Replacing RIFA X/Y MP capacitors with ceramic

[maekawa]

I'm going to remove the RIFA X/Y capacitor from my friend's Mac Plus, but like the Murata DE2, today's X/Y capacitor has become an immortal material through the transition from metallized paper to film. So I'd like to purchase ceramic X/Y capacitors or utilize capacitors scavenged from wasted ATX power supplies. What is the proper capacitance derating when replacing X/Y capacitors with ceramics?

 

[68kPlus]

You can just buy RIFA Y2 caps from anywhere which work perfectly and should work for another 30 years easily.
The ones I put in my Plus:

https://au.element14.com/kemet/pme271y447mr04/cap-4700pf-20-paper-radial/dp/2332435?ICID=I-HP-STM7REC-RP-3

 

[maekawa]

I know it is possible to purchase new MP capacitors, but since security capacitors are not filters, there is no electrical meaning to inheriting the material.

Besides the high dielectric absorption of metallized paper, I don't think it can withstand surge voltages when degraded.

 

[stepleton]

I'm confused by the request here. The thing to do is to replace X and Y line filter capacitors with X and Y line filter capacitors of the same capacitance and equivalent or greater voltage rating. Except for "looks", which is a fairly dubious justification to my mind, there's no reason I can think of why a scavenged, older safety capacitor might be preferable to a modern part, no matter how the technology inside differs.

What you must not do is replace an X-designated capacitor with a part that isn't X-designated, or a Y-designated capacitor with a part that isn't Y-designated. If you do this, you might risk damage to your friend's computer --- or to your friend.

If your friend's power supply is prone to surges, it probably doesn't make a lot of sense to address this by trying to improve the insides of their appliances? These safety capacitors are not really intended to protect against or withstand power surges. Use a well-made surge-suppressing power strip instead.

 

[cheesestraws]

Agree with @stepleton here. Safety capacitors are designed to have safe failure modes, which is extremely important where those capacitors are in the circuit. If you replace them with non-safety capacitors, you risk killing someone. To say this is not melodrama.

 

[maekawa]

I'm not talking about replacing non-X1/Y2 capacitors.
I'm considering switching to Murata DE2 SA.
DE2 SA is the X1/Y2 certified safety ceramic capacitor!

However, the capacitance of DE2 SA seems to be significantly different from that of MP/film capacitors.

 

[cheesestraws]

Oh, sorry, I misunderstood you.

I've never tried ceramic X/Y-class myself, though if the capacitance was the same, I probably would give them a go. If not as I think you're suggesting, then I'd probably be more inclined to go with a polypropylene or similar X/Y-class.

 

[maekawa]

Since the modern hardware may use ceramic safety capacitor and may not use film capacitors, I think ceramic mods are possible on the Mac as well.

There is no 0.1uF in the DE2 SA lineup. Also, ceramic capacitors tend to change their capacitance when AC voltage is applied.
I think this requires some kind of frequency simulation.

 

[AndyO]

I'm confused - mostly due to my own ignorance, but also that as far as I know, it isn't the material of the component that matters to the correct functioning of the circuit it is in, but the specification of the component. It would seem to me that if the ceramic components have different ratings, they are not likely to be a good choice of replacement.

As I said, I'm ignorant so I may be wrong, but I am puzzled as to why you would want to switch capacitor type in this use case.

 

[maekawa]

This is because I thought the leakage current would not easily increase with ceramic capacitors.
Due to the high temperature and humidity in Japan, film capacitors are not an eternal component.

 

[cheesestraws]

Also, ceramic capacitors tend to change their capacitance when AC voltage is applied.

I don't know anything about the dielectric in these, but most ceramic caps also seem to be rather more sensitive to temperature and other environmental difficulties than other capacitors. So I'd be perhaps a bit wary, especially when you know you're installing them somewhere where they won't be at room temperature.

Since the modern hardware may use ceramic safety capacitor and may not use film capacitors, I think ceramic mods are possible on the Mac as well.

I think you are probably right that it is possible. I don't think anyone has simulated or tried it yet, though. But that doesn't mean it's necessarily a bad idea.

Due to the high temperature and humidity in Japan, film capacitors are not an eternal component.

Now I understand why you're thinking about this

 

[stepleton]

I think that if a particular product line doesn't have a part in the right specifications, it will unfortunately be necessary to look to a different product line for the part you need. That may mean that a particular capacitor technology is not available to you. Particularly for a piece of safety equipment, it's probably best to duplicate the manufacturer's design as far as capacitance is concerned. You may need to use a less-durable part.

 

[AndyO]

This is because I thought the leakage current would not easily increase with ceramic capacitors.
Due to the high temperature and humidity in Japan, film capacitors are not an eternal component.

That's very helpful, thanks!

I wonder if anyone has done a study of the degradation rates of these components in adverse climates.

 

[AwkwardPotato]

As a matter of principle, you should be fine replacing film safety caps with ceramic safety ones. Quick search on Mouser says that the Murata DE2 series includes the 2200/4700pF typically used for the Y caps on these analog boards, so you should be good there. Their stability with regards to temperature appears to be roughly in line with the original RIFAs, and derating shouldn't be necessary.

As far as the 0.1uF part goes, you're pretty much stuck with a film safety capacitor there. ISTR that my Mac Plus came with a red film cap there rather than a RIFA (polypropylene? same appearance as the many other red ones on that board, but presumably X2 rated).