Capacitor Polarity Marking: The Ultimate Guide

Polarized capacitors must be connected with correct orientation, or they may fail catastrophically. Understanding polarity markings is essential for safe and reliable circuit assembly.

Polarized vs Non-Polarized Capacitors

Polarized Capacitors

These must be connected with correct polarity:

• Electrolytic (aluminum and tantalum)
• Supercapacitors
• Some specialty types

Non-Polarized Capacitors

These can be connected either way:

• Ceramic capacitors
• Film capacitors (polyester, polypropylene)
• Mica capacitors
• Most surface-mount capacitors (except tantalum)

Identifying Polarity on Through-Hole Electrolytics

Lead Length

The most common identification method for new capacitors:

Longer lead = Positive (+)
Shorter lead = Negative (-)

Note: This only works on new, uncut capacitors.

Stripe Marking

A stripe on the capacitor body indicates the NEGATIVE terminal.

• Usually gray or black stripe
• Contains minus (-) signs
• Arrow may point toward negative lead
• Located on the side near the negative lead

Body Markings

Look for these indicators:

• + symbol near positive terminal
• - symbols in the stripe area
• Some brands mark the positive end

SMD Electrolytic Capacitors

Surface-mount electrolytics use different marking systems:

Top marking: Usually indicates positive with a bar or notch.

Bottom marking: Check for + or - symbols on the pad area.

Case style: Some have a beveled corner indicating polarity.

Tantalum Capacitors

Tantalum capacitors use opposite marking convention:

Stripe or bar indicates POSITIVE terminal

This is opposite to aluminum electrolytics. The positive end is typically marked with:

• A stripe or band
• A + symbol
• A molded-in line

Critical: Tantalum capacitors are very sensitive to reverse polarity and can fail violently.

Schematic Symbols

Polarized capacitor symbol:

• One curved plate and one straight plate
• Curved plate is usually negative
• • sign shown near positive plate

Non-polarized symbol:

• Two straight parallel plates
• No polarity marking

Testing Unknown Capacitors

Multimeter Method

For electrolytics with unclear markings:

1. Discharge capacitor first
2. Set multimeter to capacitance or diode mode
3. Connect probes one way, note reading
4. Reverse probes, note reading
5. Correct polarity gives the accurate capacitance reading

Visual Inspection

Check for:

• Remaining marking fragments
• Manufacturer datasheet pictures
• Bulging or venting (indicates damaged capacitor)

What Happens with Reversed Polarity?

Aluminum electrolytic:

• Internal gas generation
• Swelling of the capacitor
• Venting through safety vent
• Possible explosion if severely abused

Tantalum:

• Can fail short circuit
• May ignite or explode
• Particularly dangerous at higher voltages

Best Practices

1. Always check polarity before soldering
2. Verify with datasheet when uncertain
3. Observe derating - use capacitors below rated voltage
4. Never force fit - if polarity is wrong, the marking system is clear
5. Double-check SMD parts - they're easy to place incorrectly

Common Mistakes

• Assuming stripe is positive (it's negative on electrolytics)
• Confusing tantalum and electrolytic conventions
• Trusting lead length on used capacitors
• Ignoring polarity on "axial" style capacitors

Conclusion

For aluminum electrolytics, the stripe marks negative. For tantalum, the stripe marks positive. When in doubt, check the datasheet or use a multimeter. Correct polarity is essential for safety and reliability.