RIFA caps?

[Doomguy42]

Hi, I got given some slot machine Psu's I'm not yet familiar with but will be using soon. I got told the RIFA's had been removed and one has an EM filter fitted in line.
I'm going to open up these supplies for a look and test the outputs but I'm not really sure about these Rifa caps. I guess I'm more inclined to just fit new ones?
What makes the best job? Can I use New RIFA's and an EM filter together? Is there any point in that?
Thanks

 

[augustinetez]

If it has a proper EMI filter fitted, then there is no need to fit a replacement for the Rifa, it will be in the filter already.

 

[shortbus=]

What type were the RIFAs? RIFA was/is just a manufacturer/brand, not a special type of cap, they made many different types. https://www.mouser.com/manufacturer/evoxrifa/

 

[augustinetez]

Any vintage Rifa cap should be replaced on sight, they are well known for exploding - and yes it is a particular brand of cap (more often fitted across the incoming mains).

 

[tomizett]

They'll want to be Polypropylene film caps. If they're going across the mans then they should be "X" rated and if they're connecting to ground then they need to be "Y" rated. Caps for this kind of application are more rugged than general purpose caps rated to the same voltage.

 

[shortbus=]

Any vintage Rifa cap should be replaced on sight, they are well known for exploding - and yes it is a particular brand of cap (more often fitted across the incoming mains).

Can you give the type that "explode" please or a link to them exploding?

Rifa isn't/wasn't the only one making "X" or "Y" caps. Those two types are the only ones I'm aware of that are used like that, or should be used for that. They are both usually a box style multi layer poly/mylar film cap, so how would they be prone to explode. Not trying to be difficult here, just wondering if I've made a mistake in a project I've been designing for many years. I bought a bunch of RIFA MPK type caps for a cap bank. And this is the first time I've seen any mention of them "exploding".

 

[AnalogKid]

These were/are very common capacitors in power line filters. Most of what I've seen and used are 0.0047 uF (4.7 nF) in a "Y" application, from line to GND an neutral to GND. Recommended by name by Vicor for their high density DC/DC converters. It is a metalized film capacitor in a plastic box. Back when switchers were ascending, along with their greater high-frequency noise, these had better than average high-frequency performance while meeting a ton of safety regs, making them the go-to part.

With this value, at 240 Vac the current through four of them in parallel (as in a balanced pi filter) is less than 3.5 mA, the max allowed GND current in UL950 / 1950 / 60950 / ...

ak

 

[tomizett]

When they fail explosively, the case might typically burst open sideways exposing layers of charred foil and plastic, emitting a loud pop and flash. The pop was often enough to cause the user to unplug and bin the appliance immediately, assuming it to be beyond repair.
So it's not as violent as an electrolytic spewing boiling electrolyte and paper, or launching its can like a bullet.

It's a curious failure, as this type of cap (modern ones, at least) are supposed to "self heal" and fail safe. If a hole in the dielectric develops then the surrounding metalisation is vapourised, clearing the short. Thus they can loose value over time, but should never fail short - that's the point of the X and Y rating.

The concerning thing is that, in order to explode, the cap must have passed significant current - and if it's in a "Y "location, to ground, then that current could have passed though a user. So I would err on the side of the caution.

 

[augustinetez]

Can you give the type that "explode" please or a link to them exploding?

If you do a 'net search on 'exploding Rifa cap', you will find a lot of info on various forums like EEVblog, DIYaudio etc.

Then do an image search with just the term 'Rifa cap', will get you plenty of images of original and expired caps - note these things are quite old now and no longer available as new stock from any reputable supplier.

They do make something of a bang and smell when they fail but the equipment usually continues working.

As for failure mode, I think that is down to moisture absorption.