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I have a couple of old power supplies in which Rifa brand metalized paper line filter capacitors (for 240V mains power) have failed - or may fail - like in this question here.

30+ years sounds pretty good for a capacitor, but researching replacements turns up lots of opinions about brands and references to different types and materials.

Similar metalized paper capacitors are still available (Evox Rifa is now Kemet), which though a bit pricey have the appeal of looking like the original part. One downside is their failure is messy!

Alternatively there are cheaper metalized film capacitors with polypropylene or polyester.

Is there evidence for a best replacement type considering performance and reliability?

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    \$\begingroup\$ I would not hesitate to use the modern equivalent (the poly... caps). Especially since they will be used for mains filtering. Then you want something good (and cheap). I guess the paper caps are only for "historically accurate" restoration. If you want something that just works, get the poly caps. Of course take care that they are of the right voltage rating and suitable for mains (240 VAC). \$\endgroup\$
    – Bimpelrekkie
    Jun 5, 2015 at 12:53
  • \$\begingroup\$ I recently replaced one of these capacitors in a speed controller for a small (sewing machine) motor, with a NOS (new, old-stock) RIFA. I replaced THAT a couple of days later for the same reason ... this time with a modern X2 cap. This time, it's working fine so far. \$\endgroup\$
    – user_1818839
    Jun 5, 2015 at 12:58
  • \$\begingroup\$ Possible answer here: electronics.stackexchange.com/q/85165/43191 \$\endgroup\$
    – EM Fields
    Jun 5, 2015 at 13:01
  • \$\begingroup\$ @Rimpelbekkie: Paper caps can sometimes exhibit microphonic behavior. If an amplifier and speaker are in the same cabinet, a paper cap in the audio path can add a mechanical feedback path which may alter the sound in a fashion unlike any other kind of cap. Someone trying to produce a "clean" amplifier would not want such behavior, but many guitarists like having their amplifiers color the sounds in interesting ways. If an guitarist likes the way microphonic effects color the sound of his amp, replacing the caps with modern ones would lose that coloration. \$\endgroup\$
    – supercat
    Jun 5, 2015 at 17:11
  • \$\begingroup\$ How large is the value of the cap? You could use ceramic caps as are used a lot in supplies now days because they have higher heat and life tolerances. \$\endgroup\$
    – Korozjin
    Jun 8, 2015 at 4:43

2 Answers 2

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As user EM Fields already suggested, I have already half-answered your question in the linked EE.SE post. Short answer is: get an X2 or Y2 capacitor (see below), these are almost always MKP (metallized polypropylene) film capacitors. Make sure you use a value close or equal to the capacitor it's replacing. Note that both paper and film resistors have very large tolerances (20% is fairly standard over operating conditions), so it doesn't matter if the value is exactly spot-on.

Your application is mains filtering, which is not a precision or high-linearity application. You do not need to worry about the exact type of capacitor. All mains-rated capacitors have similar performance, and performance is basically only dependent on their capacitance value.

What you do need to worry about is that the capacitor is safe to use. If the capacitor is connected between live and neutral, you need to use an X rated capacitor, I recommend X2 to be compliant with 99% of countries' electrical safety regulations. If the capacitor is connected between protective earth (PE) and live or neutral, use Y rated capacitors.

DO NOT use ceramic capacitors. They have very unfavourable failure modes that can cause fire or electric shock hazard.

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  • \$\begingroup\$ Thanks for the comprehensive answer. Do you know of any links/resources (even anecdotal) about expected lifetimes for MKP vs metalized paper? \$\endgroup\$
    – Nick Westgate
    Jun 14, 2015 at 10:31
  • \$\begingroup\$ @NickWestgate: Both types have lifetimes beyond anything I ever deal with. I am not aware of any differences. Most paper capacitor failures I've witnessed were because of mechanical damage causing water ingress, but hermetically sealed paper caps (as well as metallized film) can survive for decades. \$\endgroup\$
    – user36129
    Jun 14, 2015 at 10:36
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X and Y capacitors are subject to line spikes, and most will endure multiple small breakdowns over the years as a result of this stress. They fail open because local heating caused by a microscopic dielectric breakdown results in polypropylene melting, which seals off the "edge" of the breakdown.

The practical lifetime is determined more by the number of HV line spikes they absorb rather than by any aging phenomenon.

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