Capacitors can have polarity or not, depending on the type and internal structure of the capacitor.
Simply put:
Polarized capacitors:
Characteristic: There is a clear distinction between positive and negative electrodes.
Must be installed correctly: In the circuit, it must be connected according to the correct polarity (positive pole connected to high potential, negative pole connected to low potential or ground). Reversing the connection can result in performance failure and a significant increase in leakage current, and in severe cases, it can cause heating, bulging, and even explosion (especially for aluminum electrolytic capacitors).
Type:
Aluminum electrolytic capacitors: The most common type is polar capacitors. Usually packaged in cylindrical aluminum shells, the stripes or arrows marked with negative signs' - 'on the shell point to the negative pole, or the pins are one long (positive) and one short (negative). There are also patch types, usually with dark markings on the top of the shell indicating the negative electrode.
Tantalum capacitors: Surface mount packaging is the most common. It is usually a small rectangular prism with a horizontal line or a dot on one end indicating the positive electrode (note: this is marking the positive electrode! Different from aluminum electrolysis). There are also axial lead packages available.
Niobium capacitor: Similar to tantalum capacitor, the polarity marking method is usually to mark the positive electrode.
Reason: Their internal structure relies on electrochemical principles, forming a very thin layer of oxide dielectric on the surface of the anode (positive electrode) metal through the "empowerment" process during the manufacturing process. This layer of medium has unidirectional conductivity and can only withstand forward voltage (anode potential higher than cathode). Reverse voltage will damage this layer of dielectric.
Advantages: It can provide a very large capacitance (from a few uF to tens of thousands of uF or even higher) under the same volume.
Disadvantages: relatively high leakage current, relatively high ESR (equivalent series resistance), limited lifespan (especially affected by temperature), and poor frequency characteristics.
Non polarized capacitor:
Characteristic: There is no distinction between positive and negative poles. It can be installed in any direction in the circuit (unless there are special requirements for the circuit layout).
Type:
Ceramic capacitors: The most commonly used non-polar capacitors, with a wide variety of types (such as MLCC - multi-layer ceramic capacitors). From a few pF to tens of uF (large capacity usually has a larger volume).
Film capacitors: such as polyester film capacitors, polypropylene film capacitors, polystyrene film capacitors, etc. Excellent performance, commonly used in situations that require high stability and low loss.
Mica capacitor: stable performance, high accuracy, good temperature characteristics, but relatively large volume and high cost.
Paper capacitors: less commonly used nowadays.
Some special electrolytic capacitors: There are specially designed "non-polar electrolytic capacitors", which are equivalent to two polarized capacitors connected in series back-to-back, and can be used in AC situations, but the capacity to volume ratio will decrease.
Reason: Their dielectric materials (such as ceramics and plastic films) are inherently insulating, with symmetrical electrode structures and no unidirectional conductive oxide layer.
Advantages: Small leakage current, usually low ESR, good frequency characteristics (especially high frequency), long lifespan, and high reliability.
Disadvantage: Under the same volume, the capacitance is usually much smaller than that of polarized capacitors (especially aluminum electrolysis and tantalum capacitors).
Summary:
Aluminum electrolytic capacitors, tantalum capacitors, and niobium capacitors have polarity, and positive and negative poles must be distinguished during installation. Reversing them can cause damage or even danger.
Ceramic capacitors, film capacitors, and mica capacitors are non-polar and can be installed in any direction.
Supercapacitors (double-layer capacitors) usually have polarity and need to be connected correctly.
Important reminder: When repairing or designing circuits, it is necessary to confirm the polarity of the original capacitor when replacing it (observe the markings on the circuit board or the identification of the original capacitor), and select capacitors of the same type, same polarity (if any), and the same or higher specifications (withstand voltage, capacity, temperature). Changing the direction of polarity capacitors at will is very dangerous!