There are many types of capacitors depending upon their function, the dielectric material used, their shape etc. The main classification is done according to fixed and variable capacitors. The main classification is just like the above one. The fixed capacitors are the ones whose value is fixed at the time of manufacturing itself and the variable ones provide us with an option to vary the value of capacitance. Let us know something about the variable capacitors whose value alters when you vary, either electrically or mechanically.
Variable capacitors in general consists of interwoven sets of metallic plates in which one is fixed and the other is variable. These capacitors provide the capacitance values so as to vary between 10 to pF. The ganged capacitor shown here is a combination of two capacitors connected together. A single shaft is used to rotate the variable ends of these capacitors which are combined as one. The dotted line indicates that they are connected internally.
There are many uses of these variable resistors such as for tuning in LC circuits of radio receivers, for impedance matching in antennas etc. The main types of variable capacitors are Tuning capacitors and Trimmer capacitors.
Tuning capacitors are popular type of variable capacitors. They contain a stator, a rotor, a frame to support the stator and a mica capacitor. The constructional details of a tuning capacitor are shown in the following figure.
The stator is a stationary part and rotor rotates by the movement of a movable shaft. The rotor plates when moved into the slots of stator, they come close to form plates of a capacitor. The above figure shows a ganged tuning capacitor having two tuning capacitors connected in a gang. This is how a tuning capacitor works.
Capacitor and Capacitance
These capacitors generally have capacitance values from few Pico Farads to few tens of Pico Farads. These are mostly used in LC circuits in radio receivers. These are also called as Tuning Condensers. Trimmer capacitors are varied using a screwdriver. Trimmer capacitors are usually fixed in such a place where there is no need to change the value of capacitance, once fixed.
There are three leads of a trimmer capacitor, one connected to stationary plate, one to rotary and the other one is common. The movable disc is a semi-circular shaped one. A trimmer capacitor would look like the ones in the following figure.
There are two parallel conducting plates present with a dielectric in the middle. Depending upon this dielectric used, there are air trimmer capacitors and ceramic trimmer capacitors.A capacitor is a device that stores electrical energy in an electric field. It is a passive electronic component with two terminals. The effect of a capacitor is known as capacitance. While some capacitance exists between any two electrical conductors in proximity in a circuita capacitor is a component designed to add capacitance to a circuit.
The capacitor was originally known as a condenser or condensator. The physical form and construction of practical capacitors vary widely and many types of capacitor are in common use. Most capacitors contain at least two electrical conductors often in the form of metallic plates or surfaces separated by a dielectric medium.
A conductor may be a foil, thin film, sintered bead of metal, or an electrolyte. The nonconducting dielectric acts to increase the capacitor's charge capacity. Materials commonly used as dielectrics include glassceramicplastic filmpapermicaair, and oxide layers. Capacitors are widely used as parts of electrical circuits in many common electrical devices.
Unlike a resistoran ideal capacitor does not dissipate energy, although real-life capacitors do dissipate a small amount. See Non-ideal behavior When an electric potentiala voltageis applied across the terminals of a capacitor, for example when a capacitor is connected across a battery, an electric field develops across the dielectric, causing a net positive charge to collect on one plate and net negative charge to collect on the other plate.
No current actually flows through the dielectric. However, there is a flow of charge through the source circuit. If the condition is maintained sufficiently long, the current through the source circuit ceases.
If a time-varying voltage is applied across the leads of the capacitor, the source experiences an ongoing current due to the charging and discharging cycles of the capacitor.
The earliest forms of capacitors were created in the s, when European experimenters discovered that electric charge could be stored in water-filled glass jars that came to be known as Leyden jars. Today, capacitors are widely used in electronic circuits for blocking direct current while allowing alternating current to pass.
In analog filter networks, they smooth the output of power supplies. In resonant circuits they tune radios to particular frequencies. In electric power transmission systems, they stabilize voltage and power flow. In OctoberEwald Georg von Kleist of PomeraniaGermany, found that charge could be stored by connecting a high-voltage electrostatic generator by a wire to a volume of water in a hand-held glass jar. Von Kleist found that touching the wire resulted in a powerful spark, much more painful than that obtained from an electrostatic machine.
The following year, the Dutch physicist Pieter van Musschenbroek invented a similar capacitor, which was named the Leyden jarafter the University of Leiden where he worked.
Daniel Gralath was the first to combine several jars in parallel to increase the charge storage capacity. He also adopted the term "battery",   denoting the increasing of power with a row of similar units as in a battery of cannonsubsequently applied to clusters of electrochemical cells.
Leyden jars or more powerful devices employing flat glass plates alternating with foil conductors were used exclusively up until aboutwhen the invention of wireless radio created a demand for standard capacitors, and the steady move to higher frequencies required capacitors with lower inductance.A variable capacitor is a capacitor whose capacitance may be intentionally and repeatedly changed mechanically or electronically.
In mechanically controlled variable capacitors, the distance between the plates, or the amount of plate surface area which overlaps, can be changed. The most common form arranges a group of semicircular metal plates on a rotary axis " rotor " that are positioned in the gaps between a set of stationary plates " stator " so that the area of overlap can be changed by rotating the axis.
Air or plastic foils can be used as dielectric material. By choosing the shape of the rotary plates, various functions of capacitance vs. Various forms of reduction gear mechanisms are often used to achieve finer tuning control, i.
A vacuum variable capacitor uses a set of plates made from concentric cylinders that can be slid in or out of an opposing set of cylinders  sleeve and plunger. These plates are then sealed inside of a non-conductive envelope such as glass or ceramic and placed under a high vacuum. The movable part plunger is mounted on a flexible metal membrane that seals and maintains the vacuum.
A screw shaft is attached to the plunger, when the shaft is turned the plunger moves in or out of the sleeve and the value of the capacitor changes. The vacuum not only increases the working voltage and current handling capacity of the capacitorit also greatly reduces the chance of arcing across the plates.
The most common usage for vacuum variables are in high-powered transmitters such as those used for broadcastingmilitary and amateur radioas well as high-powered RF tuning networks. Vacuum variables can also be more convenient; since the elements are under a vacuum, the working voltage can be higher than an air variable the same size, allowing the size of the vacuum capacitor to be reduced. Very cheap variable capacitors are constructed from layered aluminium and plastic foils that are variably pressed together using a screw.
These so-called squeezers cannot provide a stable and reproducible capacitance, however. A variant of this structure that allows for linear movement of one set of plates to change the plate overlap area is also used and might be called a slider.
This has practical advantages for makeshift or home construction, and may be found in resonant-loop antennas or crystal radios. Small variable capacitors operated by screwdriver for instance, to precisely set a resonant frequency at the factory and then never be adjusted again are called trimmer capacitors. In addition to air and plastic, trimmers can also be made using a ceramic dielectric, such as mica. The sections can have identical or different nominal capacitances, e.
Capacitors with multiple sections often include trimmer capacitors in parallel to the variable sections, used to adjust all tuned circuits to the same frequency. A butterfly capacitor is a form of rotary variable capacitor with two independent sets of stator plates opposing each other, and a butterfly -shaped rotor arranged so that turning the rotor will vary the capacitances between the rotor and either stator equally.
Butterfly capacitors are used in symmetrical tuned circuits, e. RF power amplifier stages in push-pull configuration or symmetrical antenna tuners where the rotor needs to be "cold", i. Since the peak RF current normally flows from one stator to the other without going through wiper contacts, butterfly capacitors can handle large resonance RF currents, e. Unlike in a capacitor with several sections, the rotor plates in a split stator capacitor are mounted on opposite sides of the rotor axis.
Differential variable capacitors also have two independent stators, but unlike in the butterfly capacitor where capacities on both sides increase equally as the rotor is turned, in a differential variable capacitor one section's capacity will increase while the other section's decreases, keeping the sum of the two stator capacitances constant. Differential variable capacitors can therefore be used in capacitive potentiometric circuits.
The thickness of the depletion layer of a reverse-biased semiconductor diode varies with the DC voltage applied across the diode. Their use is limited to low signal amplitudes to avoid obvious distortions as the capacitance would be affected by the change of signal voltage, precluding their use in the input stages of high-quality RF communications receivers, where they would add unacceptable levels of intermodulation. Varicaps are used for frequency modulation of oscillators, and to make high-frequency voltage controlled oscillators VCOsthe core component in phase-locked loop PLL frequency synthesizers that are ubiquitous in modern communications equipment.A tuning capacitor is a variable capacitor used in an electronic circuit of a radio, and usually connects in parallel to a loop antenna.
There are many uses and applications for a variable capacitor; the most typical use being in an AM radio circuit. If you were building a crystal radio, then you will most definitely need this component. A typical capacitor will look like the photograph shown above.
It will usually have six pins at the back, and sometimes an additional pin at the front-side, which connects to the shaft. It is normally used in a radio capable of receiving AM and FM bands. Inside, they typically consist of four individually ganged variable capacitors, where two gangs of variable capacitance are for the AM side, and two gangs of for the FM side. The first priority is to determine which is the FM side and the AM side. There is normally some writing at the back to provide a clue to the orientation of the component.
The three pins on the lower side are usually for the FM Radio. If you are building an AM Crystal Radio, then you ignore those pins. The three pins on the top side are for the AM radio.
For a crystal radio, you need to connect the centre pin to the earthy side of your coil, and only one of the side pins to the antenna side of your coil. The schematic symbol shows four individual variable capacitors in a ganged arrangement. There are two variable capacitors for the FM section and two for the AM section. The centre pin is common to both variable capacitors.
In some variable capacitors, C1 and C2 are of equal value of around 20 pF, whilst C3 and C4 are of equal value of around pF. The centre pin, on the AM side, usually connects to the earthy side of the loop antenna. If you had a digital meter that measured capacitance, then you could double check to see which terminals exhibit the greatest capacitance.
The shaft rotates four individually separate variable capacitors. The reason for having four is because modern radios usually have an FM band and an AM band.My favourite magnetic loop for HF pedestrian mobile
In addition, each band requires two dedicated variable capacitors, where one is for station tuning and the other is for the local oscillator circuit. This means that the FM band has an antenna variable capacitor, and an oscillator variable capacitor, and the AM band has its antenna variable capacitor, and its oscillator variable capacitor.
Obviously, all of these variable capacitors within one package mean that this component will have many terminals. Many hobbyists are perturbed from using it and often end up scratching their heads trying to figure out how to connect it.Most modern electronic circuits and devices consist of different types of capacitors. Electronic newbies, experienced engineers find that these components are quite interesting due to their applications.
In radio technology, Capacitors can be categorised into fixed capacitor and variable capacitor. Fixed capacitors can be again classified into polarized or electrolytic capacitor and non-polarized capacitors. Non-polarized capacitors have small capacitance values and have small leakage current. Examples include Ceramic, mica, film capacitors, etc. A polarized capacitor has large leakage current. Electrolytic and super capacitors are examples of polarized capacitors.
There are different capacitors available depend upon their dielectric material which comes in different shapes and sizes. The most widely used capacitors are ceramic, electrolytic, tantalum and supercapacitors. Variable capacitors are nothing but works just like a potentiometer.
Basic Electronics - Variable Capacitors
This is a type of capacitor whose capacitance can be changed mechanically or electronically. It is also called as trimmer capacitor. In practical, the construction of variable capacitors, use is usually made of the first possibility i. Variable capacitors are further subdivided into capacitors for continuous variation tuning capacitorsand capacitors which only have to be adjusted occasionally trimmers.
This capacitor gives the value to vary between 10pF to pF. The types of variable capacitors are tuning and trimmer capacitors. Used for tuning in radio circuits, transmitters. An important ability of the tuning capacitor to withstand mechanical shocks or vibrations. The second group of capacitors comprises semi-fixed or trimmers. Here the capacitance is variable but not intended for frequent use. The trimmers are used only to adjust various tuned circuits.
Once these capacitors have been adjusted, they are mostly sealed with lacquer so that to all intents and purposes they are fixed capacitors.
The Air trimmer consists of a cylindrical stator in which a similar cylindrical rotor can be turned on a small screwed rod. The minimum capacitance is about 3 pF, the maximum capacitance can be 30 or 60 pF. Because air is used as the dielectric, losses in these trimmers are very low.
Adjustment is made by means of trimming key made of insulating material in the form of a box spanner. This trimmer consists of a small ceramic tube which serves as a dielectric.
The electrodes plates are formed by a tinned copper sleeve and a tinned copper pin which can be screwed into the ceramic sleeve.As I was building my transmatch, I became increasingly dissatisfied with my choice of capacitors. They were too small for more than about a hundred watts, and the ganged capacitor was in pretty sad shape. The frame was rusted and I could not remove all of the oxidation from the plates. I had a very nice Hammerlund pf that would do well for the Input Tuning, but I didn't have an equivalent ganged capacitor to use for Output Tuning.
I am temporarily disabled and have no income at the moment, so it looked as if my transmatch would either be constructed of inferior parts or put on hold indefinitely. It looked pretty simple, but I was concerned that the materials would strain my very limited budget. I did some thinking and looked around my local hardware store to check the price of materials. I figured I could manage that, so I looked for the other materials. I couldn't find the spacers, and did not have a tubing cutter to make them with.
I didn't feel the flashing would be suitable to make the contact spring for the rotor, so I found a small compression spring to use for tensioning the rotor to the contact plate.
I also decided to use nylon filled stop nuts for securing and adjusting the rotor. I had some Lexan on hand to use for endplates, but any good non-conductive material such as PlexiGlass, Teflon, Nylon, Lucite, or phenolic would work. Usable scraps are usually available at reasonable prices at most glass shops or plastics suppliers. Rather than use Anwar's measurements, I modeled my capacitor on the Hammerlund. Since it was 25 plates on the rotor and 24 plates on the stator, rated at pf, I figured that 12 rotor and 12 stator plates would give me something on the order of pf.
I measured the Hamerlund as illustrated:. This layout was drawn with Cadvance 6. One major advantage to using flashing is the ease with which you can work it. It can be easlily cut with a good pair of utilty scissors. When making your plates, lay out one of each, cut them out, and then use them as templates to layout the rest. Here are the plates, all cut out. Since no one is perfect, make sure you mark an index on each plate as you cut it out. This will insure proper alignment when you assemble the device.
Drill the endplates. It is a good idea to drill both together so the holes will match front and back.With a range of 5 pF to 27 pF, this multi-turn capacitor can be constructed using only six items. For those who enjoy building radio projects, you may have noticed that variable capacitors are not as available as they once were.
There was a time when all radio receivers contained at least one, but with the arrival of the varicap diode and frequency synthesizer, the traditional tuning capacitor is difficult to find. Fortunately, a variable capacitor is a simple device and one that you can easily construct yourself. Start by screwing the two nuts onto the screw itself. Each hex nut needs to be scratched with a one-sided razor blade, but only on one side. The scratching ensures the soldering adheres to the nut.
Be sure to hold the soldering gun onto the nut for a good 30 seconds, ensuring that the solder adheres well to each nut. You should see the solder begin to grab onto the nut after several seconds. After you have a firm dab of solder on each side of each nut, take your PCB and place it under the two nuts. Adjust the two nuts so each rests on each side of the PCB. Next, place some heavy form over your setup, ensuring it does not move.
Take your soldering gun and heat up the solder on one of the nuts.
When it becomes soft, start applying more solder to that area. Once the solder has accumulated enough, the weight of itself will fall directly onto the PCB. When the one adheres to the PCB, do the same for the other nut. Make 12 wraps of wire around your screw. Go ahead and unscrew your "coil" from the screw. Measure 16 mm up from the bottom of the head of the screw.
Take a new single nut and position it where the 16 mm is, and using your hacksaw blade, cut at that point. The new nut is placed to keep an accurate measurement while you cut.
Turn the screw all the way and take your newly-made coil and screw that onto your freshly-cut screw. The final step involves putting a piece of folded plastic underneath the screw between the two nuts. Thus, when you turn it, you will get sporadic discontinuity between the screw and the nuts. The plastic gives an upward push on the screw and the two nuts, providing a continual continuity between the two.
Lastly, bend the outside end of the coil wire toward the inside end wire and snip off the excess.