Capacitors are one of the most commonly used passive components in electronics design. They store electric charge and find widespread use for applications like filtering, energy storage, timing circuits and more. Hundreds of capacitor symbols are used in circuit schematics to denote the various types and styles available.
This comprehensive tutorial provides a full reference on identifying capacitor symbols. We examine the symbols associated with different capacitor types based on dielectric material, structure, packaging and functionality. Useful tables summarize key details and a circuit example illustrates real-world usage. Finally, the standard capacitance formula is derived along with examples calculating capacitance for different geometries.
Capacitor Symbol Overview
Capacitor symbols represent two conductors or plates separated by an insulator or dielectric. Here are the most common generic symbols:
The parallel straight lines denote two separate conductors. When packaged, dashed lines may be added:
Polarity markers are sometimes used to denote the positive and negative terminals:
Fixed Value Capacitor Symbols
When the capacitor value is known, it can be specified numerically in units of Farads:
Standard metric prefixes like micro, nano or pico are used. Eg 10nF, 47μF.
Variable Capacitor Symbols
Variable capacitors have symbols with arrows denoting tunability:
Trimmers are a type of variable capacitor tuned by a screwdriver for circuit calibration:
Polarized Capacitor Symbols
These indicate required polarity and terminal connections:
Electrolytic Capacitor Symbol
Often used in power supply filtering applications.
Tantalum Capacitor Symbol
Offers small size and reliability for surface mount boards.
Non-Polar Capacitor Symbols
These capacitors have no polarity requirements:
Ceramic Capacitor Symbol
Very common as cost effective SMD decoupling capacitors. Values up to few μFs.
Mica Capacitor Symbol
Used for stable high tolerance capacitors. Expensive compared to ceramic.
Film Capacitor Symbols
Offer very high insulation resistance and low losses. Popular as coupling and by-pass capacitors.
Capacitor Symbols by Dielectric
Dielectric material also differentiates capacitor types:
Air Capacitor Symbol
Ideal dielectric but impractical size. Used for standards.
Vacuum Capacitor Symbol
Used when minimum dielectric loss is critical. Expensive to manufacture.
Glass Capacitor
Borosilicate glass as dielectric. Low loss, temperature-stable.
Mica Capacitor
Mica sheet between plates. Stable high Q capacitor.
Ceramic Capacitor
Made from porous ceramic materials. High volume SMD type. Values up to few μFs.
Plastic Film Capacitor
Uses plastic films like polyester. Very high insulation resistance.
Electrolytic Capacitor
Uses electrolyte as dielectric to achieve high capacitance. Requires correct polarity.
Tantalum Capacitor
Uses tantalum pentoxide dielectric. Polarized, higher CV/volume ratio.
Capacitor Symbols Table Summary
| Symbol | Capacitor Type | Characteristics | Applications |
|---|---|---|---|
| Fixed Capacitor | Known capacitance value | Coupling, decoupling, filters | |
| Variable Capacitor | Tunable capacitance | Tuning circuits, trimmers | |
| Electrolytic | High capacitance, polarized | Filtering, energy storage | |
| Tantalum | Medium capacitance, polarized | Portable electronics | |
| Ceramic | Low cost, small sizes | Decoupling, bypassing | |
| Film | High stability | Signal coupling, tuning | |
| Mica | Stable, low loss | High frequency tuning |
Capacitor Symbols in Circuit Schematics
Here is an example circuit using multiple capacitor symbols:
- C1 is fixed value AC coupling capacitor
- C2 is variable trimmer capacitor
- C3 is polarized tantalum capacitor
- C4 and C5 are generic capacitors
This shows a real-world usage scenario of the various capacitor symbols in a schematic diagram.
Deriving Capacitance Formula
The capacitance value depends on physical and material aspects of the capacitor. Here we derive the basic parallel plate capacitance formula.
Consider two parallel plates of area A separated by distance d. When potential difference V is applied, charge Q gets stored:
Charge Q is proportional to voltage V:
Q = C * V
Where C is capacitance in Farads.
Capacitance C relates charge and voltage as:
C = Q/V
For parallel plates:
Q = ε * ε0 * A/d
Where:
- ε0 is permittivity of free space
- εr is dielectric constant of material between plates
Therefore:
C = ε * ε0 * A/d
This is the capacitance formula for simple parallel plate capacitors.
More complex geometries like coaxial, cylindrical and spherical require modified formulas that incorporate their dimensions.
Example Capacitance Calculations
Let’s calculate the capacitance for some sample scenarios:
Parallel Plates
- Plate Area A = 50cm x 50cm = 2500cm2
- Separation d = 2mm = 0.02cm
- Using air dielectric (εr = 1)
Plugging in values:
C = ε0 * εr * A/d C = 8.854 x 10-12 * 1 * 2500/0.02 C = 44.27nF
Cylindrical Capacitor
- Inner radius r1 = 5mm = 0.05cm
- Outer radius r2 = 10mm = 0.1cm
- Length l = 20mm = 0.2cm
- Using Teflon (εr = 2.1)
Formula is:
C = (2πεl)/ln(r2/r1)
Calculating:
C = (2*π*8.854x10-12*2.1*0.2)/ln(0.1/0.05) C = 24.2pF
Frequently Asked Questions
What does the capacitor symbol represent?
Capacitor symbols in schematics represent two conductors or plates separated by an insulator. This generic symbol is used for all capacitor types and styles.
How do you identify different types of capacitors?
Symbols may include polarity markers, value designations or specific styles to denote electrolytic, tantalum or variable capacitors. The circuit context also provides clues on likely type.
When should variable capacitor symbols be used?
Variable/tunable capacitors like trimmers have symbols with arrows on one or both plates. These are adjusted to set capacitance for tuning resonant circuits or calibration.
What is the most commonly used capacitor symbol?
The most ubiquitous capacitor symbol is the two straight parallel lines without polarity markers, representing fixed non-polarized capacitors. Common examples are ceramic disc capacitors.
What factors determine capacitance value?
Key factors affecting capacitance are plate area, separation distance between plates and the dielectric type. These geometric and material factors are incorporated in the standard capacitance formula.
