Capacitor Film: The Properties, Construction, and Application

What Is a Film Capacitor?

The capacitor film is a device (non-polarized capacitor) that has energy storage and thin plastic films as the dielectric. You can also call the specific application device a metalized capacitor because manufacturers metalize the plastic films sometimes.

Further, you’ll notice that some of the plastic films are in layers—to form a shape (rectangular or candy-like). The available dielectrics are PTFE (Polytetrafluoroethylene), PET (Polyethylene terephthalate), PP (Polypropylene), or PPS (Polyphenylene Sulfide).

So, what are the benefits of using the capacitor film?

First off, the device doesn’t wear off quickly. Hence, the device is effective for high-frequency and high-voltage applications. Second, they have remarkable frequency characteristics thanks to the device’s deficient distortion factor. Also, it serves as motor run capacitors and AC capacitors.

Features of the Film Capacitor

The capacitor film is unpolarized and has no negative temperature characteristics. So, you can use them for power supplies and AC signals that support high-power applications, and it’s also an ideal option for industrial applications.

But that’s not all.

The capacitor film has high-precision capacitor values with a high-current pulse. So, they maintain values longer than other types of capacitors with weak spots.

Also, the device has a meager failure rate, low self-inductance, dissipation factor, and Equivalent Series Resistance (ESR).

Again, the film capacitor can withstand high currents thanks to its special screw-type terminals and compact dimensions.

If you’re looking for a capacitor with a proper starting and over 200 volt-amperes capacity, you can count on it. So, it’s not a surprise that it has a long service and shelf life. And it has no poor power factor like some other capacitors.

Due to their metallic or metal foil electrodes, film capacitors can also offer high surge current pulses.

The device’s voltage rating ranges from 50V to 2KV to withstand various current pulse loads.

Construction of Film Capacitors

The first step to constructing a capacitor film is getting a thin layer of the plastic film even with an additional layer. So, the thickness you choose determines the capacitance value of the device and layer of paper.

And the thickness of your plastic film affects the distance between electrodes. Hence, if your film’s viscosity is low, your electrode distance will reduce. But your capacitance value will increase.

Typically, the capacitor’s capacitance value ranges from 1nF to 30muF. That said, when you extract your film based on your preferred breakdown voltage and capacitance value, metalize your capacitor.

And you can metalize it with zinc or aluminum. Then, proceed to make a “mother roll.” While you’re at it, ensure that you interlace your films between the aluminum sheets to create the roll.

With this, you can allow the roll to go through different processes like flattening, slitting, and winding. That way, you’ll get your preferred capacitor size, electrical features, and rectangular components.

Afterward, subject your projecting electrodes to Schoopage—a metalizing process. The procedure involves putting a protective layer on your electrodes using liquefied metals like tin, zinc, or aluminum. With this layer, your capacitor will gain a high-temperature resistance.

Next, use compressed air to spray the edges of the winding and subject it to a voltage. This process will help burn out existing defects on the electrode’s surface.

Also, moisture tends to affect the capacitor easily. But you can handle that by infusing it with an insulating fluid, e.g., silicone oil.

Then, proceed to solder your winding to the capacitor’s metallic terminals. Afterward, subject the capacitor to another round of safety coating. Then, dip the body into an external casing or protected coating.

How Does the Film Capacitor Work?

The film capacitor works the same way a capacitor works. That is, the electrode stores the electric charge and energy. Then, the device uses it alongside the inductor to create LC oscillator circuits.

So, how do you test the capacitor film? You can do this by setting a multimeter to read in Ohms (between 10K and 1m). Then, touch the multimeter’s leads connected to the capacitor’s respective leads (black to negative and red to positive). With this, your device should start reading from zero and move towards infinity slowly.

Also, you can know a film capacitor’s polarity by observing the height of the leads. The shorter terminal is the cathode or negative polarity. And the longer term is the anode or positive polarity.

Understanding the Difference Between the Film Capacitor and Other Plastic Film Dielectrics

By now, you should have an idea of the film capacitor. So, in this section, we’ll talk about the different film capacitors based on the dielectric material used.

Polyester Capacitor

This device is amongst the widely used dielectric materials. The Polyester capacitor has a high dielectric constant compared to its other counterpart, like polypropylene. Interestingly, the device’s dielectric constant permits the construction of small-sized capacitors.

Also, you can refer to this device as a Mylar capacitor. Plus, the Polyester capacitor is affordable and has excellent self-healing properties.

Further, the device dispels more power in high temperatures. Also, when the temperature reaches a low or high limit, this capacitor shows up to a 5% capacitance change. So, Polyester is not the ideal option for creating precision capacitors.

Polycarbonate Capacitor

Mostly, you’ll find polycarbonate capacitors in high-temperature applications. And it’s because the device has a dielectric constant of about 2.7. No doubt, polycarbonate film is not always available.

But the device has good electrical features over a wide temperature range, and it’s a low loss component.

Polystyrene Capacitor

The Polystyrene capacitor has a low dielectric constant. So, you can use them for high stability and typical capacitance applications. Also, it can stop capacitance stability loss (high or low) over temperature (-55⁰ to +85⁰ C).

Kapton (Polyimide) Capacitor

The Kapton is more effective for creating components for high-temperature applications. And it’s because this capacitor has a high dielectric constant of about 3.4. However, the metalized capacitors have poor self-healing capability.

PTFE Capacitor

This capacitor is yet another low-loss device that provides remarkable stability. Plus, it’s ideal for creating small-footprint components. And it’s because the PTFE isn’t a good match for high-temperature applications. Also, they have low capacitance value and are pretty pricey.

PPS Capacitor

The PPS is the perfect go-to for creating precision capacitors. And it’s all thanks to its remarkable temperature characteristics. Further, the PPS capacitor and polycarbonate capacitor have breakdown strength and similar dielectric constant. Hence, you can replace the polycarbonate with PPS in electric circuits. It also has a high self-healing capability.

What are the Types of Film Capacitors Available?

We have two types of plastic film capacitors, and they are:

1. Metallized film capacitors:

Metalized Film Capacitor

This device has a dielectric that consists of two metalized films alongside a plastic film. Also, the electrode of this capacitor comes from a thin vacuum-deposited aluminum (about 0.03µm) metallization—that coats one or both sides of the device.

Further, when there’s a short circuit between the electrodes, the component may remain intact. The drawback of this device is that it has a limited current surge rating. But you can use this capacitor to produce quality products without defects.

2. Foil Capacitors: 

The foil capacitors have a dielectric with two plastic films. Each of the electrodes has a layer of metal foil (aluminum most times). So, you can rely on this construction for an easy electrical connection to the electrodes (metal foil). Plus, the foil capacitors can handle high current surges.

How is the Film Capacitor Different from the Electrolytic Capacitor and Ceramic Capacitor?

First off, all three capacitors have different dielectrics—which determines their performance. That is, the film capacitors come in a wide range of capacitance values. However, the electrolytic and ceramic capacitors have a low distortion factor, and hence, they are ideal for circuits with low capacitance.

Second, film capacitors have thin sheets of plastic films for dielectric that are bipolar. On the other hand, the other two capacitors (ceramic & electrolytic) have sheets of ceramic material that are bipolar and oxides that are polar, respectively.

The film capacitor is best suited for high-voltage and high-frequency applications because it has a long aging time. However, the other ceramic capacitors and electrolytic capacitors have a shorter aging time.

Applications of the Capacitor Film

• Audio crossovers
• A/D convertors
• Pulsed lasers
• Power electronic devices
• Safety capacitors
• X-ray flashes
• Snubber capacitors
• Interference suppression
• Voltage smoothing capacitors

Wrapping Up

The Capacitor film is a non-polarized device that has superior features. Hence, they are ideal for power use and AC signals. And they have high-precision capacitance values. Plus, the device has a longer aging time compared to other capacitors.

So, the film capacitor is the perfect go-to if you require a reliable capacitor that can withstand high voltages. Do you need more help with the topic? Please feel free to contact us.