400V film capacitor: voltage vs capacitance - Page 1
Forum section: electronics / beginners

author: Faringdon
author_rank: Super Contributor
timestamp: October 21, 2023, 09:13:54 pm
content: Hi,Does anyone have a graph of capacitance vs voltage for an ~400V film cap? (any value)Ie, the capacitance from 0V to 400V?It would truly be amazing if any 400V film capacitor, offered the same capacitance at say 5V, as at 400V.

author: BeBuLamar
author_rank: Super Contributor
timestamp: October 21, 2023, 09:18:18 pm
content: I don't know if the capacitance varies with voltage. I always assume that the capacitance stays the same regarless of voltage.

author: Zero999
author_rank: Super Contributor
timestamp: October 21, 2023, 09:20:16 pm
content: Film capacitors are very good. They don't saturate like ceramic capacitors do.

author: TimFox
author_rank: Super Contributor
timestamp: October 21, 2023, 10:44:54 pm
content: For non-linear ceramic dielectrics (X7R and worse), look up the term "ferroelectric".

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 22, 2023, 08:50:58 am
content: Joking, butlearn to use Google.

author: Faringdon
author_rank: Super Contributor
timestamp: October 22, 2023, 09:25:44 am
content: Thanks, we have a 10nF , 400V film capacitor, in circuit it has a sine on it of peak -80V to +80V......we rely on this cap for timing....near the zero cross this cap obviously has near zero volts on it, and we still rely on it being 10nF at such low voltage...i wonder if this is possible?I looked into ferroelectric caps, and if plastic film is ferroelectric, then it may mean a kind of polarisation occurs, and maybe this messes things up near the zero cross. Still, some of the circuits work, and some dont, even though they all have the same 10nF,400V cap in them.......and sometimes the working ccts stop working, and vice versa.

author: Gyro
author_rank: Super Contributor
timestamp: October 22, 2023, 09:34:54 am
content: Of course plastic film capacitors aren't ferroelectric.

author: Faringdon
author_rank: Super Contributor
timestamp: October 22, 2023, 12:37:01 pm
content: Thanks, i actually wonder if a slight bit of moisture has got into the capacitor, and that at low voltages, this is having the effect of changing its capacitance at low voltages? Or maybe the capacitor is just old, and its chemistry has changed such that at low voltages it appears to loose capacitance?

author: temperance
author_rank: Super Contributor
timestamp: October 22, 2023, 12:48:12 pm
content: Is this cap exposed to mains transients?

author: DavidAlfa
author_rank: Super Contributor
timestamp: October 22, 2023, 01:22:58 pm
content: Why don't you expose the question/issue properly?Why would a film capacitor lose capacitance?Because they simply fail internally and part of it becomes disconnected.This is pretty common caused by moisture or high voltage trasients, normally the capacitance drops by a lot, like going from 2uF to 10nF.Replace it, carry on with something else, there's no chemical reaction going on inside MKT / MKP caps, they're made with plain metallized dielectric foil.

author: dmills
author_rank: Super Contributor
timestamp: October 22, 2023, 06:21:47 pm
content: Good film parts are very, very stable and have negligible voltage coefficient (Not quite as good as C0G, but you can get PP or PPS in higher values), that is mostly a class II ceramic trick.I have PP film pats in the several uF class carrying amps of current at 20kHz and under 100V and never see a failure and measure negligible harmonic distortion so the things are LINEAR.The failure mode you sometimes see (Old RIFA across the mains are notorious for it) is moisture penetration that then boils and blows the casing apart in a cloud of metallised plastic and steam, makes one hell of a mess, or worse the same failure but in a potted assembly where the heat generated melts the encapsulant and the steam explosion then throws horrible black gunk all over the insides of the doings.Self healing parts (Class Y and such) will internally blow parts of the metallisation around punctures open, and so loose capacitance over time, this is very much a feature not a bug.

author: TimFox
author_rank: Super Contributor
timestamp: October 22, 2023, 06:25:10 pm
content: Some film capacitors are also self-healing, where the metallization around a pinhole flaw disappears and a small permanent reduction in capacitance results.This is not a voltage effect, as in a non-linear capacitor dielectric.

author: ejeffrey
author_rank: Super Contributor
timestamp: October 22, 2023, 06:31:50 pm
content: Why don't you measure it? This is easy enough to measure if you have a capacitance meter and a bias voltage source. Put two capacitors in series and measure them with an LCR meter. Then bias the center node through a 100 k to 1 M resistor.

author: jonpaul
author_rank: Super Contributor
timestamp: October 22, 2023, 06:32:40 pm
content: photo of cap with part no?Dielectric type eg polyester, polyethelene, met paper, kynar?Your measurement technique : What V, F, bridge?Jon