# Why do capacitors short out at high frequencies? **URL:** \ **Category:** Factual Questions\ **Created:** [February 19, 2003, 2:56pm UTC](https://boards.straightdope.com/t/why-do-capacitors-short-out-at-high-frequencies/156128 "2003-02-19T14:56:22Z")\ **Posts on this page:** 15\ **Page:** 2
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [February 19, 2003, 9:27pm UTC](https://boards.straightdope.com/t/why-do-capacitors-short-out-at-high-frequencies/156128/21 "2003-02-19T21:27:51Z")
> [@](#): > > \*Originally posted by Eyer8 \* > **Actually, one of the IEEE periodicals (don’t recall which one but I could probably find it if pressed) stated that this was a case of corporate espionage gone wrong…\** No offense, but this sounds a bit fishy. Have you got a reliable cite for that? Every spec of a cap can be tested accurately, and even minimal AQL testing will reveal such a basic design flaw. I find it hard to believe that any significant cap manufacturer would fail to thoroughly test a new cap design, especially those operatiing under ISO-9000 guidlines. ---
**Author:** ![Happy\_Fun\_Ball](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/happy_fun_ball/32/17664_2.png) [@Happy\_Fun\_Ball](https://boards.straightdope.com/u/Happy_Fun_Ball)\ **Post date:** [February 19, 2003, 9:33pm UTC](https://boards.straightdope.com/t/why-do-capacitors-short-out-at-high-frequencies/156128/22 "2003-02-19T21:33:47Z")
> [@](#): > > _Originally Posted by Q.E.D._ > **Have you got a reliable cite for that?** > [@](#): > > _Originally Posted by me_ > **one of the IEEE periodicals (don’t recall which one but I could probably find it if pressed)** Give me a couple minutes… ---
**Author:** ![Happy\_Fun\_Ball](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/happy_fun_ball/32/17664_2.png) [@Happy\_Fun\_Ball](https://boards.straightdope.com/u/Happy_Fun_Ball)\ **Post date:** [February 19, 2003, 9:50pm UTC](https://boards.straightdope.com/t/why-do-capacitors-short-out-at-high-frequencies/156128/23 "2003-02-19T21:50:41Z")
Ask and you shall receive… [IEEE Spectrum Article](http://www.spectrum.ieee.org/WEBONLY/resource/feb03/ncap.html) From the article… > [@](#): > > The origins of the motherboard malaise seem a lesson in how not to commit corporate espionage. A well-placed source in Taiwan, who did not wish to be identified, largely confirmed for Spectrum accounts published in the United States that were based on sources in the Japanese electronics industry. According to the source, a scientist stole the formula for an electrolyte from his employer in Japan and began using it himself at the Chinese branch of a Taiwanese electrolyte manufacturer. > [@](#): > > \ Taiwanese capacitor makers claim they are the victims of a smear campaign by Japanese competitors who are saying that all Taiwanese capacitor makers are tainted. The price ratio of Japanese products to Taiwanese was about four to one, according to Lee, and Japanese firms may be trying to win back lost market share. What is up with the hamsters today? ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [February 19, 2003, 10:06pm UTC](https://boards.straightdope.com/t/why-do-capacitors-short-out-at-high-frequencies/156128/24 "2003-02-19T22:06:38Z")
Sounds like a lot of finger-pointing and excuse-making to me. Certainly someone screwed up, but I don’t think we’ll ever know the real truth in a case like this. ---
**Author:** ![Squink](https://avatars.discourse-cdn.com/v4/letter/s/b5e925/32.png) [@Squink](https://boards.straightdope.com/u/Squink)\ **Post date:** [February 19, 2003, 10:08pm UTC](https://boards.straightdope.com/t/why-do-capacitors-short-out-at-high-frequencies/156128/25 "2003-02-19T22:08:37Z")
> [@](#): > > \*Originally posted by Eyer8 \* > \*\*What is up with the hamsters today? \*\* In terms of its capacity, a hamster is really just a micro-ferret. To get the board up to speed really requires a full ferret or more. 😃 ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [February 19, 2003, 10:33pm UTC](https://boards.straightdope.com/t/why-do-capacitors-short-out-at-high-frequencies/156128/26 "2003-02-19T22:33:05Z")
> [@](#): > > \*Originally posted by Q.E.D. \* > \*\*Every spec of a cap can be tested accurately, and even minimal AQL testing will reveal such a basic design flaw. I find it hard to believe that any significant cap manufacturer would fail to thoroughly test a new cap design, especially those operatiing under ISO-9000 guidlines. \*\* Not necessarily. If the formula was wrong then I’m guessing the capacitors would only fail under certain conditions _over time_. “Conditions” may include high ambient temperature and/or high ripple current. This defect is easily identified when using accelerating life testing, environmental cycling, etc., but these are performed during product _development_, not on the assembly line. ---
**Author:** ![gazpacho](https://avatars.discourse-cdn.com/v4/letter/g/6f9a4e/32.png) [@gazpacho](https://boards.straightdope.com/u/gazpacho)\ **Post date:** [February 19, 2003, 10:36pm UTC](https://boards.straightdope.com/t/why-do-capacitors-short-out-at-high-frequencies/156128/27 "2003-02-19T22:36:13Z")
> [@](#): > > I find it hard to believe that any significant cap manufacturer would fail to thoroughly test a new cap design, especially those operatiing under ISO-9000 guidlines. ISO-9000 guidelines don’t say anything about testing stuff. It mostly just means you have a process and you follow the process. You can have crappy processes and get certified. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [February 19, 2003, 10:54pm UTC](https://boards.straightdope.com/t/why-do-capacitors-short-out-at-high-frequencies/156128/28 "2003-02-19T22:54:31Z")
> [@](#): > > \*Originally posted by gazpacho \* > \*\*ISO-9000 guidelines don’t say anything about testing stuff. It mostly just means you have a process and you follow the process. You can have crappy processes and get certified. \*\* Good catch, **gazpacho**. ISO-9001 is a total joke. But I guess that’s better left for IMHO… ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [February 19, 2003, 11:08pm UTC](https://boards.straightdope.com/t/why-do-capacitors-short-out-at-high-frequencies/156128/29 "2003-02-19T23:08:44Z")
Sort of. But the processes do get evaluated as well. I was an ISO auditor for a former employer, a transformer manufacturer. I got to spend three days at this cool (read “boring”) ISO-9000 training class. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [February 19, 2003, 11:47pm UTC](https://boards.straightdope.com/t/why-do-capacitors-short-out-at-high-frequencies/156128/30 "2003-02-19T23:47:30Z")
> [@](#): > > \*Originally posted by Q.E.D. \* > \*\*The formula for capacitive reactance is X[sub]c[/sub]=1/(sqr(2piFC)), where F is the frequency in hertz and C is the capacitance in farads. As you can see, as the frequency increases, the reactance decreases. At very high frequencies, the reactance approaches zero. \*\* This is just wrong. The formula is X[sub]c[/sub]=1/(j.w.C), as **N9IWP** originally said. There’s no square root in the formula. ---
**Author:** ![N9IWP](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/n9iwp/32/3154_2.png) [@N9IWP](https://boards.straightdope.com/u/N9IWP)\ **Post date:** [February 20, 2003, 12:05am UTC](https://boards.straightdope.com/t/why-do-capacitors-short-out-at-high-frequencies/156128/31 "2003-02-20T00:05:01Z")
didn’t think there was a square root, but wasn’t sure. (Hangs head in shame). Brian ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [February 20, 2003, 12:07am UTC](https://boards.straightdope.com/t/why-do-capacitors-short-out-at-high-frequencies/156128/32 "2003-02-20T00:07:22Z")
You’re correct that there is no square root (thanks for making me drag out my engineer’s reference), however, if you want to get technical, the correct formula is actually X[sub]C[/sub]=1/(\*\*-\*\*_j_wC). That minus sign is important if you’re going to factor in the imaginary component. ---
**Author:** ![Desmostylus](https://avatars.discourse-cdn.com/v4/letter/d/c57346/32.png) [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)\ **Post date:** [February 20, 2003, 12:15am UTC](https://boards.straightdope.com/t/why-do-capacitors-short-out-at-high-frequencies/156128/33 "2003-02-20T00:15:38Z")
> [@](#): > > \*Originally posted by Q.E.D. \* > **…however, if you want to get technical, the correct formula is actually X[sub]C[/sub]=1/(**-\*\*_j_wC). That minus sign is important if you’re going to factor in the imaginary component. \*\* That’s wrong, too. The correct formula is X[sub]C[/sub]=1/(_j_wC). If you wanted to explicity include the minus sign, you’d write X[sub]C[/sub]=\*\*-\*\*_j_/(wC). ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [February 20, 2003, 12:38am UTC](https://boards.straightdope.com/t/why-do-capacitors-short-out-at-high-frequencies/156128/34 "2003-02-20T00:38:58Z")
**Desmostylus** is correct; the complex impedance for an ideal capacitor is Z(jw) = 1/(jwC). The magnitude of the impedance is 1/(wC). ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [February 20, 2003, 1:51am UTC](https://boards.straightdope.com/t/why-do-capacitors-short-out-at-high-frequencies/156128/35 "2003-02-20T01:51:52Z")
I can see I need to review my formulae. It’s been a while since I’ve used this stuff in practice. :smack: [Previous page](https://boards.straightdope.com/t/why-do-capacitors-short-out-at-high-frequencies/156128.md?page=1)