Capacitor AC voltage rating - Page 1
Forum section: electronics / beginners

author: AlexK
author_rank: Newbie
timestamp: May 20, 2020, 06:58:00 pm
content: Hello,Suppose I have a capacitor with a DC voltage rating of VDC, which is subjected to a 50Hz sine wave with amplitude Vp.According to multiple sources, the AC voltage that can be applied should satisfy the condition: VACP-P< VDC.Why is that so? If the highest possible voltage across the capacitor terminals is Vp, why isn't the condition simply Vp < VDC.For example, in thisdocumentby TDK:Also, in this Kyoceraweb page:Kyocera continues,here, to say that:QuoteAgain, why is the capacitor AC voltage rating related to the peak-to-peak voltage and not just the peak voltage?Many thanks.

author: gbaddeley
author_rank: Regular Contributor
timestamp: May 21, 2020, 09:27:13 am
content: It could just be a rule of thumb. Capacitors that have a large AC voltage across them should be rated for AC Voltage. Eg. You should never use DC rated caps across AC mains. If you do, I would at least use DC rating > 2 Vpeak AC.

author: Gyro
author_rank: Super Contributor
timestamp: May 21, 2020, 09:43:15 am
content: Any capacitor used across the mains must be Class-X approved, so will automatically be AC rated.

author: gbaddeley
author_rank: Regular Contributor
timestamp: May 21, 2020, 12:06:20 pm
content: Agree. When I was young I and didn’t know better I used 630vdc poly caps as suppression across 240vac. They released their magic smoke after a couple of months.

author: AlexK
author_rank: Newbie
timestamp: May 21, 2020, 12:44:23 pm
content: Can you please explain why? What is it about AC voltage across a capacitor that stresses it more that DC?In your example a 240VAC has a peak voltage of 340V, why isn't the 630VDC rated cap enough?I understand that there is additional power dissipation related to the ESR, but that is more dominant at high frequencies and not so much at 50/60Hz, correct?

author: TimFox
author_rank: Super Contributor
timestamp: May 21, 2020, 01:31:33 pm
content: When a large AC voltage is applied to a capacitor, there will be an AC current flowing through it (called a displacement current in physics). This current also flows through the ESR resistance of the capacitor, which will cause heat and stress the capacitor. (The current through an ideal capacitor with ESR=0 does not cause heat.) The rating does work in the reverse direction, though: a capacitor rated for 200 VAC should be good for 280 VDC.ESR is not constant with frequency in practical capacitors. In film capacitors, the Q value is roughly constant so the ESR falls with frequency as the reactance falls. Electrolytic capacitors typically are dominated by the parasitic series resistance, so the ESR depends less on the reactance, but you normally don't put AC into a polarized capacitor: they are rated for ripple current, caused by a small AC voltage superimposed on the larger DC voltage in a typical power supply rectifier-filter circuit.

author: AlexK
author_rank: Newbie
timestamp: May 21, 2020, 04:01:29 pm
content: So, my assumption that heating losses at 50Hz are negligible was wrong, and the ESR has to be taken into account even at low frequencies. Is that correct?And the VACP-P< VDC is probably just a rule of thumb that should be followed when no AC rating is provided?Thank you for the informative replies.

author: Gyro
author_rank: Super Contributor
timestamp: May 21, 2020, 05:36:12 pm
content: It's not really possible to say at what frequency heating losses become negligible (and at what voltages) at it is depends on several factors...- Capacitance value (and capacitance vs package size). Like many things, the outer surface area of a capacitor tends to decrease relative to its capacitance value as the capacitance increases. This means that it has increasing difficulty in dissipating the heat caused by ESR and loss factor. 50Hz may be very relevant for a large value capacitor but insignificant for much smaller values.- Loss factor. This is dependent the dielectric used, from low loss, like polystyrene and polypropylene down through other plastics. Thermal dissipation due to dielectric losses is again strongly dependent on frequency for a given capacitance value. Bipolar electrolytics are a particularly nasty one.- Construction. This affects the parasitic inductance of the capacitor and the resistive ESR component - the heat dissipated due to resistance between the 'plate' area and terminations. The type of construction can also affect things like avoiding voids where ionisation can take place, degrading the dielectric.These are all factors (and more) which manufacturers must take into account for a particular capacitor type in order to provide usage information.Where AC mains is concerned, despite the fact that it is low frequency, is that it is also a source of low impedance, high voltage spikes. This was probably the cause of the failures suffered by gbaddeley. To be able to withstand these spikes, Class X capacitors are of thin metallised film construction, where, when the dielectric fails, the metallisation evaporates back to 'heal' the capacitor (loosing some value in the process) rather than shorting. These have specific flammability ratings too.EDIT: Unfortunately there are lots of rules of thumb, which work most of the time, but then suddenly fail in a specific situation.EDIT1: Looking back, I just noticed that the TDK document you linked relates specifically to Multilayer Ceramic Chip Capacitors (MLCCs). There are all sorts of factors in these, compared to, say, a polypropylene film. It's very difficult to generalise.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 21, 2020, 07:05:10 pm
content: Hi,This depends on the technology used to make the capacitor. In a film there can be voids in the construction. Corona will happen in the voids and this will degrade the insulation until it fails.The corona inception voltage is about 200V. So an AC capacitor is typically constructed with two or more sections in series.This is two capacitors in series. If one of the capacitors breaks down, there is another one in series.A high voltage dc capacitor may be a single capacitor.Reference from WimaRegards,Jay_Diddy_B