The unfortunate shrinkage of capacitors - Page 1
Forum section: electronics / general

author: IanB
author_rank: Super Contributor
timestamp: January 11, 2022, 03:45:37 am
content: I just replaced the motor run capacitor on my furnace blower, due to the gradual failure of the old one. It is rated 7.5 µF, 440 V. The size is kinda sorta 1 inch x 2 inches x 3 inches. It measured 2.2 µF when I pulled it out.I remember, back in the 1970's, I had a fluorescent tube ballast capacitor, which was described as 8.5 µF, 250 V AC. It was probably 2x or 3x the size.I really don't know if material science has advanced that much that they can shrink down the volume that much today compared to the old days? I've got to think it is cost cutting.From what I understand, run capacitors are considered to be a consumable now, they are guaranteed for up to 5 years, and are a regular replacement item. I'd rather pay twice as much, and not have to worry about replacing it.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: January 11, 2022, 03:57:15 am
content: I've got to do the one for my compressor in my A/C and I know the replacement I got won't be as good and the original that has lasted about 12 years.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: January 11, 2022, 04:03:38 am
content: Quote from: IanB on January 11, 2022, 03:45:37 amTo address this specifically, these were included here in Oz to counteract the effect of inductance. The particulars in the issue have be explored at length elsewhere, but I think in recent years before LED 'flouros', the installed cap had only to be good enough to pass a test at the compliance level. The bloody thing didn't last 5 minutes. It used to annoy me that they would often fail short.

author: IanB
author_rank: Super Contributor
timestamp: January 11, 2022, 04:09:46 am
content: Quote from: Ed.Kloonk on January 11, 2022, 04:03:38 amYes, I think they were for power factor correction. Given that all industrial and commercial premises were lit with fluorescent tubes using magnetic ballast, I imagine this was a matter of some importance.

author: IanB
author_rank: Super Contributor
timestamp: January 11, 2022, 04:11:17 am
content: Quote from: Ed.Kloonk on January 11, 2022, 03:57:15 amI suppose I should check the one in my A/C unit. However, it seems to work fine and has been in use for at least 20 years.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: January 11, 2022, 04:16:54 am
content: Quote from: IanB on January 11, 2022, 04:11:17 amIf it's starting OK and running OK (no vibrations), then get a brush and gently clean it a bit. Look for any rust. Ensure the drain isn't clogged. You're in business.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 11, 2022, 04:21:51 am
content: If it's across the line, it's probably exposed to a lot of transients, maybe a MOV across will do? Then choose voltage rating to meet/exceed MOV peak (surge) voltage, and it should last "forever". Shouldn't be a problem where it's behind a winding (run cap?, ferroresonant, etc.), the inductance should do well as filtering?Tim

author: floobydust
author_rank: Super Contributor
timestamp: January 11, 2022, 04:37:21 am
content: Was it an electrolytic or polypropylene run cap?Dielectric heating affects lifetime, they do experience have high currents. Electrolytics just dry out as well, the rubber bung is huge and not a great seal I think.If a run cap goes completely open, there is a pressure switch inside to prevent drama.

author: Martin Miranda
author_rank: Regular Contributor
timestamp: January 11, 2022, 04:44:26 am
content: they have a solution for that.

author: james_s
author_rank: Super Contributor
timestamp: January 11, 2022, 04:47:16 am
content: Quote from: IanB on January 11, 2022, 04:11:17 amMine died when it was about 12 years old. A mouse had got in there and fried itself across the terminals and in the process it peed on top of the capacitor and that rusted the terminals off.

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: January 11, 2022, 04:56:24 am
content: Quote from: james_s on January 11, 2022, 04:47:16 amOccasional visual inspection can save ya. One of my A/C's on the other side of the house has 10 more years seniority and needed it's TLC a number of years ago.Poking around I found evidence of Jerry the mouse who had recently moved in nearby. There was no Tom the cat, but some strategically placed baits sure sorted him out and prolly saved me a mint cos we found a bit of power cable being gnawed on.

author: IanB
author_rank: Super Contributor
timestamp: January 11, 2022, 04:58:33 am
content: Quote from: T3sl4co1l on January 11, 2022, 04:21:51 amThis was a run cap for the furnace blower motor. It had definitely failed, the motor was sluggish to start and would sometimes stall. The cap was rated at 7.5 µF but measured 2.2 µF. After the cap replacement the motor runs so vigorously it is like a hurricane.Quote from: floobydust on January 11, 2022, 04:37:21 amI'm not sure it is electrolytic. I think they are made from aluminized plastic film and filled with castor oilI looked at a video where someone disassembled a failed capacitor, and it had visible bulging and distortion of the internal structure.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 11, 2022, 09:26:57 am
content: The run capacitors are usally non electrolytic, to day polypropylene. In the old time paper (with oil) capacitors were used and these were considerably larger.It looks like they improved (more consistent) the production of thiner foils and this way allowed a smaller form factor. A foil that is 10% thinner would be a 20% reduction in volume as less area is needed.The starter capacitors are sometimes non polarized electrolytic types as they are not used that long and often need higher capacitance. With reduced size for the PP types it can be possible to use a film type instead.As far As I understand it the capacitors suffer from overvoltage spikes, e.g. when the motor is switched off.Chances are the loss in capacitance in nonlinear in time - the smaller the capacitance the easier the capacitance can rise too high. So once too small the detoriation can speed up.

author: magic
author_rank: Super Contributor
timestamp: January 11, 2022, 10:22:28 am
content: Quote from: IanB on January 11, 2022, 03:45:37 amThere may be a way if you are willing to pay 4x more

author: TERRA Operative
author_rank: Super Contributor
timestamp: January 11, 2022, 01:00:24 pm
content: You can always use a new capacitor with a higher voltage rating to get something with thicker plastic laminations inside that will last longer.

author: IanB
author_rank: Super Contributor
timestamp: January 11, 2022, 02:51:13 pm
content: Quote from: magic on January 11, 2022, 10:22:28 amIt makes mounting and wiring a bit more complicated though...

author: IanB
author_rank: Super Contributor
timestamp: January 11, 2022, 02:52:38 pm
content: Quote from: TERRA Operative on January 11, 2022, 01:00:24 pmI am using a 440 V capacitor on a 120 V system. I understand 600 V capacitors are available, but they are harder to source.

author: cvanc
author_rank: Frequent Contributor
timestamp: January 11, 2022, 03:49:17 pm
content: I have used polypropylenes from Digikey to replace both of my furnace run caps. Works fine.I don't think it's motor spikes killing these, I think it's powerline spikes. One way to increase life is to use the highest voltage cap you can find (or fit). These are self healing types so a lifetime of power glitches eventually reduces the capacitance. A higher voltage rating slows this down.And I bet adding a varistor across the furnace's AC line input would help, too?

author: coppercone2
author_rank: Super Contributor
timestamp: January 12, 2022, 03:20:48 am
content: I think the key to this might be the improvement of QC practices for film materials.I am not sure on the specifics (heard something about radiation casks being used as test equipment for aluminized films), but the idea might be that the material is the same but the increased quality of testing and production methods lead to designers being more confident in thinner materials.But of course proliferation of intense bean counters is a known thing, so its unlikely to be entirely a positive change.

author: jonpaul
author_rank: Super Contributor
timestamp: January 12, 2022, 10:51:51 am
content: Bonjour, our wise Solons and The mandarins with sinecures in Wash DC banned the best capacitor oils due to PCBs decades ago.The newer subs are not as good.Finally all old line USA cap mfg like Sangamo, Mallory and Aerovox long closed.The current "USA" ballast firms like Magnetek offshored ALL capacitor and other manufacture to China were the materials, quality control, fabrication and seals are the cheapest possible.Look for old 1950s...1980s parts,Jon

author: Nauris
author_rank: Regular Contributor
timestamp: January 12, 2022, 05:02:54 pm
content: These oil filled Shizuki caps are good I have not yet seen one failBut the usual dry polypropylene are all bad, even the good brand ones don't last long

author: mazurov
author_rank: Frequent Contributor
timestamp: January 12, 2022, 10:08:43 pm
content: Quote from: coppercone2 on January 12, 2022, 03:20:48 amI'd say that improvement of QC practices (process control rather) is the cause of modern caps being less robust. Devices that in the past required 30% margin to have a desired yield now are produced with 2%. Consequently, when a manufacturer says the device will maintain its specs for 5 years, 95% of the devices will fail between 5.1- 5.5 and the rest in 6.This is not specific to electronics. In a bag of screws all 100 will have their diameter 0.1% above minimum, milk will go bad 2 days after exp.date, even when unopened, etc. The meaning of the word "rating" has changed.

author: SilverSolder
author_rank: Super Contributor
timestamp: January 12, 2022, 10:17:36 pm
content: Quote from: mazurov on January 12, 2022, 10:08:43 pmThat definitely rings true. So if you want 30% margin, you now have to spec that up front when you select the component?

author: coppercone2
author_rank: Super Contributor
timestamp: January 13, 2022, 06:47:17 am
content: Quote from: mazurov on January 12, 2022, 10:08:43 pmthat is true, people want to get rich off the 'margin' at equipment owner expense. Add 30% cost increase for repairs and duds for every purchase you make. The illusion of cheapness to most, only really short term users benefit.