The standard non polarised capacitor symbol consists of two parallel, straight lines of equal length separated by a small gap, representing the two conductive plates of the component. Because the dielectric material between the plates is physically symmetrical, the symbol features no positive (+) or negative (-) terminal indicators. This means the physical component—whether a ceramic disc, multilayer ceramic chip (MLCC), or polypropylene film capacitor—can be soldered into the circuit in either direction without risk of catastrophic failure or reversed polarity damage.
The Complete Capacitor Symbol Reference Table
Before wiring a board or debugging a schematic, you must correctly identify the capacitor type. The table below maps the standard symbols to their real-world components, contrasting the two dominant global standards. Read the 'Practical Application' column to understand what the symbol demands from your parts bin.
| Component Type | IEC 60617 Symbol | IEEE 315 / ANSI Symbol | Practical Application & Bench Notes |
|---|---|---|---|
| Non-Polarised (Fixed) | Two straight parallel lines | Two straight parallel lines | Use MLCC, ceramic, mica, or film caps. Can be inserted in any orientation. Standard for high-frequency decoupling and audio signal coupling. |
| Polarised (Fixed) | One straight line, one curved line, '+' on straight side | One straight line, one curved line, '+' on straight side | Electrolytic or tantalum. The curved plate is the negative cathode. Reversing polarity risks venting, explosion, or short-circuit failure. |
| Variable | Two straight lines with a diagonal arrow crossing them | Two straight lines with a diagonal arrow crossing them | Air or vacuum variable capacitors. Used in RF tuning circuits and antenna matching networks. Non-polarised. |
| Trimmer | Two straight lines with a 'T' shaped arrow | Two straight lines with a 'T' shaped arrow | Small PCB-mount variable caps for factory calibration. Adjust with a non-metallic (ceramic or plastic) tool to avoid shifting capacitance. |
| Feedthrough | Two parallel lines with a perpendicular line striking through the gap | Similar to IEC, sometimes shown as a cap with a ground shield | 3-terminal EMI filter. The through-line carries the signal; the outer shell grounds to the chassis. Do not substitute with a 2-lead MLCC. |
| Ganged (Multiple) | Multiple non-polarised symbols linked by a dashed mechanical line | Multiple symbols linked by a dashed line | Multi-section tuning capacitors. Common in vintage superheterodyne AM/FM radios for simultaneous oscillator and RF stage tuning. |
Regional Standards and the 'Rows People Get Wrong'
Capacitor symbols are governed primarily by two standards: IEC 60617 (dominant in Europe, Asia, and modern global designs) and IEEE 315 / ANSI Y32.2 (historically dominant in North America). While modern schematics have largely harmonized around the IEC style, legacy designs and regional habits still cause confusion on the workbench.
The 'Curved Plate' Legacy Trap
The most common mistake occurs when reading older North American schematics or vintage UK designs (BS 3939). Prior to the 1980s, American draftsmen frequently used one straight plate and one curved plate to represent all capacitors, regardless of polarity. The curve simply represented the outer foil of a rolled paper capacitor. Today, under both modern IEEE 315 and IEC 60617, a curved plate strictly designates the negative terminal of a polarised capacitor. If you see a curved plate on a modern schematic, do not populate it with a non-polarised film or ceramic capacitor; the design expects an electrolytic or tantalum part.
The Feedthrough Misidentification
A symbol showing two parallel plates with a perpendicular line striking through the center is frequently mistaken for a standard non-polarised bypass capacitor with an unusual draft. It is actually a feedthrough capacitor. These are 3-terminal devices where the signal passes through the center conductor while the capacitor body grounds directly to a metal chassis or ground plane to shunt high-frequency EMI. Installing a standard 2-lead non-polarised MLCC in this footprint will leave the ground plane floating and cause the device to fail EMC compliance testing.
Class 1 vs. Class 2 Ceramic Substitutions
While the non polarised capacitor symbol does not specify the dielectric material, the circuit's function does. If the symbol is in an audio signal path or a precision timing oscillator, you must use a Class 1 ceramic (C0G/NP0) or film capacitor. Substituting a Class 2 ceramic (X7R or Y5V) just because it fits the footprint will introduce microphonics, piezoelectric noise, and severe capacitance drop-off under DC bias.
Safe Interpretation When Markings Are Faded or Missing
When repairing vintage amplifiers, reverse-engineering proprietary hardware, or working with poorly documented open-source boards, the silkscreen polarity markers are often burned off, scraped away, or entirely missing. Here is how to safely verify a non-polarised footprint without relying on the PCB legend.
- Analyze the Footprint Geometry: Non-polarised SMD pads are geometrically identical (e.g., two matching 0805 or 1206 rectangles). Polarised SMD footprints often feature a larger anode pad, a distinct cathode stripe on the silkscreen, or a chamfered corner on the component outline. For through-hole boards, polarised caps usually have a square pad for pin 1 (+) and a circular pad for pin 2 (-). If both pads are identical circles or identical squares, the designer intended a non-polarised part.
- Inspect the Physical Dielectric: If you have the loose component, look at the casing. Ceramic discs (brown/orange epoxy), MLCCs (tan/grey unmarked blocks), and film capacitors (epoxy-dipped or plastic box) are inherently non-polarised. Aluminum cans with a contrasting grey stripe and a minus sign are polarised. Solid tantalum beads are polarised and usually have a printed line indicating the positive anode (the opposite of aluminum electrolytics).
- Bench Verification with an LCR Meter: If the component is unmarked and you suspect it might be a polarised capacitor installed backwards, use an LCR meter. Set the test signal to 1V AC at 1kHz. If you are testing an unknown polarised capacitor with a DC bias applied backwards, the internal leakage current will cause the dissipation factor (D) or equivalent series resistance (ESR) reading to skew wildly. A true non-polarised capacitor will yield stable capacitance and D readings regardless of how you swap the test leads.
Frequently Asked Questions
Does a non polarised capacitor symbol ever have a plus or minus sign?
No. By definition, the non polarised capacitor symbol lacks polarity markers. The physical components it represents rely on symmetric dielectrics—such as ceramic, mica, glass, or plastic film—that do not form a chemical oxide layer dependent on voltage direction. If a schematic symbol includes a '+' or '-' sign, or a solid bar indicating a positive terminal, it is a polarised capacitor (like aluminum electrolytic or tantalum) and must be installed in the correct orientation.
What is the exact difference between polarised and non polarised capacitor symbols?
The primary visual difference is the shape of the plates. A non polarised capacitor symbol uses two straight, parallel lines of equal length. A modern polarised capacitor symbol uses one straight line (representing the positive anode) and one curved line (representing the negative cathode or outer foil). Additionally, polarised symbols will almost always include a '+' sign next to the straight plate or a shaded bar to explicitly denote the positive terminal, whereas non-polarised symbols remain unmarked.
Can I replace a polarised capacitor with a non polarised one if the symbol is unclear?
Electrically, yes, but practically, it is limited by physics. A non-polarised film or ceramic capacitor can safely replace a polarised electrolytic capacitor if the capacitance value, voltage rating, and physical dimensions match. Non-polarised capacitors are inherently safer because they cannot be installed backwards. However, non-polarised dielectrics have a much lower volumetric efficiency. You can easily find a 0.1µF non-polarised MLCC to replace a polarised part, but finding a non-polarised 1000µF capacitor rated for 50V is physically impossible for standard PCB mounting. For bulk power supply filtering, you must stick to polarised electrolytics.
