non polarised capacitor - Page 1
Forum section: electronics / beginners

author: paulhm81
author_rank: Regular Contributor
timestamp: March 05, 2016, 11:58:24 am
content: HelloI would like to fix a night light that works with an led at mains and I discovered that the capacitor used to divide the voltage was dead. I searched my junk box and found one and it works with it but it's too big so I went to my local store to buy one smaller and they asked me if it want a dc or ac 250v cap. I thought they didn't know what they were talking about and bought one that is small enough but it says 250-e7 and went on my way but then I started thinking that maybe the 250 is the dc limit and I didn't think about the peak voltage. Am I right in thinking that that is the difference?

author: bktemp
author_rank: Super Contributor
timestamp: March 05, 2016, 12:09:40 pm
content: When a cap says 250V without ~ or AC it is the DC rating and it is not suited for 230V AC.A 400Vdc cap might work, but better get a cap specifically rated for mains operation. Those are X2 rated.

author: Brumby
author_rank: Unknown
timestamp: March 05, 2016, 01:05:02 pm
content: This:Quote from: bktemp on March 05, 2016, 12:09:40 pmAside from the voltage aspect, X2 rated caps have other safety measures designed into them that make them more appropriate in high energy circuits - such as mains.

author: Seekonk
author_rank: Super Contributor
timestamp: March 05, 2016, 01:09:30 pm
content: Without a country indicator we do not know what your line voltage is or if that cap is suitable.

author: Visher
author_rank: Contributor
timestamp: March 05, 2016, 01:30:31 pm
content: Keep in mind that if your mains are eg. 230 V rated (so 230 V is AC RMS value), peak AC value is square root two times RMS value. Of course for safety reasons we put higher voltage capacitors (12 VAC RMS -> 16 V cap, 24 VAC RMS -> 35 V cap - and that's including the minimum safety coefficient).

author: wraper
author_rank: Unknown
timestamp: March 05, 2016, 01:47:49 pm
content: show a photo of this cap.

author: wraper
author_rank: Unknown
timestamp: March 05, 2016, 01:49:53 pm
content: Quote from: paulhm81 on March 05, 2016, 11:58:24 amIt sounds like they did know what they talked about but you were clueless.

author: Simon
author_rank: Unknown
timestamp: March 05, 2016, 01:53:01 pm
content: but if its an LED surely there is a diode as well to block the negative cycles. But its very pulsy DC so I suppose an ac cap won't hurt.

author: wraper
author_rank: Unknown
timestamp: March 05, 2016, 01:59:39 pm
content: Quote from: Simon on March 05, 2016, 01:53:01 pmYou cannot use capacitor dropper with pulsed DC (after rectifier or in series with one diode). It will just charge and not pass any current anymore. In capacitive dropper circuits capacitor is used on AC side in series with rectifier.

author: Simon
author_rank: Unknown
timestamp: March 05, 2016, 02:00:19 pm
content: true

author: paulhm81
author_rank: Regular Contributor
timestamp: March 05, 2016, 03:20:24 pm
content: thank you for the answers. This is the original type of cap that was gone and I found an identical one in some old dead laptop charger and it's working now.http://kepfeltoltes.hu/141110/470nf_www.kepfeltoltes.hu_.jpgso... it's not X2 but it's 400V. The X2 that they wanted to give me at the store was huge and I wouldn't have been able to close the lid of the lamp with it.I'm not designing anything, I'm just fixing a lamp so It's not just an led. It's a bit more complex and it has even a photo cell and it lights up the led when it's dark.so... it's not X2 but it's 400V

author: bktemp
author_rank: Super Contributor
timestamp: March 05, 2016, 03:35:14 pm
content: Quote from: paulhm81 on March 05, 2016, 03:20:24 pmMaybe that is the cause why it failed.Maybe 2/3rds of the caps I have replaced were much smaller than the replacements. Because they are often not rated for direct connection to mains, they lose their capacity quickly and the device fails after some years.

author: paulhm81
author_rank: Regular Contributor
timestamp: March 05, 2016, 03:39:01 pm
content: Quote from: bktemp on March 05, 2016, 03:35:14 pmI'm sure you're right. I'm happy if it works a few years. I read about reactance and I know that the value of the cap is important. The old one had a quarter of what it was supposed to have.

author: wraper
author_rank: Unknown
timestamp: March 05, 2016, 03:52:25 pm
content: For use under 230V AC it needs to be X2 ~250V safety cap or at least 630 V DC regular film cap. 400 VDC capacitors will work for some time, but very often will eventually fail.

author: wraper
author_rank: Unknown
timestamp: March 05, 2016, 03:54:20 pm
content: Quote from: paulhm81 on March 05, 2016, 03:20:24 pmThat means they have a clue about what they sell.

author: paulhm81
author_rank: Regular Contributor
timestamp: March 05, 2016, 04:05:11 pm
content: Quote from: wraper on March 05, 2016, 03:54:20 pmYes, that's what it means. I was not beeing a smart ass but the initial question when you're buying a non polarised cap if you want ac or dc was strange to me. They have no knowlege about electronics, just sales but they picked up a lot over the years. Actually I asked the a wile ago for a mosfet to repair a crt tv and they asked me if I wanted the cap with it. They said everyone who bought this mosfet also bought a certain cap ( I don't remember wich one) and sure enough the cap was indeed bad. SO yeah, I guess they learned a lot!