Capacitor (film)
Capacitor
A capacitor using a plastic film dielectric. Non-polarised, very stable, low loss and self-healing — the go-to choice for audio, timing, snubbers and anything touching mains.
Schematic symbol
Capacitor (non-polarised)
How capacitor (film) appears in a circuit diagram.
Types & variants
Key specs
Capacitance
FTypically 1 nF – 10 µF; larger than that gets physically big.
Voltage rating
VCommonly 63 V – 630 V DC; X2 types rated 275 V AC.
Dielectric
PP = lowest loss and best stability; PET = smaller and cheaper.
Tolerance
%±5% or ±10% typical — far better than electrolytics.
Laws that govern it
- Law of capacitance — capacitance formula Q = CVQ = C × V, i = C·dv/dt, E = ½CV²
Very stable Q = CV with low loss, so film caps are the ones used in timing, filters and audio where the value must not drift.
- Gauss's lawΦ(E) = Q(enc) / ε₀
The plastic dielectric's permittivity and thickness set both the capacitance and the voltage the field can be pushed to before puncture.
Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.
How it compares
- Ceramic vs Film Capacitor
Side by side with Capacitor (ceramic / MLCC) — specs, variants and which one the job calls for.
Markings
Value printed directly or as a 3-digit pF code (e.g. 474 = 470 nF) plus voltage; X2/Y2 safety marks on mains-rated parts.
Standard values
E6/E12 values from 1 nF to 10 µF. 100 nF/630 V and 470 nF X2 are bench staples.
How to choose
Polypropylene for audio, snubbers and precision timing; polyester for general coupling. Across mains, only use X2 (line-to-line) or Y2 (line-to-earth) safety-rated parts.
Pinout & package
2 leads, not polarised. Boxed (radial) or dipped; big motor-run types come in cans with tabs.
Example circuits
- Mains input filter (X2) in a power supply
- Loudspeaker crossover
- RC snubber across a relay contact
Common failures
Very reliable. Metallised types self-heal from small punctures, losing a little capacitance over time; X2 caps across mains slowly drift down in value.
How to test
Capacitance mode on a multimeter (discharge first). Should read within tolerance; a big drop in an X2 filter cap explains a dead capacitive-dropper supply.
Substitutes
Another film cap of equal or better voltage rating; ceramic (C0G) works for small values. Never substitute a plain cap where X2/Y2 safety rating is required.