Why electrolytic capacitor required in this zero crossing circuit? - Page 1
Forum section: electronics / beginners

author: czk9527
author_rank: Contributor
timestamp: August 11, 2022, 08:13:49 am
content: Hi guys,Just came across this zero crossing circuit. Figure 8, in Link:https://sound-au.com/appnotes/an005.htmIt says "the circuit needs an electrolytic capacitor"...The circuit actually originated from this link :https://dextrel.net/dextrel-start-page/design-ideas-2/mains-zero-crossing-detectorIt says "The voltage across the capacitor never exceeds 10V"I don't really understand why it needs to be electrolytic.The voltage across it is low. The current through it is low as well.Can someone give me some hint ?Cheers~~

author: MrAl
author_rank: Super Contributor
timestamp: August 11, 2022, 08:54:21 am
content: Quote from: czk9527 on August 11, 2022, 08:13:49 amHi,That's probably because the value is as high as 10uf which would be high for a ceramic.Electro's are cheaper usually.

author: czk9527
author_rank: Contributor
timestamp: August 11, 2022, 10:17:59 am
content: Hi MrAl,Hmm... I'm thinking it's got something to do with some characteristics of electrolytics apart from the cost.

author: wraper
author_rank: Unknown
timestamp: August 11, 2022, 10:25:28 am
content: No specific characteristics required. It's just the most economic and compact option that suits very well. But If you use MLCC, you likely will need to use capacitor with larger capacitance due to capacitance loss under voltage potential and temperature increase. And avoid capacitors with Z5U and Y5V dialectic type because they have the same problems as X5R and X7R but so much worse. I don't think that a decent electrolytic capacitor will have any lifetime issues in this circuit. Capacitor with 50V or higher voltage rating should be used.

author: EPAIII
author_rank: Super Contributor
timestamp: August 11, 2022, 10:32:15 am
content: In reading the article I could not find any reference to "electrolytic capacitor". And a search did not find it either.Just where is that stated?Edit: Oh I see. It wasn't in the article. I think the person commenting on it just assumed that a 10uF capacitor would need to be an electrolytic. Back in the 1960s or 1970s that may have been substantially true and a 10uF capacitor would have been either quite large or very difficult to source or both. Oh, and probably expensive too. But today the manufacture of capacitors has progressed. I think you can ignore that comment and use whatever 10 uF capacitor you can find.

author: czk9527
author_rank: Contributor
timestamp: August 11, 2022, 10:35:04 am
content: Quote from: EPAIII on August 11, 2022, 10:32:15 amIt's in the first link. I actually attached the screenshot of the highlighted keyword.

author: czk9527
author_rank: Contributor
timestamp: August 11, 2022, 10:35:43 am
content: Quote from: wraper on August 11, 2022, 10:25:28 amYep makes more sense to use electrolytic. Thanks wraper.

author: wraper
author_rank: Unknown
timestamp: August 11, 2022, 10:51:21 am
content: Quote from: EPAIII on August 11, 2022, 10:32:15 amNot really, if you use 10uF 50V MLCC there likely will be 2-5uF left under working voltage (~30V) and room temperature depending on type (will be more for caps of large size and better dielectric). And temperature can make it even worse. MLCC ageing is a thing too and they can lose a large part of initial capacitance with time. Reheating above Curie point (reflow for SMD parts) resets the process.

author: chris_leyson
author_rank: Super Contributor
timestamp: August 11, 2022, 11:30:01 am
content: Use this circuit from EDN and no electrolytic needed. 470nF cap can be reduced to 100n 50V.lhttps://www.edn.com/mains-driven-zero-crossing-detector-uses-only-a-few-high-voltage-parts/

author: czk9527
author_rank: Contributor
timestamp: August 11, 2022, 12:00:48 pm
content: Quote from: wraper on August 11, 2022, 10:51:21 amInteresting curves. Learned something new. Thanks for sharing.

author: czk9527
author_rank: Contributor
timestamp: August 11, 2022, 12:01:53 pm
content: Quote from: chris_leyson on August 11, 2022, 11:30:01 amAnother interesting circuit