Potting an electrolytic capacitor will make it  last longer?  - Page 1
Forum section: electronics / beginners

author: ocset
author_rank: Super Contributor
timestamp: August 25, 2018, 05:30:43 pm
content: Hello,The following Philips Xitanium LED driver has a big “wet” electrolytic capacitor in it……and several smaller ones…..but they are all totally surrounded in potting compound. Do you believe that this will make them last longer than if they were not buried in potting compound?Philips Xitanium LED driver (75W)…http://www.docs.lighting.philips.com/en_gb/oem/download/xitanium/Xi_LP_75W_0.2-0.7A_S1_230V_S240_sXt_929000963206.pdf

author: Zero999
author_rank: Super Contributor
timestamp: August 25, 2018, 05:46:33 pm
content: I don't see how it will make it last longer, more like the reverse if the potting compound reduces the capacitor's ability to dissipate heat. It's also a potential safety issue. If the capacitor fails, the pressure will build up to a higher level, than it would do if the vent were allowed to pop open and vent the gas.

author: ocset
author_rank: Super Contributor
timestamp: August 25, 2018, 06:21:10 pm
content: Thanks, but i assume that in a totally enclosed enclosure, the potting compound would always make it easier for heat to get out of the capacitor?Yes i did think of the need-to-vent too....but philips potted over the can top......probably only about 5mm of potting covering the can top though....so could still explode out.....presumably no 'lytic explosion would be enough to fracture the 1.5mm thinck plastic enclosure

author: Zero999
author_rank: Super Contributor
timestamp: August 25, 2018, 06:27:29 pm
content: Potting stops thermal radiation and convection, leaving only conduction, which will be dependant on the compound, so I would expect it would act as a better thermal insulator, than free air.As far as explosion is concerned, I think it will depend on the hardness of the potting compound, with the softer, more elastic compounds being safer, than the harder more rigid ones.

author: voltsandjolts
author_rank: Unknown
timestamp: August 25, 2018, 06:59:35 pm
content: There are thermally conductive potting compounds such as Dow Corning TC6020 with conductivity of 2.72 W/mK, whereas RTV3120 is around 0.001 W/mK.Still air is around 0.05 W/mKSo, potting could make thermal issues worse or better. It depends.

author: Zero999
author_rank: Super Contributor
timestamp: August 25, 2018, 07:41:32 pm
content: Quote from: voltsandjolts on August 25, 2018, 06:59:35 pmYes there are some relatively high thermal conductivity compounds available, but how do they compare to convection and radiation through air? I know for power components, it's definitely better to use a heat sink, than relying on any potting compound, but I wouldn't know about something like a capacitor, which shouldn't get that hot.Improved heat transfer between the capacitor and other components might also be a bad thing, if there are other high power components on the board, which will heat the capacitor up more.

author: mariush
author_rank: Super Contributor
timestamp: August 25, 2018, 07:59:48 pm
content: I think those are potted more for better reliability, to prevent humidity and vibrations, to make them better for outside use... see the notes in datasheet ..Application•Road and street lighting•Area lighting•Tunnel lighting•Industrial lightingThey're also designed more or less to be quickly and easily replaced with spare parts so they don't care if you can or can not desolder a capacitor, or that the capacitor blows after 1-2 years of operation... it's probably considered low enough failure percentage and potting them is probably cheaper than conformal coating and water proofing the case and everything else.\

author: David Hess
author_rank: Super Contributor
timestamp: August 25, 2018, 09:36:41 pm
content: Quote from: Hero999 on August 25, 2018, 07:41:32 pmRadiation is insignificant compared to convection and conduction unless the temperature is much higher than you will want to operate the capacitor so it can be ignored.

author: ocset
author_rank: Super Contributor
timestamp: August 25, 2018, 09:48:15 pm
content: thanks, i thought that the potting compound can help conduct the heat from the components to the inside of the enclosure, where it can then conduct through the enclosure wall, and then convection currents can move the heat away from the outer surface of the enclosure (even if its a plastic enclosure).....in this way, and in conjunction with a potting compound like the 2.73W/mk mentioned kindly above by voltsandjolts, a potted power supply would be more thermally optimised than a non potted one?

author: David Hess
author_rank: Super Contributor
timestamp: August 25, 2018, 10:31:55 pm
content: Yes, using a high thermal conductivity potting compound would be better than air. Sometimes they use thermal pads between the parts and enclosure or even heat pipes.

author: Kirkhaan
author_rank: Frequent Contributor
timestamp: September 03, 2018, 06:00:40 pm
content: Quote from: Hero999 on August 25, 2018, 06:27:29 pmIndeed, and when applying the more harder/rigid potting compound, often special rubber caps are placed on top of the ELCAP, leaving some room for the pressure relief valve to open and electrolytic to get out. This way no excess pressure is build up and the ELCAP can die in a controlled (and safe) way...

author: Wolfram
author_rank: Frequent Contributor
timestamp: September 04, 2018, 03:31:07 pm
content: Quote from: voltsandjolts on August 25, 2018, 06:59:35 pmThis sounds unrealistically low. The only number I found in this range were in this datasheet:http://media.erapol.com.au/wp-content/uploads/files/products/silicone/condensation/Dow%20Corning%20Silastic%203110.pdf, but given in the units of calories per second centimeter celcius, corresponding to 0.24 W/mK. Other sources place RTV silicone in this ballpark as well (around room temperature):https://www.sciencedirect.com/science/article/abs/pii/S001122759700146X

author: Kjelt
author_rank: Super Contributor
timestamp: September 04, 2018, 03:46:28 pm
content: They are potted because they are outdoor drivers.Besides the driver has a thermal protection, an ntc measured by the microcontroller.Most metal indoor drivers from Philips have a thermal spot printed on the housing, that is where the temp is measured.Normally potting would distribute the heat over the device.

author: In Vacuo Veritas
author_rank: Frequent Contributor
timestamp: September 04, 2018, 04:46:52 pm
content: Quote from: treez on August 25, 2018, 06:21:10 pmAre you really an engineer? Which breakfast cereal did your degree come with?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 04, 2018, 04:56:46 pm
content: Potting probably reduces the diffusion of electrolyte, but I wouldn't count on it to help significantly, you're stopping rubber with rubber.Also, that electrolyte has to go somewhere. Most of it will remain diffused inside the potting, but some of it may condense in pores and interfaces in and around the potting, which could be an electrolysis/corrosion hazard just the same as any leaky cap is (but more limited, to such locations).Conductivity: air sucks, particularly when the device is compact and there is little room for airflow within the enclosure. Typical figure is 0.045 W/mK, but this is going to be a hand-wave because heat diffuses rather than conducts, and some convection always occurs.It can be much higher where convection (chimney effect) takes place, but this requires more room, or more temperature differential.Speaking of, temperature differential. Convection only requires that, and gravity. Putting some components inside a closed metal box incurs two convection steps, so the peak internal temp rise is almost double (very roughly speaking!), compared to the case for an open or well-ventilated enclosure.A fan can be desirable inside a sealed enclosure, for this reason. Better is just to design it so everything sinks heat to the enclosure directly.These may be of interest,http://www.cde.com/resources/catalogs/MLP.pdfbut good luck budgeting for itTim

author: floobydust
author_rank: Super Contributor
timestamp: September 04, 2018, 05:12:41 pm
content: Old Sangamo electrolytics had a hard epoxy seal.I see they crack with age or due to some pressure building up within the capacitor.I've never seen another cap manufacturer do this.edit: These are from the 1980's when there were no safety vents on electrolytic capacitors.

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2018, 05:23:42 pm
content: The inverter style backup generator I have has the entire inverter board potted to half way up several large snap-in aluminum electrolytic capacitors but with their tops are exposed, presumably they would rupture safely there.

author: Gyro
author_rank: Super Contributor
timestamp: September 04, 2018, 05:59:45 pm
content: Quote from: In Vacuo Veritas on September 04, 2018, 04:46:52 pmtreez may have his faults but your stupid sarky one-line comments don't help anyone. If you're going to make such comments, then FFS include some useful information!

author: In Vacuo Veritas
author_rank: Frequent Contributor
timestamp: September 04, 2018, 06:23:34 pm
content: Quote from: Gyro on September 04, 2018, 05:59:45 pmThe only faults he has are in his circuits. How this man-child got close to electricity is a mystery.OK, here's some useful information:Oh, and I believe the word is "snarky".

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 04, 2018, 06:30:36 pm
content: Another one,https://www.vishay.com/docs/42055/678d.pdf

author: Gyro
author_rank: Super Contributor
timestamp: September 04, 2018, 06:38:04 pm
content: Quote from: In Vacuo Veritas on September 04, 2018, 06:23:34 pmNo it's sarky, short for sarcastic in the UK... and I still see no useful contribution.

author: iainwhite
author_rank: Unknown
timestamp: September 04, 2018, 06:38:59 pm
content: Quote from: In Vacuo Veritas on September 04, 2018, 06:23:34 pmSarky is a British-English slang word for "sarcastic"

author: In Vacuo Veritas
author_rank: Frequent Contributor
timestamp: September 04, 2018, 06:39:53 pm
content: Quote from: iainwhite on September 04, 2018, 06:38:59 pmWell it's what we called Nicolas Sarkozy.

author: In Vacuo Veritas
author_rank: Frequent Contributor
timestamp: September 04, 2018, 06:42:03 pm
content: Quote from: Gyro on September 04, 2018, 06:38:04 pmI disagree, I provided useful nutritional information. Treez is a bit sugared-up and needs to cut down on the carbs I think.

author: ogden
author_rank: Super Contributor
timestamp: September 04, 2018, 07:45:46 pm
content: Quote from: Gyro on September 04, 2018, 07:33:31 pmRight. Potting electronics in high TC epoxy is good stuff unless you have forced air flow option.

author: Simon
author_rank: Unknown
timestamp: September 04, 2018, 08:01:04 pm
content: Quote from: In Vacuo Veritas on September 04, 2018, 04:46:52 pmCalm down, it's a valid question. Perhaps you can enlighten us all as judging from your reply you are quite the expert but have neglected to share such expertise that allows you to be so dismissive of the question.Edit, having seen the rest of your sarky posts i have decided to give you a few days to reflect as well as calm down!

author: ocset
author_rank: Super Contributor
timestamp: September 05, 2018, 05:14:52 am
content: QuoteThanks, i thought the same......but there are plenty of non potted outdoor drivers.I assume potting prevents sulfur laden atmospheres from getting round the components and corroding them?Also, the potting stops moisture from getting round the components.

author: Kirkhaan
author_rank: Frequent Contributor
timestamp: September 05, 2018, 05:47:03 am
content: Another reason for potting a driver is that creepage and clearance distances can potentially be reduced in the design.Such drivers are mostly mains isolated and the galvanic isolation barrier could become smaller if potting material is applied.Don't get me wrong, its not just adding potting material and you're done. Careful circuit design and PCB layout is needed, but in the end it can lead to significant miniaturization.

author: Kjelt
author_rank: Super Contributor
timestamp: September 05, 2018, 07:04:10 am
content: Quote from: treez on September 05, 2018, 05:14:52 amShow me one Extreme outdoor driver from Philips that is not potted ?That other chinese manufacturers do not pot their outdoor drivers is their (bad) business, I can show you products designed that will not last 1 year after power is connected. The extreme series of drivers have always had high requirements, should survive lighting strikes in the vicinity, bad mains, brown out etc. etc.Outdoor environments can be hars, damp, heat, cold, ice, and so on.

author: Kjelt
author_rank: Super Contributor
timestamp: September 08, 2018, 09:08:17 am
content: And HV testinghttps://youtu.be/JenQYBuqJWk

author: ocset
author_rank: Super Contributor
timestamp: September 08, 2018, 02:45:32 pm
content: Thanks, i was at a radar company once...and they said they had problems with a certain potting compound, where it would put pressure on components as it cured, and damage them.I hope to avoid PSUs with that potting compound inside them.

author: Gyro
author_rank: Super Contributor
timestamp: September 08, 2018, 05:59:56 pm
content: According to nctnico in the following thread, Epoxy compound shrinks during curing, PU doesn't...https://www.eevblog.com/forum/projects/temperature-probe-failure-analysis/msg277766/#msg277766

author: coppercone2
author_rank: Super Contributor
timestamp: September 08, 2018, 06:21:04 pm
content: there will exist some kind of internal strain in the solid potted assembly that is not present in the nonpotted with any kind of surface contact. You can't compare free air to any kind of solid enclosure. Different geometries will be effected differently. The more gummy it is the more frictional losses of mechanical there will be to mechanical strain by temperature change. It's basically creating a bimetalic strip everyone is familiar of to a lesser degree.I do wonder if there are any tricks, like putting a buncha straws to draw potting compound away (creating voids) from non populated PCB areas or something, so they can deform. It's too complicated for me to imagine.I don't like the idea of potting myself. It seems like a brute force solution to a delicate problem.

author: xani
author_rank: Frequent Contributor
timestamp: September 08, 2018, 07:11:29 pm
content: Quote from: coppercone2 on September 08, 2018, 06:21:04 pmMix a styrofoam granules with potting compound ? Would also make whole thing lighter and use less potting compound. And probably a lot worse for heat conduction

author: coppercone2
author_rank: Super Contributor
timestamp: September 08, 2018, 07:17:11 pm
content: I thought you can put straws in strategic places though