Q:
We are often asked if certain components have polarity, which is not always obvious. This article will share some practical experiences around some common components that are often asked about polarity.
In the electrical context, polarity is defined as the direction in which current flows in a circuit. In the DC system, there may be a positive extreme and an electrical neutral point called grounding, namely 0V point. There is a bipolar DC system which includes both positive and negative voltage sides. By convention, DC current flows from the positive terminal to the negative terminal (or ground). The electrons move in the opposite direction. The polarity of alternating current is not consistent, because the polarity usually fluctuates between positive and negative at 50 or 60Hz.
Polarity of passive components
01 resistance
In electrical analysis, people usually assume that the resistance has polarity, so that it is easier to intuitively understand the current flow during design. In my opinion, this practice often leads people to believe that resistance must have polarity. In reality, resistance can be regarded as a kind of wire with great effect, with specific rated power and resistance. The physical characteristics of the resistance completely depend on the materials used inside, and its role is to block or reduce the current in both directions. Therefore, in terms of design, structure and physical characteristics, any type of basic resistance is non-polar.
02 inductance
The second most frequently asked component is inductance. Inductors are somewhat similar to resistors, except they are designed to store electrical energy in the form of magnetic energy. Inductors can be simplified as fancy coil wires surrounding ferromagnetic materials (or other magnetic materials). For reasons of physical characteristics, design and structure, inductance, like resistance, does not care about its placement. Therefore, I think this confusion comes from the analysis practice and some applications. In these cases, when it comes to the direction of the current, the polarity must be considered. Even if the inductor has no polarity, the specific direction of current flow will change the magnetic poles inside and around the inductor. For example, the coil on the relay has a specific polarity mark on the circuit diagram, but this is only to indicate the correct current direction of triggering the relay, which does not mean that the coil itself is polar. The magnetic pole may be affected by the electrode, but it is a completely different concept.
03 resistance
Capacitance is a little more complicated in terms of polarity. The dielectric material in the capacitor and the overall design of the capacitor determine whether it has polarity. for instance:
Aluminum electrolytic capacitor: this capacitor is almost always polar, unless the specification or parameters specifically indicate whether they are bipolar. The same is true of aluminum polymer capacitors.
Ceramic capacitor: no polarity; We have not seen anything with polarity.
Film capacitance: It seems to be non-polar, but if you are not sure, please check the specification. The factory usually indicates whether a component has polarity, as this is critical for applications requiring polarity. If it is not clearly stated in the specification (according to the specification I have seen), it is likely to be non-polar.
Tantalum capacitor: inherently polar parts, reverse voltage will cause damage.
Mica capacitance: generally non-polar.
PTFE capacitors: These capacitors have a thin film design in technology, so they also tend to be non-polar.
04 resistance
For any other type of passive device, if the polarity is not clearly indicated, it is likely that there is no polarity. Switches and metal contacts have no polarity at all, because everything is mechanical, and metal can conduct electricity anyway. Polarity is an attribute type that determines or destroys the element and/or design, so it must be included in the specification. An example of this type of element is the reed switch, which is basically a simple electromechanical switch with simple metal contacts moved by a magnetic field, so it has no polarity
Active device
Active components are almost always polar because they require a power supply to work.
However, depending on the type of active device, in some aspects (such as input and output), the direction of the current may not be concerned. Active components are much more complex than passive components. Therefore, if you have questions about such components, you are recommended to read the corresponding product specifications.
Here are some examples of active devices:
Diodes: typical polar components, depending on the type (Zener diodes are special cases, they can have bipolar, depending on the voltage).
Transistor: for power supply, it has polarity, because specific positive source or negative source is required; For conduction current, it depends on the type and mode of transistor used.
AD-DC converter: the input is bipolar; The output can be unipolar (full positive or full negative voltage and grounding reference, or bipolar, that is, bidirectional positive and negative voltage output with grounding reference). The power supply is inherently polar.
Crystal oscillator/resonator: If the crystal/resonator is a two pin version, it is usually non-polar, and they can be powered in either way. For multi pin version, please be sure to check the specification.
