The universal symbol of a capacitor consists of two parallel lines representing conductive plates separated by a dielectric. In North America (IEEE 315), both lines are typically straight for non-polarized types, while international (IEC 60617) and modern polarized representations use one straight and one curved line to denote the negative (outer foil) plate. Below is the master reference for interpreting these schematic variations.
The Complete Capacitor Symbol Reference Table
| Component Type | IEEE 315 (US) Symbol Description | IEC 60617 (Intl) Symbol Description | Polarity | Common Use Case |
|---|---|---|---|---|
| Non-Polarized | Two parallel straight lines | Two parallel straight lines | None | AC coupling, high-frequency decoupling, RF filtering |
| Polarized (Electrolytic) | One straight line, one curved line (plus sign on straight side) | One straight line, one curved line (plus sign or filled block on straight side) | Strict (Anode/Cathode) | Bulk power supply filtering, low-frequency coupling |
| Variable | Two parallel lines with a diagonal arrow crossing through them | Two parallel lines with a diagonal arrow crossing through them | None (usually) | User-tunable RF circuits, analog radio tuning |
| Trimmer | Two parallel lines with a T-bar or flat-headed arrow | Two parallel lines with a T-bar or flat-headed arrow | None | Factory calibration, oscillator fine-tuning |
| Feedthrough | Capacitor symbol with a continuous line passing through the center | Capacitor symbol enclosed in a grounded shield box | None | EMI suppression on panel penetrations, high-frequency bypass |
| Differential / Ganged | Two variable capacitor symbols linked by a dashed mechanical line | Two variable capacitor symbols linked by a dashed mechanical line | None | Simultaneous tuning of multiple LC tank circuits |
Regional Standards and the "Rows People Get Wrong"
When reading schematics, you must first identify which drafting standard the engineer used. The IEEE 315 standard dominates legacy North American designs and military hardware, while the IEC 60617 standard is the default for modern European, Asian, and international open-source hardware. While the basic symbol of a capacitor remains largely consistent, the nuances in the drawing style trip up many technicians.
Here are the specific rows and symbol variants that cause the most misinterpretations on the bench:
- The Curved Line Myth: Many hobbyists assume a curved line always indicates a polarized electrolytic capacitor. In older IEEE schematics, a non-polarized capacitor drawn with one curved line simply indicated the "outer foil" connection. Connecting the outer foil to the lower-impedance node (usually ground) shields the inner foil from electromagnetic interference. If you replace it with a polarized electrolytic based on that assumption, you will destroy the circuit upon power-up.
- Variable vs. Trimmer Arrows: The arrowhead matters. A standard diagonal arrow means the component is user-adjustable (a variable capacitor with a shaft). An arrow terminating in a flat horizontal bar or a T-shape indicates a trimmer—a component meant to be set once at the factory with a small ceramic screwdriver and left alone.
- Feedthrough Confusion: A feedthrough capacitor is often drawn as a standard non-polarized symbol with a wire passing straight through the middle, or with both plates tied to ground. Technicians frequently misidentify this as a standard bypass capacitor. Mechanically, a feedthrough is a bulkhead component; electrically, it provides ultra-low inductance grounding for GHz-range EMI suppression.
Safe Interpretation When Physical Markings Are Faded
Schematics are ideal, but on the repair bench, you frequently encounter baked, faded, or physically damaged capacitors where the ink has entirely evaporated. When the physical markings are gone and the schematic is unavailable, follow this diagnostic framework:
- Determine the Role via Circuit Tracing: If the capacitor is connected directly across a DC power rail and ground, it is bulk decoupling. You can safely substitute a low-ESR electrolytic or polymer capacitor of 10µF to 100µF, rated for at least double the rail's nominal voltage. If it is in series with an audio signal, it is an AC coupling capacitor; a 1µF to 10µF non-polarized film capacitor is usually a safe substitute.
- Measure Health In-Circuit: Use an ESR meter (such as the Peak Atlas ESR70) to check the component without desoldering. An ESR reading above 5 ohms on a bulk filter capacitor indicates the electrolyte has dried out, regardless of what the faded label originally claimed.
- Extract and Measure Precise Values: For timing circuits (like a 555 timer RC network) where exact capacitance dictates frequency, you must desolder the component. Measure it using a dedicated LCR meter (like the DER EE DE-5000) at 100Hz for electrolytics or 1kHz for ceramics. Multimeter capacitance functions are often inaccurate for values below 100nF or for high-ESR degraded parts.
- Decode Ceramic SMDs: If you are looking at a faded surface-mount ceramic capacitor, physical size is your best clue. A 0402 package is almost certainly under 100nF, while an 1812 package in a power supply is likely in the 1µF to 10µF range. For through-hole ceramics with faded 3-digit EIA codes, remember that the first two digits are significant figures and the third is the multiplier in picofarads (e.g., "104" = 10 x 10^4 pF = 100,000 pF = 100nF).
For deep-dive application notes on voltage derating and temperature coefficients, refer to manufacturer resources like the Vishay capacitor application guidelines or Electronics Tutorials on capacitor symbols.
Frequently Asked Questions
What does the plus sign on a capacitor symbol mean?
The plus sign (or sometimes a shaded block in IEC standards) designates the anode (positive) terminal of a polarized capacitor, such as an aluminum electrolytic or tantalum. Connecting the anode to a lower voltage than the cathode reverses the internal dielectric oxide layer, causing rapid internal heating, gas generation, and eventual venting or short-circuit failure. Always verify the anode connects to the higher DC potential.
Why does my schematic show a capacitor symbol with a third line?
A capacitor symbol drawn with a third line parallel to the main plates usually represents a shielded capacitor or a specific type of feedthrough capacitor. In RF and precision analog circuits, the third line represents a grounded electrostatic shield (often a copper foil wrap on the physical component) used to prevent capacitive coupling of external noise into the high-impedance circuit node.
How do I read the 3-digit code on a ceramic capacitor symbol?
Small ceramic capacitors use the EIA 3-digit coding system. The first two digits represent the significant figures, and the third digit represents the multiplier (number of zeros) in picofarads (pF). For example, a marking of "473" means 47 followed by three zeros (47,000 pF), which equates to 47 nF or 0.047 µF. If the code includes a letter at the end (like "104K"), the letter denotes the tolerance (K = ±10%, M = ±20%, J = ±5%).
Is the symbol of a capacitor different for AC vs DC circuits?
The schematic symbol itself does not change based on whether the circuit is AC or DC; a non-polarized capacitor is drawn as two parallel lines in both contexts. However, the type of capacitor chosen for the physical build differs drastically. DC circuits frequently use polarized electrolytics for high capacitance density, while AC circuits (like motor run capacitors or AC line filters) must use non-polarized film or ceramic capacitors rated specifically for AC voltage (VAC), as DC voltage ratings do not account for the continuous dielectric stress of alternating polarity.
