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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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16th Dec 2018, 11:08 pm | #41 |
Octode
Join Date: Dec 2008
Location: Wincanton, Somerset, UK.
Posts: 1,783
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Re: Would you trust these caps?
OK, so we now have some interesing results from Waveydipole's caps.
He sent me two samples of 470pF and 4n7 respectively. Both withstood 2kV with no issues.
The capacitance of our hi-K cap is then calculated using the standard forumula. The attached graph says it all. John |
17th Dec 2018, 8:48 am | #42 |
Dekatron
Join Date: Jun 2016
Location: London, UK.
Posts: 3,687
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Re: Would you trust these caps?
Interesting results. Thanks for sharing.
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17th Dec 2018, 4:23 pm | #43 |
Nonode
Join Date: Mar 2013
Location: Leicester, UK
Posts: 2,535
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Re: Would you trust these caps?
John, thanks for taking the time and trouble to perform these tests and posting the results. I am glad your bridge survived the ordeal! At only 250v the capacitance is down by some 70%, and by 1kv down over 90%! Quite shocking.
I came across this article, which, while talking about SMD components, shows how the characteristics of parts with the same dielectric can vary significantly as component size shrinks. His observations were recorded at much lower voltages: https://www.maximintegrated.com/en/a...ex.mvp/id/5527 Regarding these eBay parts that I purchased, they look pretty much identical, like they could have come from the same production line and were made with the same materials, yet your temperature observations alone (which I have now also carried out on my DER LCR meter and can confirm) would seem to indicate that they behave quite differently. Whether this suggests that they are made with different materials, or whether as per the article its down to the smaller size relative to capacitance, I'm not sure. It doesn't seem to make sense that something having 10x the capacitance, with the same voltage rating and using the same materials could be manufactured in a smaller package without something being compromised. Still, at least they did seem to withstand 2kv across them, even if, by that point, the capacitance was a small fraction of the nominal rating. The conclusion of the article makes a good point about checking datasheets, which, of course is not usually possible with components purchased from eBay as sellers do not usually provide links to them and rarely, if ever, post anything but the most basic information. If the component is branded, it may be possible to find a datasheet elsewhere, but if it is unbranded, there is simply no way to be sure of anything. Looking at your graph, if I was expecting something like the nominal capacitance I probably wouldn't use these for anything much above 100v, or at least if I did, then I would need to de-rate the capacitance accordingly. I guess this goes to show that just because things come in identical packages, it does not mean that they are made the same, and for anything critical, check the datasheet and use bona-fide sources. |
18th Dec 2018, 5:16 pm | #44 |
Octode
Join Date: Dec 2008
Location: Wincanton, Somerset, UK.
Posts: 1,783
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Re: Would you trust these caps?
For completeness, here's the plot of one of the 470p capacitors: clearly a very different dielectric.
John |
18th Dec 2018, 5:40 pm | #45 | |
Dekatron
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
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Re: Would you trust these caps?
Quote:
Pops up in many places, didn’t expect it here though! I love it!
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Al |
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