Capacitors 101: What are they and how do they work?


Capacitors are essential components in the electronic industry, and recent shortages in the marketplace have led to steep price hikes. As the intricacy of existing smart devices continues to increase, so too does the quantitative need for capacitors to power them. Manufacturers are at the mercy of companies that now prioritize production of high-margin MLCC’s over traditional, low-margin capacitors.

There’s a mad scramble underway by car manufacturers, smartphone makers, Internet of Things startups and any other company whose products use electronics. That’s because capacitors, the most basic electronic components, are getting more expensive and harder to find.

Whether you tinker with circuits at home or design products for a living, capacitors are an everyday part of the electronics toolkit. But what are they? How do they work? And why are they getting so hard to find?

What is a capacitor?

Western scientists first discovered capacitors in the mid-1700s. One of those scientists, Benjamin Franklin, conducted capacitor experiments three decades before the American Revolution. Franklin even coined the term “battery” to describe the way a row of capacitors resembled a battery of canons.

Although the physics are different from a modern battery, a capacitor is a device that stores electrical charge. The physics of a capacitor also create other properties — blocking a direct current, for example, or passing changing currents.

What does a capacitor do?

Batteries are great for supplying constant flows of electric current. Capacitors show up when you need a specific amount of power delivered to a specific part of a circuit for a specific period.

Take a camera’s flash, for example. The slow flow of power from the phone’s battery can’t deliver enough electricity fast enough to power the flash. When you press the shutter button, it’s the camera’s capacitors that provide the short burst of electricity needed to light up the scene.

When electronics tear-down specialists iFixit took apart a Nikon camera a few years back, they found a capacitor the length of a finger that powered the camera’s flash. The iFixit writer also points out that, without the right tools, touching a capacitor that size could kill you — so don’t try this at home.

Another reason capacitors are used in circuit designs is to modify signals the circuit carries. This takes advantage of the way capacitors block direct current but pass the signal’s variable current.

By selecting the right properties, you can choose which frequencies pass through the capacitor. TVs and stereo equipment, for example, use a lot of capacitors to filter noise from power supplies and produce high-quality audio.

How does a capacitor work?

You can build a simple capacitor from a sheet of paper, two sheets of aluminum, a few wires, a resistor and a battery. Instructable user jwmiller’s Aluminum Foil Plat Capacitor lets you try it yourself. Just be careful: not following the instructions could give you a nasty shock.

The aluminum sheets are conductors. If they were allowed to touch each other, the current from the battery would flow straight through. The paper, on the other hand, is an insulator that blocks the current.

With nowhere else to go, the electrons pile up on one side of the paper gap and their negative charge pushes electrons away from the other side of the divide. The positively charged foil and the negatively charged foil are separated by the piece of paper that stops the growing charge from going anywhere.

As long as current is supplied to the capacitor, the charge will keep building until one of two things happens. In the worst case scenario, jwmiller has a clear warning: “Do not exceed 1400 V or the capacitor may ignite.”

With a commercial capacitor built into the right circuit design, the charging rate slows until the capacitor can’t hold anymore. It reaches its capacitance.

How do you calculate capacitance?

The DIY paper-and-aluminum capacitor is a real-world example of the parallel plate capacitor model you’ll see in textbooks. Two parallel conductive plates (the aluminum foil) are separated by a gap filled with another material called a dielectric (the sheet of paper).

The plates with surface area (A) are separated by a distance (d). The final element in the capacitor equation is a property of the dielectric material called its permittivity, 𝛆, which defines how easily the dielectric material forms an electric field.

This simple model is where the capacitor symbol used in circuit diagrams comes from. The circuit trace runs perpendicular into two lines (the plates) separated by a gap (the dielectric).

To calculate the capacitance (C) of a parallel plate capacitor, you’ll use the capacitor equation which looks like this:

C = 𝛆 (A/d)

The unit of capacitance is called the Farad. Most capacitors used in electronics have capacitance levels in the microfarads (µF) or picofarads (pF).

The capacitor equation tells you how design choices give capacitors different properties. You get higher capacitance, for example, as you shrink the gap between the two conductors.

You also get higher capacitance with larger areas. Of course, electronics wouldn’t be very mobile if capacitors used giant sheets of aluminum. Instead, capacitor manufacturers roll or fold the conductors and dielectrics into small packages. You get all the advantages of a large surface area crammed into a small space.

Your dielectric’s permittivity directly affects the capacitance. The lower the permittivity, the more the material resists forming an electric field and the smaller the charge the capacitor can hold. Dielectrics are measured against vacuum which has the worst permittivity at 8.85pF/m.

The polyethylene used in film capacitors is three times better than a vacuum while the titanium dioxide used in electrolytic capacitors is 86-173 times greater than a vacuum.

Ceramic capacitors using barium titanate have permittivities 1,200 to 10,000 times better than a vacuum.

Why is there a capacitor shortage in the market?

On the demand side of the equation, popular digital devices flooding the consumer market (tablets, laptops, etc.) are using exponentially more capacitors.

A recent report found that the iPhone X possesses a total of 1000 multi-layer ceramic conductors (MLCCs) compared to the iPhone 6’s 500, for example. Products that also traditionally lacked complex electronic interfaces are quickly becoming more reliant on digital technologies. The same report showed that a Tesla Model 5 uses 3-4 times as many MLCCs (in the neighborhood of 10,000) as a traditional car!

MLCCs are in high demand from manufacturers because of their ability to pack high capacitance in small forms. An MLCC, compared to jwmiller’s simplified capacitor experiment, contains thousands of alternating dielectric and conductor layers.

*For your curiosity, the capacitor equation for an MLCC (with n referring to number of layers) looks like this: C = 𝛆 (nA/d)

Only a handful of companies have the high-precision technology to make MLCCs — and they’re certainly making the most of it. Several of these aforementioned companies have discontinued or raised the prices on popular (but low-margin) capacitors to focus on their high-margin MLCC products. Not surprisingly, Seeking Alpha, a crowd-driven news aggregator for investors, expects tight MLCC supply for several more years to come.

Fortunately, the lasting impact of MLCC shortages on innovative circuit benders won’t likely be as severe. Unlike rigid manufacturers, circuit benders have the flexibility to alter existing circuit designs to integrate other types of capacitors. Startups and entrepreneurs, on the other hand, face tougher challenges. If their flagship products require the use of these limited available capacitors, prolonged shortages could shut them down entirely.

In a recent Stack Exchange discussion about the capacitor shortage, community members advised taking a less-optimized approach to picking capacitors.

One option is to switch to capacitors with a larger package size or a different dielectric. Another popular suggestion is to change the circuit design to use two or more capacitors in the place of one. While this method allows you to utilize capacitors with stronger supply, these approaches do have tradeoffs — namely in lower efficiency and higher production costs.

Have you experienced difficulties securing capacitors for your projects? Have you found a workaround to dampen the impact of shortages within the industry? Have any tips or advice you’d like to share? Let us know down in the comments!