What is a Variable Capacitor?

Jessica Reed

A variable capacitor is a special type of capacitor, most commonly used for tuning radios, which allows the amount of electrical charge it can hold to be altered over a certain range, measured in a unit known as farads. Regular capacitors build up and store an electrical charge until it's ready to use. While a variable capacitor stores the charge in the same fashion, it can be adjusted as many times as desired to store different amounts of electricity. Since the most common use for the variable capacitor is in the tuning mechanisms of radios and older TV sets, it often goes by the name tuning capacitor or variable tuning capacitor.

Capacitance refers to the amount of energy a capacitor can store.
Capacitance refers to the amount of energy a capacitor can store.

When altering a variable capacitor, the user is actually changing its capacitance. Capacitance means the amount of energy the capacitor can store. A bigger capacitance means more stored energy. This energy is measured in farads, but because a variable capacitor typically has a very small capacitance, a smaller unit known as a picofarad is used instead.

Variable capacitors, which can be found on most vintage commercial and field radios, allow operators to tune into different frequencies without having to change capacitors.
Variable capacitors, which can be found on most vintage commercial and field radios, allow operators to tune into different frequencies without having to change capacitors.

Two types of variable capacitors include air variable capacitors and vacuum variable capacitors. While each performs the same function, one uses a high vacuum instead of air to insulate the capacitor. This allows for a higher capacitance to be produced in a smaller-sized capacitor. Variable capacitors can also be controlled mechanically or electronically. The electronically controlled capacitors change their capacitance based on DC voltage applied to it, while the mechanically controlled versions are designed so the parts can be moved to increase or decrease capacitance.

One of the most common uses for variable capacitors is in radios to allow the radio to tune to different stations. The capacitor is part of an LC circuit where the L stands for an inductor and the C stands for a capacitor. This inductor/capacitor combination uses the variable capacitor to alter the frequency passing through the LC circuit and thus connect with radio stations, each of which operates on a different frequency that the LC circuit must match to receive.

The ability to change the amount of electrical charge it can hold is the variable capacitor's main advantage over a regular capacitor. It allows the user to adjust the capacitor on objects, such as radios, which constantly need to connect with different frequencies. To change back and forth like this without variable capacitors would require a different capacitor for each frequency and would be impractical, if possible at all. The major disadvantage is the relatively small range they can cover. Typically they only change over a limited range and these values are of small capacitance to begin with.

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Discussion Comments


Where I tend to see the most mechanical capacitors used is on effects pedals for musicians. Also, the very computer keyboards we type on use capacitors to convert our typing to electronic signals. Capacitors can also be great fun for home experiments. Making high voltage capacitors is a simple process of separating a conductive material, like aluminum foil, with an insulator, usually plastic. The first similar capacitor was the Leyden jar invented in 1745. This was a jar with an outer and inner metal layer. The glass separated the two conductive layers. Many tutorials and videos are available online that will help you build the capacitor you like. But, be extremely cautious as they are high voltage and can cause quite a shock!


Sporkasia -Variable capacitors can appear complicated. The best way to learn about them is to get your hands on the inner working elements of a radio and see how everything works together. By the way, plastic foils are sometimes used as dielectrics (insulators), as is air, in mechanically controlled variable capacitors.

While all variable capacitors are designed to do the same job, there is a bit of variation in the individual products. If you should decide to buy a variable capacitor, do your research and have an idea what you are looking for before going to make the purchase.


So that's how radios store electricity and pick up stations. I've always been the one who wanted to take things apart, and seldom was I able to put those things back together in working form. Among the many items I disassembled were several radios.

I've been trying to learn more about radios lately. I still don't have a total working concept in my head, but the article clears up some of my questions. I think plastic foil is used in some way with the high voltage capacitors.

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