Some really burned RIFA capacitors, but not X class - Page 1
Forum section: electronics / general

author: cybermaus
author_rank: Frequent Contributor
timestamp: October 07, 2021, 06:45:44 pm
content: So my brother trades in old machinery, and he recently got two Bridgeport CNC routers from ' 89Pretty early for CNC machines in fact. They had been in use until very recently at a university, where he picked them up himself.Firing the first one up, after 20 minutes of fiddling with its mechanics, it released the magic smoke. And *lots* of it.Being in an enclosure, it took some time to detect, and some more to respond and douse the fire, see for yourself.Checking the 2nd machine, with a IR camera at the ready pointing at the same components, indeed, the capacitor themselves very quickly shot to 50C, and crawled to 80C in less then a minute. Normally not a really temp, but it was too quick, and nothing else was even getting warm yet.Of course, it is always the question when finding a burned component: was this the cause, or a consequence.Well, giventhis videoandthis thread, and the fact nothing else got hot, we think/hope this is the cause. They will be replaced for sure, all 12 of them.Slightly worry however is that I measured the non-burned ones, and they measure a nice, exactly 0.1uF and 0mA leakage at 360V (that is as high as my leakage test setup goes)So they measures very well. Except for the ESR, that seems a bit high to me, but I am unsure what is normal for a film capacitor like this.So question: what would be a normal ESR value for a 80'ies RIFA 0.1uF non-X class film capacitor? Or any such film cap from that era?I did check some equivalent modern ones, and they are lower, but I am not sure if I should compare 2010 devices to 1980 devices.Thanks

author: Gyro
author_rank: Super Contributor
timestamp: October 07, 2021, 08:06:54 pm
content: The failure mode of Rifa MP (MetallisedPaper) capacitors is moisture ingress due to the epoxy package cracking. As you would expect, this is most likely to occur after a period of storage. If there is visible cracking then you have no choice but to replace them as a matter of urgency.With values as low as 100nF, it isn't really a matter of measuring ESR, but Q and dissipation factor [Edit: damp dielectric is likely to cause increased dissipation I suspect]. In this case, it looks as if the capacitors are being used in RC snubbers across the SCRs, where the resistor should swamp such factors. Metallised paper caps have a slightly higher dV/dt withstand capability than metallized film, but again it shouldn't matter in this case.Either way, you're probably going to want to replace the caps with MKP Polypropylene film - although you do also have the option of using newly manufactured Rifa MPs which will probably be reliable for a few more decades of active use.

author: eti
author_rank: Super Contributor
timestamp: October 08, 2021, 12:21:45 am
content: I see a smoking RIFA 😄 but as opposed to a reefer, one doesn’t get a choice, and it’s potentially far from worse for one’s health.

author: cybermaus
author_rank: Frequent Contributor
timestamp: October 08, 2021, 06:29:01 am
content: Quote from: Gyro on October 07, 2021, 08:06:54 pmcorrect.Quote from: Gyro on October 07, 2021, 08:06:54 pmOk, got some studying to do, those terms are new to me (in the context of capacitors)My reasoning of suspecting the ESR is/was (other then they measure a little high, but really not extremely so):To get hot, there must be energy dissipated, and a high resistance in combination with current constantly flowing in- and out of the capacitor (either AC or spikes) gives a simple I2RAnyway, obviously I already have polyfilm's on order (I only had 450V rated ones lying about)Edit:relevant link

author: richard.cs
author_rank: Super Contributor
timestamp: October 08, 2021, 05:46:49 pm
content: Quote from: cybermaus on October 08, 2021, 06:29:01 amThese normally fail the other way - the leakage resistance inparallelwith the capacitance causes enough current to flow for heating. The ones that measure no-leakage are probably OK for now, but will inevitably fail the same way at some point, this type are infamous for it.

author: cbutlera
author_rank: Regular Contributor
timestamp: October 08, 2021, 07:57:43 pm
content: Quote from: richard.cs on October 08, 2021, 05:46:49 pmI tested a number of old and newish RIFAs a few years back. All of them, both new and old, had a leakage resistance that approximately halved for each 3.5 to 4oC rise in temperature. The newish ones had a room temperature leakage resistance well over 1G ohms, so this reduction with temperature is inconsequential. At the other extreme, anything below about 10M ohms at room temperature is primed to explode due to thermal runaway. This won't happen immediately when power is applied, but once the self heating feedback loop gets going there can only be one outcome.Depending on the room temperature leakage resistance, they have what is effectively a critical temperature. Go beyond this temperature, and thermal runaway starts. The initial temperature rise often just comes from the equipment warming up.I wouldn't continue to use any RIFA if the room temperature leakage is less than about 100M ohms.