What are fixed capacitors? 

Fixed capacitors, like fixed resistors, are special capacitor devices designed to store a fixed amount of electrical charge as pre-determined during the time of manufacturing. Since the value of electrical charge or capacitance is fixed, it cannot be adjusted. Thus, a fixed capacitor helps in maintaining a constant charge in electric appliances and enables them to generate a constant amount of energy output. 

Types of fixed capacitors

Fixed capacitors are available in different types. Broadly, they can be classified in two major types, namely polarised fixed capacitors and non-polarised fixed capacitors. 

• Polarised fixed ccapacitors (polar capacitors): In these types of fixed capacitors, the terminals or electrodes of the device possess a specific polarity, i.e., either positive or negative. 
• Non-polarised capacitors  (Non-polar capacitors): In these types of fixed capacitors, the terminals are devoid of any particular polarity, making them suitable for use in both AC & DC circuits. 
   

Depending on the dielectric material used to construct a fixed capacitor device, they can be generally classified into different types. The most common ones include: 

• Paper capacitors, having wax covered paper as their dielectric material and placed alternately between aluminum sheets. 
• Plastic capacitors, where the dielectric material is made from different types of plastic and are available in two varieties - film foil capacitors and metalised film capacitors. 
• Ceramic capacitors, which uses a ceramic material as the insulator or dielectric material and is particularly useful for limited space. 
• Mica capacitors, possessing plain or silver coated mica between aluminum or copper sheets, and offering better stability, reliability and performance. 
• Electrolytic capacitors, where aluminum oxide sheets are present as the dielectric material and a paper soaked with electrolytic fluid is present as a conductor.

Applications of fixed capacitors

Fixed capacitors have extensive applications across the electronics and electrical industries. Their common applications can be found in:

• Resonant circuits of transmitter stations
• General-purpose capacitors
• DC motors for RF noise reduction 
• DC motor brushes
• Switching mode power supply
• Low pass filters for input and output voltage smoothening
• Power electronic devices
• X-ray machines
• Phase shifters
• Pulsed lasers
• Decoupling capacitors and filters
• A/D converters