What Each Symbol Means
Capacitor symbols on circuit diagrams tell you whether the part is polarized and what type it is. Misreading a symbol can mean installing a polarized capacitor backwards — which ends with a popped vent and a dead board.
The Basic Symbols
Fixed Non-Polarized Capacitor
Two parallel lines of equal length. Generic symbol for ceramic, film, NP0 MLCC. Install either direction.
Polarized Capacitor
One straight plate and one curved plate. The curved plate is negative. Universal symbol for aluminum electrolytic and tantalum capacitors. On the physical part, the negative lead has a stripe. On SMD aluminum electrolytics, the black mark indicates negative.
Variable Capacitor
An arrow through the parallel plates. Used for tuning capacitors and trimmer caps.
Feedthrough Capacitor
Three terminals: signal passes through with capacitance to ground on both sides. Used for EMI filtering at enclosure boundaries.
Physical Polarity Markings
The trap that catches beginners: aluminum electrolytics mark negative, tantalums mark positive. Verify which type you are holding:
- Radial aluminum electrolytic: Stripe with minus signs marks negative. Shorter lead is also negative
- SMD aluminum electrolytic: Black mark or stripe on top indicates negative
- Tantalum (through-hole): Plus mark near positive lead
- SMD tantalum: Beveled or painted end is positive
Measuring Polarity When the Marking Is Gone
- Use a multimeter on diode mode: The oxide layer forms a tiny diode. One polarity reads slightly higher voltage drop
- Check the PCB: Square pad is usually positive (verify against other capacitors on the same board)
- Trace the circuit: If one side connects to ground, that is negative — no guesswork needed
Key Rules
- Aluminum electrolytic and tantalum are polarized — respect the symbol
- Ceramic and film are non-polarized — install either way
- Aluminum electrolytics mark NEGATIVE. Tantalums mark POSITIVE. Never confuse them
- When in doubt, check the datasheet
