Types of capacitors

Ceramic capacitors

Ceramic capacitors are typically disk-shaped and come in a wide variety of physical sizes, ranging from 1/8 inch to 1 inch in diameter. They are made from two or more plates of copper or other metal with a layer of ceramic material between. In AC circuits (such as radio circuits) they are suitable for moderately high frequencies.

Plastic film capacitors

These capacitors are typically made from thin layers of polyester, polystyrene, polycarbonate or another plastic with thin layers of metal (usually aluminum) vacuum deposited on the surface. Leads are connected to the metal and the plastic is rolled tightly to make a compact size. The final product is often dipped in epoxy or another plastic. Different plastics give the capacitor different electrical, frequency and temperature characteristics.

Polarized plastic film capacitors

Some plastic film capacitors are marked with a band to indicate which lead connects to the outermost foil layer. When such capacitors are connected to ground, this is the lead that should connect to ground. This provides shielding to prevent AC noise from reaching the inner layers of foil and thus the ungrounded side of the circuit.

Electrolytic capacitors

Electrolytic capacitors. Tantalum capacitors on the left, solid capacitors across the top, surface mount (SMD) on the left middle. The silver colored one to the bottom left is also a tantalum capacitor. The rest are traditional aluminum foil with liquid electrolyte.

An electrolytic capacitor is typically two layers of aluminum foil with a layer of paper soaked in an electrolyte solution (typically boric acid or sodium borate with ethylene glycol added to retard evaporation). The foil and paper is rolled in to a convenient size and packaged, usually in a small aluminum can. Electrical current is passed through the capacitor, which forms a thin layer of aluminum oxide on one of the layers of foil. This thin oxide layer, with conducting electrolyte on one side and aluminum on the other, facilitates a high capacitance. Electrolytic capacitors have the highest capacitance to size ratio of any capacitor, especially EDLCs.

An electrolytic capacitor is polarized. This means that the voltage must be placed across the capacitor in only one polarity. All electrolytic capacitors are marked to show which side is positive and which is negative. If voltage is placed across an electrolytic capacitor with the wrong polarity, the insulating oxide layer will break down causing a short circuit. The oxide layer also converts to oxygen gas and can cause the capacitor to explode.

Axial lead electrolytic capacitors. The indent on the can (as well as the plus sign) indicates the positive end.

 

Radial lead electrolytic capacitors. The indent is useless to indicate the polarity so the negative lead is clearly marked with a thick black stripe.

Electrolytic capacitors tend to dry out over time. This is a perennial problem with old and not-so-old electronics. Old non-functional equipment can often be fixed by simply replacing all the electrolytic capacitors.

In the early 2000s there was a common problem with computers and peripherals failing because of leaking electrolytic capacitors. This phenomenon was often called capacitor plague.

Leaking defective electrolytic capacitors

This was due to a huge number of faulty electrolytic capacitors being manufactured by a small number of companies. Because of the low cost these capacitors ended up in countless devices made by small and major manufactures alike. The bad capacitors should have disappeared from the market in approximately 2007. However, they will still show up in equipment made in the early 2000s.

Non-polarized electrolytic capacitors

A non-polarized electrolytic capacitor. Indents at each end of the can indicate that the capacitor is non polarized.

 

Construction of a non-polarized electrolytic capacitor—two regular electrolytic capacitors end-to-end.

Some applications (particularly speaker crossover networks) require high capacitance non-polarized capacitors. The problem is, to get the required capacitance in a small package you need electrolytic capacitors, but electrolytic capacitors are polarized. What is the solution? If two electrolytic capacitors are connected in series with their negative ends together, they will make a non-polarized electrolytic capacitor. The individual capacitors are designed to withstand reverse voltage. When current is flowing one direction through the pair, one is reverse polarized and the other is correctly polarized. The reverse polarized capacitor becomes a short circuit and only the correctly polarized capacitor operates. When the current is reversed the capacitors switch as to which one is a short circuit and which is operating normally.

Tantalum capacitors

Tantalum capacitors are electrolytic capacitors that are made from the element tantalum. A typical tantalum capacitor is made from a cylindrical piece of sintered tantalum coated with magnesium dioxide which is then coated with silver. This assembly is dipped in epoxy giving a tantalum capacitor a somewhat teardrop shape. The porous surface of the tantalum yields a very large surface area and the magnesium dioxide forms a very thin insulating layer. This gives a tantalum capacitor a very high value in a very small size.

ELDCs

 

Electric double-layer capacitors (EDLCs) use electric double-layer effect. This is what keeps fat droplets in milk from coagulating into butter. The capacitors are made from activated carbon, carbon nanotubes, aerogels or other materials that have a very large surface area. The electric double-layer effect provides a very thin insulating layer. This provides the highest capacity to size ratio of any capacitor. Their disadvantage is that they have low voltage ratings and self-discharge relatively quickly.

Solid electrolytic capacitors (polymer capacitors)

 
Solid electrolytic capacitors have an electrolyte made of a solid polymer. They have become popular as a replacement for liquid electrolytic capacitors because they don't dry out over time.

Variable capacitors

A two-stage ganged variable capacitor. This is two variable capacitors that adjust in unison (see Ganging switches (and other devices) above)

 

This type of variable capacitor changes the overlaping area of the plates as the shaft is rotated.

There are two styles of variable capacitor. Most use one or more stationary plates (the stator) and one or more rotating plates (the rotor) separated by air. The rotor plates interleave with the stator plates. Rotating the shaft changes how much of the rotor plates are interleaved with the stator plates. More interleave means closer proximity and more capacitance. The less common style has plates that are squeezed together by a screw, changing how close the plates are to each other.

A ceramic base mica insulated trim capacitor

These are called trim capacitors. They vary in size but are comparatively small and have low values. They are often used in conjunction with larger fixed capacitors to fine tune the capacitance.