What Does a Capacitor Do?

We have capacitors in pretty much everything. They’re in your electronics to act as filtering mechanisms and they’re in your air conditioners and industrial equipment. What could be so versatile that it’s basically everywhere?

Storing a Charge

Capacitors are essentially short term batteries. They generally store small amounts of power, charge quickly, and discharge quickly. In cameras, they’re used to create massive charges that power the camera’s flash. The battery cannot provide power fast enough, but it does have enough power. A capacitor is capable of discharging it’s full charge in fractions of a second, making it ideal to provide a boost to the flash.

Why can’t batteries work like capacitors?

Capacitors don’t store a charge chemically. They store their charges electro-statically. There’s two metals inside a capacitor, separated by non-conductive material. Charge flows into one metal, creates an electromagnetic field, and the other metal converts that field back into electricity. If this sounds like a transformer, you’re on the right path.

The key difference here is design. The capacitor isn’t designed to change voltages. It’s designed to store power in a magnetic field. This is essentially what makes it so fast to charge and discharge. You can have power built up inside that just needs a way out. With batteries, the challenge is getting the power out, electrons can only move so fast in the dense battery chemistry. That issue isn’t present here.

Use in Air Conditioners

Inside an air conditioner, there’s usually two capacitors: a start capacitor and a run capacitor. These are usually beefy devices, about the size of a soda can on the smaller end for your compressor. Their job is to alter the current going into the windings, inducing a rotating magnetic field, which causes the motor to spin.

The start capacitor’s specific job is to increase the power going to the motor. This increased power will create a massive amount of torque. This increased torque allows the motor to overcome any friction and the load against it quickly and get up to running speed. The downside is that it’s horribly inefficient. Imagine diving on a highway, in first gear, at 50 miles an hour. Your engine has to be rev’d hard, burning a ton of gas to spin fast enough to turn first gear and get you moving. At that point, you don’t need all that torque to keep moving.

This is where the run capacitor comes in. A switch is triggered that will move the motor’s power from the start capacitor to the run capacitor. The run capacitor is designed differently. Rather than maximize torque, it just has to provide enough power to keep the fan spinning. The different design increases the efficiency. It’s not supplying a ton of gas, it’s making the motor sip and run at a reasonable speed.

Use Elsewhere

Other capacitors are used to filter and decouple electronics from variations in electrical power. For example, have you ever had a brief power outtage, fractions of a second, but your electronics stayed running? Blame the capacitors. Most electronic devices use capacitors to account for variances in how much power they receive, allowing them to remain functional if power briefly dips or if the electrical power is not entirely up to spec, perhaps 59.9995hz instead of the required 60hz.

Capacitors’ ability to hold and manage a charge allows them to absorb and smooth out changes in power, over tiny spans of time. Combined with additional components like resistors, and capacitors are a key component in filtering out electrical noise, cleaning up electrical signals. This makes them essential for things like modern networking and clean sounding audio from your home theater.

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