The schematic capacitor symbol represents two conductive plates separated by a dielectric material. While the basic non-polarized symbol consists of two parallel lines, variations in polarity, adjustability, and regional drafting standards (IEEE vs. IEC) fundamentally change how the symbol is drawn and interpreted on the bench.
The Complete Schematic Capacitor Symbol Reference Table
Reference this table when reading schematics or drafting your own PCB layouts. The visual descriptions below map directly to standard EDA tools like KiCad, Altium, and Eagle.
| Symbol Name | Visual Description | Polarity | Typical Component & Application |
|---|---|---|---|
| Non-Polarized | Two parallel straight lines of equal length | None | Ceramic, Film. Used for AC coupling, high-frequency decoupling, and snubber circuits. |
| Polarized (IEEE) | One straight line, one curved line | Strict | Aluminum Electrolytic. Used for bulk power supply filtering and low-frequency coupling. |
| Polarized (IEC) | Two parallel straight lines with a '+' sign on one side | Strict | Aluminum Electrolytic. Standard in European and international IEC 60617 documentation. |
| Bipolar / Non-Polarized Electrolytic | Two curved lines facing away from each other (or two straight with no polarity marks) | None | Bipolar Electrolytic. Used in audio crossover networks and AC motor start circuits. |
| Variable | Non-polarized symbol with a diagonal arrow crossing through it | None | Variable tuning capacitor. Used in RF oscillators and analog radio tuning circuits. |
| Trimmer | Non-polarized symbol with a T-shaped or flat-head adjustment line crossing it | None | Trimmer capacitor. Used for one-time factory calibration or precise RF impedance matching. |
| Feedthrough | Capacitor symbol with a continuous line passing through the center of both plates | None | Feedthrough capacitor. Used for EMI filtering where a signal/power line passes directly through the grounded chassis. |
| Ganged / Multiple | Multiple capacitor symbols linked by a dashed mechanical line | Varies | Multi-gang variable capacitors. Used in superheterodyne receivers to tune RF and local oscillator stages simultaneously. |
Regional Standards: IEEE 315 vs. IEC 60617 vs. Old UK BS
Unlike electrical wiring which is governed by the NEC (US) or IEC 60364 (International), schematic symbols are governed by drafting standards. Knowing which standard your schematic follows prevents catastrophic reverse-polarity wiring on the bench.
IEEE 315 (North America Standard)
The IEEE 315 standard uses the curved line to denote a polarized capacitor. The critical rule here is that the curved line always represents the negative terminal (or the terminal connected to the lower DC potential/ground). The straight line is the positive terminal. This visually mimics the physical construction of an aluminum electrolytic capacitor, where the outer aluminum can (negative) is curved, and the internal foil (positive) is flat.
IEC 60617 (International Standard)
The IEC standard rejects the curved line entirely to avoid confusion with physical component variations. In IEC schematics, all capacitors use two straight, parallel lines. Polarity is indicated exclusively by a '+' sign next to the positive terminal, and sometimes a thick filled rectangle on the negative terminal. If you are reading a schematic from a European manufacturer (like STMicroelectronics or Infineon), expect IEC notation.
Old British Standard (BS 3939)
Largely obsolete but still found in legacy UK military and broadcast equipment schematics, BS 3939 used a filled solid rectangle for one plate and an outlined rectangle for the other. The filled rectangle denoted the negative terminal. Modern UK designs have fully harmonized with IEC 60617, but repair technicians working on vintage audio gear still encounter BS 3939 symbols.
Symbols, Rows, and Markings People Get Wrong
Misinterpreting a schematic capacitor symbol or its physical counterpart is one of the most common causes of component failure, ranging from popped dielectrics to violent electrolytic venting.
Mistake 1: Reversing the IEEE Curved Line Orientation
Beginners often assume the curved line represents the positive terminal because it looks 'larger' or more prominent. In IEEE 315, the curved line is strictly negative. If you wire VCC to the curved line and GND to the straight line, the aluminum electrolytic capacitor will experience reverse bias. At voltages above 1.5V, this causes the dielectric oxide layer to break down, generating hydrogen gas and leading to a ruptured vent.
Mistake 2: Confusing Tantalum and Aluminum Physical Markings
While the schematic symbol for a polarized capacitor is identical for both aluminum electrolytic and tantalum types, their physical markings are exact opposites.
- Aluminum Electrolytic: The physical stripe with minus signs (-) denotes the negative lead.
- Tantalum: The physical colored bar or line denotes the positive lead.
Never guess the voltage rating or capacitance of a physical capacitor when the silkscreen text is faded, burned, or missing. Guessing the voltage rating and applying mains or high DC bus voltage can cause an explosion.
- Use an LCR meter to measure the actual capacitance and Equivalent Series Resistance (ESR).
- Trace the PCB copper to determine the maximum DC bias voltage applied to that node.
- Apply a minimum 20% voltage derating rule (e.g., if the node sees 25V DC, install a replacement rated for at least 35V or 50V).
Mistake 3: Using Polarized Symbols for High-Frequency Decoupling
If a schematic shows a non-polarized symbol (two straight lines) next to an IC's VCC pin, do not substitute an electrolytic capacitor just because you have one with the right uF rating. The non-polarized symbol specifically calls for a ceramic or film capacitor, which has the low ESL (Equivalent Series Inductance) and high SRF (Self-Resonant Frequency) required to shunt high-frequency digital switching noise to ground.
Frequently Asked Questions
Which side of the curved capacitor symbol is positive?
In the North American IEEE 315 standard, the straight line is the positive terminal, and the curved line is the negative terminal. The curved line should always be oriented toward ground or the lower voltage potential in the circuit. If you are working with an IEC 60617 schematic, there are no curved lines; instead, look for the explicit '+' sign next to one of the two straight parallel lines to identify the positive terminal.
What does a capacitor symbol with a diagonal arrow through it mean?
A diagonal arrow crossing through a standard non-polarized capacitor symbol indicates a variable capacitor. This component allows the user to mechanically adjust the distance or overlap area between the plates to change the capacitance value. They are primarily used in RF applications, such as tuning the resonant frequency of an LC oscillator in a ham radio transmitter or an analog FM receiver. If the arrow is replaced by a flat T-shaped line, it indicates a trimmer capacitor, which is meant for precision calibration rather than continuous user adjustment.
How do I safely replace a capacitor when the physical markings are completely faded?
When physical markings are illegible, you must rely on circuit analysis and bench measurement. First, safely discharge the circuit and desolder the component. Use a calibrated LCR meter (like a Keysight U1733C or a DER EE DE-5000) to measure the capacitance and ESR. Next, consult the device's service manual or trace the PCB to identify the maximum voltage present at that node. For electrolytic replacements, always apply a 20% to 50% voltage derating margin. If the original was a 470µF cap on a 12V rail, replace it with a 470µF 25V or 35V low-ESR capacitor (such as a Panasonic FR or Rubycon ZL series) to ensure longevity and handle ripple current safely.
