Complete guide to polyester capacitors (Mylar/PET): key specs, temperature coefficient limits, uses in coupling and timing, and when to choose polypropylene instead.

There is a capacitor type that sits quietly on tens of millions of boards, rarely causes trouble, never gets discussed in design reviews, and costs so little that engineers barely notice it on the BOM. That is the polyester capacitor — also called a Mylar capacitor, PET capacitor, or MKT capacitor depending on who is writing the datasheet. It is not glamorous. It does not have the precision of a C0G ceramic or the volumetric density of a tantalum. But for a wide range of general-purpose circuit functions, the polyester capacitor is exactly the right tool: inexpensive, robust, non-polarized, tolerant of voltage spikes, and stable enough for most applications that do not need laboratory-grade accuracy.

If you have been specifying X7R ceramics or tantalum electrolytics for everything by habit, this guide will help you see where the polyester capacitor actually belongs — and where its real limitations start.

What Is a Polyester Capacitor?

A polyester capacitor is a type of film capacitor that uses polyethylene terephthalate (PET) as its dielectric material. PET is better known by its trade names — DuPont’s Mylar and Hoechst’s Hostaphan are the most widely cited — which is why these parts are so often called Mylar capacitors in the field. The terms are used interchangeably, and in European documentation you will also see the abbreviations KT (film/foil construction) and MKT (metallized film construction).

The dielectric constant (relative permittivity) of polyester is approximately 3.2, which is higher than polypropylene (2.2) but lower than ferroelectric ceramics. That relatively high permittivity — combined with the ability to manufacture PET film down to 0.7 µm thick — is what allows polyester capacitors to achieve compact physical sizes for a given capacitance value. This is why polyester has captured roughly a 40% share of the global film capacitor market, second only to polypropylene.

Film/Foil vs. Metallized Construction

Polyester capacitors come in two internal construction variants, and understanding the difference matters for some applications.

Film/foil (KT) types use separate aluminum foil electrodes wound with the PET film. These have slightly lower ESR and can handle higher instantaneous current surges, making them preferred for pulse applications.

Metallized film (MKT) types deposit a thin aluminum or zinc electrode directly onto the PET film. The key benefit is self-healing: when a local breakdown occurs in the dielectric, the energy of the arc vaporizes the thin metallization around the fault, isolating it without destroying the part. Metallized types are more compact for a given capacitance, lower cost, and the most common type found in general electronics.

Key Electrical Specifications of Polyester Capacitors

Understanding the numbers on a datasheet helps you pick the right part and avoid misapplication.

Parameter	Typical Value / Range
Dielectric Material	Polyethylene Terephthalate (PET / Mylar)
Capacitance Range	1 nF to 15 µF
Voltage Rating (DC)	50 V to 1,500 V (common range)
Tolerance	±5%, ±10%, ±20%
Operating Temperature Range	−55°C to +125°C
Temperature Coefficient	Non-linear; up to +5% to +6% change (25°C to 85°C)
Dielectric Absorption	0.2% to 0.5%
Dissipation Factor (tan δ)	~0.5% to 1% at 1 kHz
Insulation Resistance	>10,000 MΩ (typ.)
Polarity	Non-polarized
Package	Leaded only (through-hole); not available in SMD for standard PET
Self-Healing	Yes (metallized types)

Temperature Coefficient: The Critical Limitation

The non-linear temperature coefficient is the specification PCB engineers most often underestimate. Between 25°C and 85°C, the capacitance of a polyester capacitor increases with rising temperature — up to +5% or +6% total change. At low temperatures approaching −40°C to −55°C, it dips back down. This non-linear, somewhat unpredictable behavior is acceptable for general-purpose blocking and bypass functions, but it disqualifies the polyester capacitor from precision timing, precision filtering, and frequency-determining circuits where the value needs to stay tight across temperature.

If your application needs stable capacitance over temperature, polypropylene (PP) is the right film capacitor choice, with a much more stable −2.5% linear temperature coefficient over its operating range. The polyester type trades that stability for smaller size and lower cost.

Dielectric Absorption: Why It Matters in Precision Circuits

Dielectric absorption — sometimes called “soakage” or voltage memory — is a phenomenon where the dielectric retains a partial charge after the capacitor is fully discharged. In polyester capacitors, this is typically 0.2% to 0.5%, compared to 0.02% for polypropylene and near zero for C0G ceramics. In most circuits this is irrelevant. But in sample-and-hold circuits, charge redistribution networks, precision integrators, and DAC/ADC front ends, dielectric absorption introduces a residual voltage error that can corrupt measurements. For those applications, reach for C0G ceramic or polypropylene film instead.

Polyester Capacitor vs. Other Film Types: Side-by-Side Comparison

Property	Polyester (PET/Mylar)	Polypropylene (PP)	Polystyrene (PS)
Dielectric Constant (εr)	3.2	2.2	2.5
Capacitance per Volume	High	Lower	Lower
Temperature Coefficient	Non-linear, up to +6%	Linear, ±2.5%	Negative, very low
Dielectric Absorption	0.2–0.5%	~0.02%	~0.05%
Max Operating Temp	+125°C	+105°C	+85°C
SMD Availability	Limited (PEN preferred for SMD)	No	No
Cost	Low	Medium	High / hard to source
Best For	General purpose, coupling, blocking	Precision, audio, RF snubbers	Precision (now largely obsolete)
Available Max Voltage	Up to 1,500 V	Up to 3,000 V+	Low-voltage

The conclusion from this table is clear: polyester wins on cost, size, and temperature ceiling. Polypropylene wins on precision, low loss, and dielectric absorption. For a seasoned engineer, this decision usually takes about five seconds once you know what the capacitor is doing in the circuit.

What Polyester Capacitors Are Good For

General-Purpose Decoupling and Bypass

For decoupling on circuits where exact capacitance is not critical and the operating frequency is below the self-resonant point of the part (typically below 1 MHz for common leaded polyester values), polyester capacitors offer a low-cost, robust solution. Unlike aluminum electrolytic capacitors, they have no polarity concern, no aging mechanism, and no electrolyte to evaporate. For bypass duties in analog circuits running at audio frequencies, a polyester capacitor does the job reliably for the life of the product.

For understanding how bypass capacitor placement on a PCB affects decoupling performance, the PCB capacitor design guide at RayPCB provides practical layout guidance that applies equally to film capacitor placement.

Signal Coupling and DC Blocking

High insulation resistance — often exceeding 10,000 MΩ — makes polyester capacitors well suited for DC blocking and signal coupling. In audio preamplifiers, tone control stages, and interstage coupling in analog circuits operating below 100 kHz, a polyester capacitor passes the signal cleanly while blocking any DC offset. The self-healing property of metallized types provides additional robustness if the circuit is subjected to transient voltages beyond normal operating conditions.

Semi-Critical Timing Circuits

Where timing accuracy requirements are moderate — say, ±5% to ±10% is acceptable — polyester capacitors work well in RC timing networks. Oscillators and monostable circuits in non-precision instruments, consumer timers, and basic control circuits all fall into this category. The key word is “moderate.” If your timing window needs to hold tighter than ±2% across the full operating temperature range, move to polypropylene or C0G ceramic.

Audio Circuits: Acceptable for Many Roles, Not All

Polyester capacitors are widely used in non-critical audio coupling positions. For power supply bypassing, interstage coupling in budget gear, and tone control networks, they are entirely adequate and cost-effective. Their dielectric absorption of 0.2–0.5% introduces a small but measurable harmonic distortion component, particularly at lower frequencies. In high-end audio designs — where engineers argue seriously about dielectric materials — polyester is the type most often cited as introducing a slightly gritty or harsh character compared to polypropylene or polystyrene. That comparison matters mainly in the signal path of quality audio equipment. In power supply filtering and non-signal-path roles, the audio distinction is irrelevant.

Noise Suppression and EMI Filtering

Metallized polyester film capacitors are used in across-the-line (X-capacitor) applications and in line-to-ground (Y-capacitor) applications in AC power entry EMI filters. For X-capacitor positions — connected directly across AC line voltage — safety-rated types with appropriate class certification (X1, X2) are specifically designed for this role. The self-healing property of metallized film is essential here, as the capacitor may experience transient overvoltage events from the AC line. Do not use non-safety-rated polyester capacitors in X or Y positions; use parts explicitly safety-certified for mains voltage application.

Replacing Tantalum in Some Low-Voltage Roles

For simple higher-frequency filter circuits, or in voltage regulator or voltage doubler circuits, low-cost metallized polyester film capacitors can provide long-term stability and replace more expensive tantalum electrolytic capacitors — particularly where the capacitance values needed fall within the polyester range (up to ~1–4.7 µF practically) and there is no polarity concern.

Where Polyester Capacitors Do Not Belong

Being explicit about limitations prevents misapplication.

High-frequency circuits above 1 MHz: The wound construction of a leaded polyester capacitor introduces significant series inductance. Above a few hundred kHz, the impedance behavior starts to depart from ideal. For RF bypass, high-frequency decoupling, or filtering at switching frequencies above 500 kHz, SMD ceramic capacitors are the correct choice.

Precision timing and frequency-determining circuits: The non-linear temperature coefficient up to +6% rules polyester out. Use C0G ceramic or polypropylene film.

Precision sample-and-hold circuits and integrators: Dielectric absorption of 0.2–0.5% introduces charge memory errors. Use polypropylene or C0G.

Direct AC mains connection (without safety rating): Standard polyester capacitors are not rated for mains-connected applications. Use safety-certified X/Y class types.

Tight-tolerance applications: Polyester is available in ±5% at best for most catalog parts. Polypropylene reaches ±1% and better.

How to Identify a Polyester Capacitor on a Board or in Stock

Recognizing polyester capacitors by appearance prevents substitution errors during repair or assembly verification.

Feature	Typical Appearance
Body shape	Rectangular or oval block, sometimes disc-shaped for low values
Body color	Yellow, orange, green, or blue (varies by manufacturer)
Lead style	Radial (both leads from same end) or axial (one lead each end)
Markings	Capacitance in nF or µF, voltage rating, tolerance code
Standard codes	“MKT” or “KT” in series name; “PET” or “polyester” on label

Common manufacturer color conventions: WIMA polyester capacitors are typically yellow; Epcos/TDK and Panasonic types appear in various colors. Polypropylene types from WIMA are their iconic red. Be careful not to assume color alone indicates dielectric type across brands.

Reading Polyester Capacitor Value Markings

Leaded polyester capacitors use a three-digit code similar to resistors, where the value is in picofarads:

Marking	Decoded Value
104	10 × 10⁴ pF = 100,000 pF = 100 nF = 0.1 µF
473	47 × 10³ pF = 47,000 pF = 47 nF
222	22 × 10² pF = 2,200 pF = 2.2 nF
105	10 × 10⁵ pF = 1,000,000 pF = 1 µF

The tolerance code follows (J = ±5%, K = ±10%, M = ±20%), followed by the voltage rating.

Useful Resources for Polyester Capacitor Selection

Resource	What It Offers	Link
WIMA Product Catalog	Technical data, tolerances, and application notes for MKS/MKT series	wima.com
Vishay Film Capacitor Selector	Parametric search for Vishay polyester and film capacitor series	vishay.com
Panasonic ECQ-E / ECQ-B Series	Widely stocked polyester and polypropylene film capacitors	industry.panasonic.com
KEMET Film Capacitors	R82, R60 series polyester and polypropylene, with simulation tools	kemet.com
DigiKey Parametric Search	Filter by dielectric (PET/polyester), voltage, capacitance, lead style	digikey.com
Mouser Electronics	Wide inventory with datasheet access, filter by MKT/KT/film type	mouser.com
Passive Components EU	Deep technical articles on film capacitor dielectrics and construction	passive-components.eu

Frequently Asked Questions About Polyester Capacitors

1. Is a Mylar capacitor the same as a polyester capacitor?

Yes, completely. Mylar is the trade name that DuPont registered for its biaxially-oriented polyethylene terephthalate (BoPET) film. When a manufacturer calls a capacitor a Mylar capacitor, the dielectric is PET — the same material used in polyester capacitors. The two names describe the same component, and they appear interchangeably in datasheets, BOMs, and component databases. You will also see “PET capacitor” and “MKT capacitor” (in European nomenclature) referring to the same part.

2. Can I use a polyester capacitor in place of a ceramic capacitor?

Often, but not always — it depends heavily on what the capacitor is doing. For DC blocking, general coupling, and low-frequency bypass, a non-polarized polyester capacitor is a workable substitute for a ceramic in the same value and voltage range. The important caveats are physical size (leaded polyester capacitors are larger than equivalent SMD ceramics), frequency performance (ceramic outperforms at high frequencies), and temperature stability (X7R ceramic and polyester are broadly comparable, but C0G ceramic is significantly more stable than polyester). Never substitute polyester for a ceramic in a surface-mount footprint — polyester capacitors are not available in standard SMD packages.

3. Why are polyester capacitors not available in SMD packages?

Standard PET film has a maximum rated temperature of 125°C, but the reflow soldering process for SMD components typically peaks at 260°C — well above what PET film can survive without dimensional distortion that degrades the dielectric. Polyphenylene naphthalate (PEN) film has better high-temperature stability and is used for SMD film capacitors. Standard polyester capacitors are made for through-hole assembly only.

4. When should I choose polypropylene over polyester film?

Choose polypropylene when any of these conditions apply: your circuit requires stable capacitance over the operating temperature range (PP changes about −3% from 25°C to 100°C versus +6% for PET); you need low dielectric absorption for precision sample-and-hold, integrator, or DAC applications; you are designing an audio signal path where distortion introduced by dielectric absorption is audible; or you need high-voltage ratings above 1 kV. The tradeoff is that polypropylene capacitors are physically larger for the same capacitance and cost more. For general-purpose coupling and bypass where these factors do not matter, polyester remains the more practical choice.

5. Do polyester capacitors have a polarity?

No. Polyester capacitors are non-polarized, just like ceramic capacitors. They can be installed in either orientation without any effect on performance or longevity. This is a significant practical advantage over aluminum electrolytic or tantalum capacitors, which require correct polarity and will fail if reversed. In any circuit position where an AC signal or alternating voltage is present — coupling capacitors, crossover networks, tone controls — the non-polarized nature of the polyester capacitor is a straightforward advantage.

Summary: Where the Polyester Capacitor Fits

The polyester capacitor earns its enormous market share by being genuinely excellent at a specific, well-defined set of tasks: general-purpose DC blocking, signal coupling at audio and sub-RF frequencies, semi-critical timing in non-precision circuits, noise suppression in AC power entry filters, and low-frequency bypassing in analog circuits. It does these jobs reliably, inexpensively, without polarity concerns, without aging, and without the failure modes associated with electrolytic types.

It is not a precision component. Its non-linear temperature coefficient and comparatively high dielectric absorption mean it does not belong in frequency-determining networks, precision integrators, or the signal path of high-end audio equipment. In those positions, polypropylene film or C0G ceramic is the engineering-correct choice.

Use the polyester capacitor deliberately — knowing exactly what function it serves and what performance margins your circuit tolerates — and it will likely be one of the most trouble-free parts on your board.