If you are wondering which is positive and negative in capacitor symbol diagrams, the direct answer is: in a standard polarized capacitor symbol, the straight plate represents the positive (+) terminal, and the curved (or bowed) plate represents the negative (-) terminal. Alternatively, if the symbol features two parallel straight lines, look for a plus sign (+) next to one of the plates to indicate the positive lead. Non-polarized capacitors (like ceramics) use two identical parallel straight lines with no polarity markings.
The Complete Capacitor Symbol & Polarity Reference Table
Before wiring up a board or reading a vintage schematic, cross-reference the symbol on the diagram with the physical markings on the component. The table below maps schematic symbols to their real-world counterparts.
| Schematic Symbol | Physical Component Marking | Capacitor Type | Polarity Rule |
|---|---|---|---|
| One straight plate, one curved plate | Stripe with minus (-) signs and arrows on the can | Aluminum Electrolytic (Through-hole & SMD) | Straight plate = Positive (+) Curved plate = Negative (-) |
| Two straight plates with a "+" sign | Stripe, dot, or notch on the component body | Tantalum & Niobium Electrolytic | "+" sign points to Positive (+) Warning: Stripe/dot is POSITIVE here |
| Two identical parallel straight plates | No polarity markings (often symmetrical) | Ceramic, Film, Mica, Glass | Non-polarized. Can be inserted in either direction. |
| Two plates with an arrow striking through them | Shaft or tuning screw on top | Variable / Tuning Capacitor | Usually non-polarized, but the arrow indicates adjustable capacitance. |
Regional Standards and Faded Markings
While the basic physics of capacitors don't change across borders, the drafting standards for schematics do. If you are reading a schematic, you need to know which standard the engineer used.
IEC vs. ANSI/IEEE Standards
Globally, the IEC 60617-4 standard dictates that the curved plate always represents the negative terminal of a polarized capacitor. This is the default in modern CAD tools like Altium, KiCad, and Eagle. In North America, the IEEE 315 / ANSI Y32.2 standard historically allowed for two straight parallel plates with a distinct "+" sign drawn next to the positive terminal. While modern US schematics largely default to the IEC curved-plate style to avoid ambiguity, you will frequently see the IEEE "+" style in older American military, aerospace, and vintage audio schematics.
Safe Interpretation When Markings are Faded or Missing
On the bench, you will inevitably encounter an aluminum electrolytic capacitor where the negative stripe has rubbed off, or a salvaged part with clipped leads. Never guess polarity; reverse-biasing an electrolytic capacitor causes internal dielectric breakdown and rapid gas generation.
- The Bottom Indent Trick: Flip the capacitor over. On most radial aluminum electrolytics, the rubber bung on the bottom has a physical groove or indent on the side corresponding to the negative terminal.
- The SMD Chamfer: For surface-mount aluminum electrolytics, the black plastic base usually has a chamfered (cut) corner or a painted semi-circle indicating the positive terminal.
- The Multimeter Leakage Test: If markings are entirely gone, set your multimeter to measure current (mA/µA range) in series with a low-voltage DC source (like a 3V coin cell). Apply voltage in one direction, note the leakage current, then reverse it. Electrolytics exhibit significantly lower leakage current when biased correctly. The orientation with the lower current reading is the correct polarity.
Rows People Get Wrong (and How to Avoid Blowing Them Up)
Even experienced makers make catastrophic mistakes when assuming all capacitors follow the same visual rules. Here are the most common traps that lead to popped components and damaged PCBs.
The most dangerous mistake in capacitor wiring involves tantalum capacitors. On a standard aluminum electrolytic capacitor, the painted stripe with minus signs indicates the negative cathode. However, on a tantalum capacitor, the painted stripe, dot, or notch indicates the positive anode. Wiring a tantalum cap backward using the aluminum rule will cause it to fail short-circuit, often resulting in a violent spark, smoke, and a dead power rail. Always look for the "+" sign on the schematic when dealing with tantalums, and remember: Tantalum stripe = Positive.
The "Long Leg" Fallacy
Manufacturers ship through-hole electrolytic capacitors with the positive lead pre-cut longer than the negative lead. Many hobbyists memorize "long leg is positive." This rule instantly fails the moment you or a previous technician trims the leads to fit a tight enclosure or after desoldering a part from a donor board. Always rely on the can stripe or the bottom bung indent, not the lead length.
Ignoring the Voltage Derating Curve
Polarity isn't the only thing that matters; voltage rating does too. A common mistake is placing a 16V capacitor on a 12V rail. While 16V is technically higher than 12V, aluminum electrolytics suffer from reduced lifespan and increased Equivalent Series Resistance (ESR) when operated near their maximum rated voltage. As a rule of thumb, derate your capacitors by at least 20-30%. For a 12V rail, use a 25V or 35V rated capacitor.
Frequently Asked Questions
What happens if I wire an electrolytic capacitor backward?
If you reverse-bias an aluminum electrolytic capacitor, the internal aluminum oxide dielectric layer begins to break down. This triggers an electrochemical reaction that generates hydrogen gas. As pressure builds inside the aluminum can, the internal temperature spikes. If the voltage is high enough, the capacitor will vent through its scored top cross (the "X" or "Y" stamp), releasing acrid white smoke. In severe cases, or if the vent fails to open, the casing can rupture explosively. If you catch a reverse-wired cap early (low voltage, short duration), it might survive, but its leakage current and ESR will be permanently degraded. Replace it.
Does the curved line always mean negative in a capacitor symbol?
Yes, in modern standard schematic symbols (IEC 60617), the curved or bowed line universally represents the negative terminal of a polarized capacitor. The straight line represents the positive terminal. The only exception you might encounter is in highly stylized or non-standard hand-drawn diagrams, which is why engineers usually add a redundant "+" sign next to the straight plate to eliminate any doubt. For non-polarized capacitors, both lines are drawn perfectly straight and parallel.
How do I read the 3-digit code on a small ceramic capacitor?
Ceramic capacitors are non-polarized (two straight parallel lines on a schematic), so you don't need to worry about positive or negative. However, you do need to read their capacitance value, which is printed in picofarads (pF) using a 3-digit EIA code. The first two digits are the significant figures, and the third digit is the multiplier (number of zeros). For example, a cap marked 104 means 10 followed by 4 zeros: 100,000 pF. This converts to 100 nF or 0.1 µF. A cap marked 222 is 2200 pF (2.2 nF). Always verify with an LCR meter if the printing is ambiguous.
