight across the Live and Neutral pins on the plug.

For now I've plugged it into a fused multiplug on its own

Jim

[G8HQP Dave]

I don't remember seeing a fuse in mine either, but I think there was a small resistor in series. Hopefully this will act as an unofficial fuse. Probably intended to limit the brief spike current.

[G4_Pete]

Hi,
Just to add further info to this thread.

The attached photoes are from a failed X2 capacitor from a mains to low voltage converter embeded in a comercial system. ( Good quality cap manufacturer not cheap unknown ) Being curious as to why an X2 capacitor marked 0.68uF was reading 0.32uF I disassembled it and found the film damaged with what appeared to be tracking across the film.

The rating is 275v ac but obviously something spikes it.

Magnification X 200 setting on a simple Dyno-Lite USB magnifyer.
Pete

[sp10mk11]

I hope this is not too far of topic.

If I use X/Y capacitors across mains switches they do there job and cut the spike, but if the circuit has a power on LED it stays illuminated albeit slightly dimmer, I tried putting the same value caps in with out a built in resistor and sure enough the LED goes off, but the spike returns.

I was looking at a Delta cap with varistor across the mains wondering if this would cure the spike and yet not allow a trickle voltage through so it lights the LED.

Gary

[G8HQP Dave]

Putting an X cap across a switch allows some current through, as you have found. Put the X cap on the output side of the switch instead (or from live out to neutral if it is a single pole switch). This has the same effect in suppressing arcs, but also means that Off means Off and the cap lasts longer as it only faces mains voltage when switched on.

[sp10mk11]

Thanks for that Dave, this is a double pole switch, so single X cap across the live and neutral on the output of the switch is what you mean. sorry about the bad drawing

Gary

[G8HQP Dave]

Yes.

[sp10mk11]

G8HQP Dave
Thanks for that works a treat

Gary

[Radio Wrangler]

The loss of metallisation is deliberate.

Many moons ago Rifa sent some of their technical peole round to HP to give a run through of their capacitor products (They were part of Ericsson, a large customer of ours at the time)

Their problem was that mains-borne transients were as ood as limitless and any dielectric which could pack enough C into an acceptable size would experience occasional flash-through. What the had to do was to let the foil melt back away from the point of failure so that the capacitr did not fail short circuit. Extreme dv/dt would cause extreme surge currents and could also cause melting.

So these capacitors progressively decrease in capacitance as they keep experiencing line transients from switching, lightning etc.

Me? I think networks ought to be plastered with surge arrestors sufficient to do the job.

THe worst RFI generators seem to have suppression components not fittedand bridged-out, so I don't suppose it matters what sort of capacitor would have been fitted but wasn't

David

[G8HQP Dave]

Originally Posted by Radio Wrangler

THe worst RFI generators seem to have suppression components not fittedand bridged-out, so I don't suppose it matters what sort of capacitor would have been fitted but wasn't

It matters a great deal. It is the quality of the unfitted capacitor which gains the "China Export" label for the back panel. Otherwise we might be forced to conclude that the item was non-compliant even at the design stage.

[Leon Crampin]

I remember giving the Rifa rep a real grilling over the miserable performance his devices gave when subjected to my thermal cycling tests. The ability of the dielectric to survive a foil melt-through was not especially critical to my intended application, but both sealing integrity and the ability of the foil terminations to withstand rated lead-pull and thermal cycling (simultaneously) most certainly were.

He was a nice man, but I'm not convinced he understood the significance of the tests I had performed which revealed, in my view a fatally flawed design. I'm also less than convinced that my findings got back to base. I think the passing years have proved my point regarding the utility of these capacitors.

Philips and NEC got my business - no problems with either. The hard epoxy coating used by Rifa killed them in my tests.

Leon.

[Humber888]

I've been reading this thread with interest. It seems most of the discussion has centred on plastic film capacitors and their loss of capacity. So, what about Class X and Y disc ceramics? I realise these will only likely be available in the smaller values used for mains suppression, but do these lose capacity in the same way?

Mike

[kalee20]

I avoid hard epoxy Rifas like the plague. The plastic boxed ones seem OK though.

Ceramics? I have no hard experience of X and Y ceramic capacitors. I do remember a Rifa rep steering me away from ceramics (well he would, wouldn't he) because to get the capacitance and voltage in a small size, high-K dielectric ceramic is used which has crummy tolerance and tempco (it does). But how good is reliability? No idea.

[Herald1360]

How much does crummy tolerance and tempco actually matter for a capacitor to swallow noise and spikes?

[kalee20]

Well if capacitance drops by 50% then it can make the difference betwen not causing interference, and causing it. And that's not at all unrealistic! I have been the victim of Z5U dielectric capacitors dropping out when the temperature rose from 25 to 60 deg C.