Safety Capacitor Replacement

[Laval]

Hello,

I need to replace two safety caps in a SMPS. They are X2 class Rifa and one of them was cracked and had started leaking brown goo. Looking at the schematics, it seems like these are more then just capacitors, they are shown to have an internal resistor of 22 ohms integrated. The picture shows the part of the schematics where they are used just before the rectifier bridge. They are designated by U3103 and U3093.

Looking at these capacitors (pictures below) on Mouser, it doesn't look like they have anything special. The writings on these caps are just certification logos, climate indication, voltage and capacity ratings as well as the series and tolerance. I know safety caps are required to have an internal parallel resistor to discharge fast to make sure the mains plug doesn't stay energized long after being disconnected. But I can't see anything about a series resistor. The one that is still good measures and ESR of 8.7 ohms and a parallel resistance of 46.5 k.

I guess my question is: if I replace them by safety caps with the same rating (capacitance, voltage and X2 class), do I have to solder a 22 ohms resistor in series with them ? If so, what type of resistor would be the best ?

[BrokenYugo]

They're designated with a note **, what does it say?

AFAIK the only caps with built in resistors (bleeder, not series) are microwave oven caps and I think some motor caps. Safety caps (X or Y) are just metalized capacitors designed to fail open in directly across the line (X) or line to ground/chassis (Y) applications, if you want/need a bleeder on one you add it in circuit.

[selcuk]

As far as I remember, Rifas with integrated series resistors have both capacitor and resistor values written on top of device packaging explicitly. They are for snubber usage. Yours look like safety X2s. They are different than schematics. Were they replaced before?

[Laval]

Thank you for answering.

The note says Safety part. See parts list. But the parts list doesn't give more information. For some components it gives a range of part numbers or it says things like the IC may not be installed on all models but nothing special for this one. One thing I find suspicious though, when looking at the board diagram, they don't look like what I have removed. The Rifa caps I have removed had been installed using soldered extended leads with sleeves, one on each side of the board. Maybe they have been replaced in the past, this unit is old (25-30 year old) and it looks like it has been serviced a few times. RC filter would not be wired like that either whether low pass or high pass.

You are right, Microwave oven caps have bleeder resistors but they are not series. I'm not sure what I should do, probably just replace them with other good quality safety caps I guess.

[Vovk_Z]

Pictured caps look like usual X2 caps but not more special snubber caps. (They don't have 22 R on them). And PME271 series seems to be a usual X2 class. Possiibly, somebody replaced them earlier, or it was even a manufacturer? 
Any X2 cap having its rated voltage 275VAC or more should do I guess.
(I gueass 275 VAC rated X2 caps are popular now instead of old 250 VAC ones).

[Laval]

Pictured caps look like usual X2 caps but not more special snubber caps. (They don't have 22 R on them). Possiibly, somebody replaced them earlier?

That's what I think looking at the schematics and the board diagram, it is clear they don't match. The unit was working fine with the Rifa though, even with one being blown. I guess I could just replace them with regular safety caps ...

Yes, all the ones I see are 275V which is fine of course. I think they may have been replaced too. I'll just install regular safety caps with proper ratings and see.

[floobydust]

Rifa PME271M are available today, Kemet bought them years ago. Between the epoxy cracking and the paper dielectric ballooning out, don't use them. I thought they had oddball lead spacings, so measure that.
They have no series resistor as your schematic shows - you can get those type of parts usually for arc suppression but they are crazy expensive. Example Cornell Dubilier Electronics (CDE) Quencharc 104M06QC22 $21.24 !

Consider Kemet R47 series X1 0.1uF 440VAC 15mm regular safety cap but the resistor does increase Q and prevent resonances occurring. For a suitable 22R resistor, it must be flameproof fusible type.

Does the scope have any?

[Laval]

Rifa PME271M are available today, Kemet bought them years ago. Between the epoxy cracking and the paper dielectric ballooning out, don't use them. I thought they had oddball lead spacings, so measure that.
They have no series resistor as your schematic shows - you can get those type of parts usually for arc suppression but they are crazy expensive. Example Cornell Dubilier Electronics (CDE) Quencharc 104M06QC22 $21.24 !

Consider Kemet R47 series X1 0.1uF 440VAC 15mm regular safety cap but the resistor does increase Q and prevent resonances occurring. For a suitable 22R resistor, it must be flameproof fusible type.

Does the scope have any?

No, it doesn't have melf on the board.

Yes, these capacitors are expensive. I guess the question is, are they absolutely necessary. I guess not since the ones in place are not but maybe it is worth to change them for the right type.

Thanks to everyone for all the info, it's very interesting and helped a lot.

[floobydust]

I would say the RC was the designer's intent but manufacturing or cost or conducted EMI concerns- kaiboshed that and they settled for just a cap.
If SMPS switching noise and harmonics are showing up, it's a really sensitive instrument I might go through the extra work to add them. 22R 1/4W fusible FP is enough for 120VAC mains I would say, 1/2W if they can fit. If you're a bit lazy then just replace the Rifa caps with vanilla X or Y-caps.

[Laval]

I would say the RC was the designer's intent but manufacturing or cost or conducted EMI concerns- kaiboshed that and they settled for just a cap.
If SMPS switching noise and harmonics are showing up, it's a really sensitive instrument I might go through the extra work to add them. 22R 1/4W fusible FP is enough for 120VAC mains I would say, 1/2W if they can fit. If you're a bit lazy then just replace the Rifa caps with vanilla X or Y-caps.

It's a 21 GHz spectrum analyser so yes, it is a sensitive instrument but it was working fine before so I'll try the standard X2 cap and see what happens. If there is a noise problem, I'll add resistor or get a pair of those CDE capacitors. That sounds like a reasonable course of action, Thanks.

[Laval]

I just wanted to add an update.

I reassembled the unit after tuning the switching frequency as indicated in the service manual. I then checked all the voltages and they were all either spot on or very close with very low ripple. The calibration of the instrument is spot on too. The noise floor is also lower then before. These oldies are impressive.

Thanks to everyone for the wisdom and thanks to those who make these old service manuals available. It was invaluable.