The cap that just won't die - Page 1
Forum section: electronics / projects

author: taydin
author_rank: Frequent Contributor
timestamp: October 25, 2019, 07:20:28 pm
content: I have had the chance to play around with a 5 kV megger (Megger MIT525). One of the tests that I did was to test the voltage rating of a 3.3 nF, 630V film capacitor.First I measured the capacitance and the ESR at 100 KHz and wrote it down.Then I did a ramp test on this cap going to 1 kV. And I was kinda surprised to see that the cap survived this test. I measured the capacitance and ESR after the ramp test and the values were almost exactly the same.Then I did a ramp test to 2.5 kV. And I was more surprised to see that the cap survived this test, too! I measured the capacitance and ESR after the ramp test and the values were almost exactly the same.Then I did a ramp test to 5 kV. And I was shocked to see that the cap survived this test, too! I measured the capacitance and ESR after the ramp test and the values were almost exactly the same.So my question is, how can a cap rated 630V stay intact when I apply 5 kV across it? I have read that metallic film type capacitors have self healing property, but after so many arcs and healing cycles, the capacitance or the ESR should have changed at least a little bit. But they are pretty much the same! So I think there is no arcing and self healing going on, the cap just can withstand the 5 kV! How is this possible?

author: capt bullshot
author_rank: Super Contributor
timestamp: October 25, 2019, 07:27:04 pm
content: Most probably this is the reserve for ageing and environment conditions.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 25, 2019, 07:27:06 pm
content: Is it actually reaching nominal voltage, or is there a voltage drop across a resistance that it's not reading? (Pretty sure a megger is supposed to measure the terminal voltage so this should be a stupid question.)Tim

author: taydin
author_rank: Frequent Contributor
timestamp: October 25, 2019, 07:38:30 pm
content: Quote from: T3sl4co1l on October 25, 2019, 07:27:06 pmI didn't have a series resistance, just the cap across the megger terminals. When measuring insulation on larger capacitors (microfarads), it is necessary to use a 1 MΩ ish series "quieting resistor" to avoid environment noise from confusing the megger, but for this 3.3 nF cap it wasn't necessary and i was getting stable insulation resistance reading in 2.5 kV range.I have also confirmed using a high voltage probe that there is the specified voltage at the megger terminals. The megger can deliver up to 3 mA in short circuit.

author: taydin
author_rank: Frequent Contributor
timestamp: October 25, 2019, 07:40:39 pm
content: I did measure the insulation resistance of this cap at 2.5 kV and it was around 2.5 TΩ. I have read that it's common for film type capacitors to have PTFE dielectric so this very high insulation resistance makes sense.I have also repeatedly tested this cap by applying 5 kV directly. At 5 kV, the insulation resistance was measured to be 780 GΩ. Not sure why the discrepancy. But I tend to believe the 2.5 TΩ number, because when I measured at 500 V and 1 kV range, the megger was out of range and just indicated "> 2 TΩ".

author: mzzj
author_rank: Super Contributor
timestamp: October 25, 2019, 07:45:19 pm
content: Not that suprising, there needs to be pretty good margin for temperature and aging. Try maximum temp and 1000h and your results are probably lot different.

author: taydin
author_rank: Frequent Contributor
timestamp: October 25, 2019, 07:48:36 pm
content: Quote from: mzzj on October 25, 2019, 07:45:19 pmYes could be. It is still surprising that this would require a voltage margin that is 8 times the rated value

author: taydin
author_rank: Frequent Contributor
timestamp: October 25, 2019, 07:50:21 pm
content: Another thing that I learned while experimenting with this megger is that isolation resistance increases with voltage. So measuring IR at 2 kV and 2.5 kV gives different results (both as stable as expected), bu the 2.5 kV resistance is higher. This is explained by the fact that the electrons are pulled away from the insulator, leaving a region where there are extremely small number of free electrons left.But I suppose at 5 kV another effect kicks in and this time the IR goes down quite a bit.

author: mcovington
author_rank: Regular Contributor
timestamp: October 26, 2019, 04:18:05 am
content: Does this manufacturer offer capacitors rated higher than 630V? I'm guessing 630V might actually be their highest rating, often applied to products that are actually good at much higher voltages but haven't been thoroughly tested to prove it.

author: taydin
author_rank: Frequent Contributor
timestamp: October 26, 2019, 06:17:05 am
content: Quote from: mcovington on October 26, 2019, 04:18:05 amI don't know the manufacturer. This is a generic cheap Chinese cap, no markings other than the picture above.

author: Gyro
author_rank: Super Contributor
timestamp: October 26, 2019, 12:36:57 pm
content: The capacitor voltage rating also has to hold for the full device temperature rating. Its breakdown voltage would almost certainly be lower at its maximum rated temperature.

author: 001
author_rank: Super Contributor
timestamp: October 26, 2019, 02:58:40 pm
content: Quote from: Gyro on October 26, 2019, 12:36:57 pmWhat is "maximum rated temperature" for noname cap?

author: mzzj
author_rank: Super Contributor
timestamp: October 26, 2019, 03:36:21 pm
content: I played with electric fly swatter and some caps that I had lying around:63v 1uF noname(possibly wima): ~230V, short circuits100v 2.2uF philips MKC: starts to crackle at 550v

author: Gyro
author_rank: Super Contributor
timestamp: October 26, 2019, 04:52:14 pm
content: Quote from: 001 on October 26, 2019, 02:58:40 pmNo idea, but it has device markings and presumably some sort of spec somewhere. Irrelevant anyway, it clearly has a large safety margin for 630V.

author: taydin
author_rank: Frequent Contributor
timestamp: October 26, 2019, 08:56:28 pm
content: Quote from: mzzj on October 26, 2019, 03:36:21 pmI tested a 150nF 63V polyester cap today, and it started to crackle at around 250V. But it didn't fail short. It ended up with a permanent 1.6 kΩ resistance.

author: Gyro
author_rank: Super Contributor
timestamp: October 26, 2019, 09:03:26 pm
content: Quote from: taydin on October 26, 2019, 08:56:28 pmIt's a relatively low value capacitor, so there probably wasn't enough energy in the system for it to completely self-heal. If you hit is with 60-100V at reasonable current (eg. discharge a bigger capacitor into it), it might self-heal to a slightly lower capacitance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 26, 2019, 10:03:04 pm
content: Also you were probably "cooking" it quite literally. Polyester tends to melt into a nasty goober and that probably lead to carbonization or shorting of metallization.I have an ancient video* where I tested polyester caps in a resonant circuit; it worked fine for a few seconds, then a hotspot formed in one capacitor, forcing bubbling goo and smoke out a hole, and so on.*Which over time, YouTube has compressed down to an unintelligible bitrate. Suppose I should go dig up the archived copy and repost it for posterity, but meh. Heh, it would also be quite reasonable as a GIF these days (which itself is often webm, not GIF).Tim

author: Electro Detective
author_rank: Super Contributor
timestamp: October 26, 2019, 10:51:20 pm
content: Quote from: taydin on October 25, 2019, 07:38:30 pm"quieting resistor"in parallel or series?fwiw the megger is ramping up to those higher voltages,the survivability might drop a bit if 2.5 and 5kv suddenly got dumped into the capat the speed of flight

author: taydin
author_rank: Frequent Contributor
timestamp: October 27, 2019, 06:28:37 am
content: Quote from: Electro Detective on October 26, 2019, 10:51:20 pmI have hit the above film cap with 5 kV directly, too. In fact, many times. It just charges up at a few uA, and then the current drops until it stabilizes at around 2 nA. It takes around 3 minutes for the resistance reading to stabilize. The starting capacitance (fresh out of the part bin) was 3.355 nF. After all the whacking and ramping with 5 kV, the final capacitance was 3.364 nF. So, an increase of 9 pFIf there would have been self healing, the cap was supposed to lose capacitance.

author: taydin
author_rank: Frequent Contributor
timestamp: October 27, 2019, 07:02:58 am
content: Quote from: T3sl4co1l on October 26, 2019, 10:03:04 pmThe reason it gets cooked, rather than a more dramatic effect, is probably the 3 mA current limit of the megger.

author: taydin
author_rank: Frequent Contributor
timestamp: October 27, 2019, 07:10:18 am
content: Quote from: Electro Detective on October 26, 2019, 10:51:20 pmWhen I initially tried to measure the insulation resistance of another cap, I was getting erratic readings between 10 GΩ and 100 GΩ (first graph). But after I have added a 10 MΩ quieting resistor in series, I got much more stable readings around 24 GΩ (second graph).The explanation for this is in the document below. This is from a company that makes a very high end megaohmmeter that can measure up to 100 TΩ with 0.5% accuracy.https://www.ietlabs.com/pdf/application_notes/1865_IET/measuring_insulation_resistance_of_capacitors.pdf

author: taydin
author_rank: Frequent Contributor
timestamp: October 27, 2019, 07:16:18 am
content: And here are a few fun insulation resistance measurementsFirst picture, a tootpick measured at 2.5 kV. IR = 400 GΩSecond picture, a sheet of paper measured at 2.5 kV. IR = 52.5 GΩ

author: taydin
author_rank: Frequent Contributor
timestamp: October 27, 2019, 07:21:37 am
content: Insulation resistance of some multimeter probes, measured in worst case scenario (twisted together), at 1200V$2 POS multimeter (in the picture), IR = 72 GΩKeysight U1282A, IR = 17 GΩRigol DM3068, IR = above 2 TΩ(Please don't take this too literally. The twisting tightness was not the same, the probe lengths are different, and in the end, it doesn't really matter. All of them have insulation resistance that is much more than enough)

author: taydin
author_rank: Frequent Contributor
timestamp: October 27, 2019, 07:40:32 am
content: Quote from: MasterTech on October 27, 2019, 07:32:33 amYes, I have seen the same comment in other forums. And I am working on a 3 kV DC power supply and will definitely try this after it's working. But here is what doesn't make sense to me:Let's assume the cap would have failed with a 5 kV, high energy source. Then I would expect the following observation with the megger. I start the test at 5 kV, and the cap instantly starts to draw 3 mA (the current limit) and the megger would stop the test because it considers excessive current draw to be a breakdown. But this doesn't happen. The maximum current draw is a few microamps at the beginning and then the current drops exponentially. It basically exhibits the standart capacitor charge curve.The megger has two modes of operation, "breakdown" and "burn". In breakdown mode, excessive current draw stops the test. In "burn" mode, excess current doesn't stop the test. I had tested the 150 nF 63V polyester cap above in "burn" mode and it cooked. I have tested the 630V cap that just won't die in both breakdown and burn mode, it just isn't being affected in any way.Quote from: MasterTech on October 27, 2019, 07:32:33 amYes, and that's the only sensible way to measure. With AC there are just too many variables which cannot be controlled to get a repeatable measurement.

author: mzzj
author_rank: Super Contributor
timestamp: October 27, 2019, 09:53:33 pm
content: Try ceramic MLCC capacitors if you want to see some "silly" large margins. Some of them survive 150x rated voltage, ie 1500v for 10v cap.

author: najrao
author_rank: Regular Contributor
timestamp: November 05, 2021, 03:11:48 pm
content: Old post this, but I have to say this:As a school boy, I had a 'tractor engine magneto', which was a 10kg monster producing spark plug voltagedirectly. It even had a four cylinder distributor built into it. Could draw half inch sparks by hand spinning the shaft stub!Now for the interesting part: I also had a 'distributor condenser', released from my father's car. This was a metal can the size of a thumb, and had but one terminal insulated from the can by what I thought was a glass seal. I would charge it up by shortened sparks from the magneto, god knows to what voltage*, and gingerly carry it to school. Just hand it to a mate, and watch the fun!* the cap itself would hold a quatrter of an inch spark.