Query about Safety Capacitor ratings

[GhostlyPea]

Hi all

I've spent a few days searching but can't find anything solid on the ratings for X Class Safety Caps. The general jist I get is that most caps are rated by their DC voltage, so for example, UK being '240v' RMS x 1.4 = about 336 peak. Now, I was looking at making a dropper simply for learning and got confused by the way the X2s I have are rated. Some results seem to indicate that they should still be rated for about 400V to take care of the peak, but some other sources indicate that as its an X2 it's already had the peak taken in to account. As the cap I have here is rated at 275VAC specifically, the latter seems to make sense and lines up with the last post in this thread:

https://www.eevblog.com/forum/beginners/capacitor-calc-for-powering-110vac-60hz-device-from-230vac-50hz/msg1548218/#msg1548218

Picture of the cap specifying VAC attached.

Would appreciate some feedback and preferbaly a decent link to something to read on this if available

TIA

[Vovk_Z]

What do you want from that cap? Do you want to use it in the DC circuit? I think it has to work up to about 275*1,41 VDC (But if I'm not wrong it is certified only for AC circuits).
X- and Y- caps are VAC rated because they are designed for AC circuits.

[GhostlyPea]

Hi

As mentioned I want to make a dropper PSU (Cap Dropper ofcourse in this case), so the intention is that cap would be in series with the 240vac Live wire before the rectifier. Essentially the confusion is that although it's rated at 240VAC, I am seeing a lot of mentions of it still not being within the peak range (i.e. 336v), however, as its specifically rated VAC not VDC it feels like that is already taken in to account and there forre should be fine with headroom.

Take the new attached photo (this is an example, not the one I wanted to build specifically). As you can see it's 240v mains, but the cap represented is 400V, which looking at the tech sheet is a VDC rating, which would I guess be fine for 240VAC as it's within tolerance. The cap I pictured is only 275V rated BUT it is rated as VAC not VDC as in this case

[GhostlyPea]

Thanks Blueskull

I found the charts that indicated the x1/x2 & y1/y2 peak safety ratings, it does leave me a little confused as to why there would be different values on the X class caps if it doesn't mean much - I found many at 140VAC, 275VAC and 400VAC which appear to somewhat correspond with ratings in various contries for 120v and 240v mains, but if its regulatory then I assum it's simply a demand - what would be the impact of using a 140VAC X2 on a 240V mains supply - from your wording I assume none?

I did try searching for some material to read up on about htis but couldn't find muh, I don't suppose you have a link to anything of use?

Thanks

[Gyro]

Hi

As mentioned I want to make a dropper PSU (Cap Dropper ofcourse in this case), so the intention is that cap would be in series with the 240vac Live wire before the rectifier. Essentially the confusion is that although it's rated at 240VAC, I am seeing a lot of mentions of it still not being within the peak range (i.e. 336v), however, as its specifically rated VAC not VDC it feels like that is already taken in to account and there forre should be fine with headroom.

Take the new attached photo (this is an example, not the one I wanted to build specifically). As you can see it's 240v mains, but the cap represented is 400V, which looking at the tech sheet is a VDC rating, which would I guess be fine for 240VAC as it's within tolerance. The cap I pictured is only 275V rated BUT it is rated as VAC not VDC as in this case

It's normal - in fact pretty much essential, to put a resistor in series with the capacitor. Whilst the capacitor will pass a certain known current on a normal sinusoidal mains waveform, it will pass a much higher current on sharp edged or high frequency interference spikes. A resistor of a few hundred ohms will absorb these spikes and prevent extra stresses on the rectifiers (the spikes won't make it past the electrolytic capacitor) and also unnecessary self healing of the series capacitor (which will quickly reduce its value).

It's best practice to make the series resistor a fusible type, so that it will break the circuit in the face of serious repetitive spikes, or in the case where the capacitor fails to self-heal properly.

[GhostlyPea]

Thanks both for the replies, just what I was after. I'll try and do a but more reading but otherwise happy

Thanks again 

[Alti]

I think you should also answer a question:
Why to use safety rated capacitor in a capacitive dropper?

AFAIK the idea of your supply is that it does not offer any protection, thus no safety rating needed. Well, had you used insulated capacitive dropper, then maybe this could have its place but otherwise - I do not see the point.

[wraper]

Some results seem to indicate that they should still be rated for about 400V to take care of the peak, but some other sources indicate that as its an X2 it's already had the peak taken in to account.

If you use regular film capacitor rated for 400 VDC it will fail quite fast. In general X2 capacitors can be considered to have somewhere around 630 VDC rating. Some X2 capacitors have VDC rating in datasheet, some don't.

[Vovk_Z]

As mentioned I want to make a dropper PSU (Cap Dropper ofcourse in this case)

- I think you know now that X2 caps will work fine in such a circuit.

[Gyro]

I think you should also answer a question:
Why to use safety rated capacitor in a capacitive dropper?

AFAIK the idea of your supply is that it does not offer any protection, thus no safety rating needed. Well, had you used insulated capacitive dropper, then maybe this could have its place but otherwise - I do not see the point.

Yes, interesting question. The capacitor doesn't actually need to be X rated as it's not a direct across the line application. A 630V rated capacitor would serve in this application. However it is important to ensure that the implementation is safe (as safe as a non-isolated PSU can be) particularly fire etc. An X rated capacitor carries agency approvals, which provides some level of assurance of flammability etc. even though it might have thinner metalisation and therefore more prone to capacitance loss through self-healing. I mentioned also, the use of a fusible resistor. Again, it's important that the implementation is as safe as possible.

With regard to the 'Insulated' PSU that you linked. That's a difficult one. Yes, having a series capacitor in both legs does make it L/N orientation insensitive, but it also guarantees that the circuit is floating at half-mains potential in all cases (with significant current capability) - It also means that the capacitors will be twice the value, further increasing the current capability in a contact to ground situation. Obviously, no part of the circuit or its inputs / outputs can be user accessible, they need to be treated as being at mains potential.

I think 'Insulated' is the wrong term for this circuit. it only provides any insulation with DC on the input (and of course no output in that situation). It is most certainly not insulated when driven by AC mains. [EDIT: The article also says "totally isolated from mains supply", which it is certainly NOT - this is a dangerous statement!]

As mentioned, it does make the circuit L/N insensitive which may help in some countries with non-polarised mains connectors, but as the OP is in the UK, with defined polarity on outlets, I think that the disadvantages outweigh the single advantage.

[Alti]

(..)I think 'Insulated' is the wrong term for this circuit. it only provides any insulation with DC on the input (and of course no output in that situation). It is most certainly not insulated when driven by AC mains.

If you remove the transformer from raw AC/DC supply (flyback), you are left with Y capacitor accross primary and secondary. Take two of those supplies, connect secondaries and you have insulated capacitive dropper from the schematic. 

[Gyro]

(..)I think 'Insulated' is the wrong term for this circuit. it only provides any insulation with DC on the input (and of course no output in that situation). It is most certainly not insulated when driven by AC mains.

If you remove the transformer from raw AC/DC supply (flyback), you are left with Y capacitor accross primary and secondary. Take two of those supplies, connect secondaries and you have insulated capacitive dropper from the schematic. 

Y caps are typically 2.2 to 4.7 nanoFarads,  The capacitors in that schematic are 2.2 microFarads.  A little tingle from a Y cap doesn't really compare with the Cardiac Fibrillating, RCD blowing current associated with a 2.2uF to live mains (approx. 165mA)! 

That supply is not insulated or isolated.


EDIT: Your referenced schematic attached for convenience...