non polarised capacitor

[paulhm81]

Hello

I 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? 

[bktemp]

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.

[Brumby]

This:

... but better get a cap specifically rated for mains operation. Those are X2 rated.

Aside 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.

[Seekonk]

Without  a country indicator we do not know what your line voltage is or if that cap is suitable.

[Visher]

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).

[wraper]

show a photo of this cap.

[wraper]

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

It sounds like they did know what they talked about but you were clueless.

[Simon]

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.

[wraper]

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.

You 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.

[Simon]

true

[paulhm81]

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_.jpg

so... 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

[bktemp]

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_.jpg

so... 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.

Maybe 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.

[paulhm81]

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_.jpg

so... 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.

Maybe 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.

I'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.

[wraper]

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.

[wraper]

so... 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.

That means they have a clue about what they sell.

[paulhm81]

so... 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.

That means they have a clue about what they sell.

Yes, 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!