We are often asked if parts have a polarity or not and it isn’t always immediately apparent. This post will cover common components that we get asked about on whether they have polarity or not. The definition of polarity in electrical context is the direction of current flow in an electrical circuit. In a direct current system most likely there is a positive side and an electrically neutral point called ground which is the 0V point. There are bi-polar DC systems with positive and negative voltage sides present. DC current prefers to flow from the positive terminal to the negative terminal (or ground) by convention. Electrons flow the opposite direction. Alternating current doesn’t have a consistent polarity since the polarity fluctuates between positive and negative at 50 or 60Hz typically.
It starts to get somewhat vague when it comes to components of electrical systems for a few reasons. Lets start with passive parts that we often get asked about, for example, a resistor.
In electrical analysis it is a common practice to assume resistors have polarity to make it easier to visuailze how one might think current will flow when designing something. This practice, I believe, often gives to the thought that resistors must be polarized. In reality, resistors could be thought of as glorified wires with very specific ratings on wattage and resistance. The physics of a resistor all depends on what material is used inside to impede or reduce the electrical flow in either direction. Basic resistors of any style are therefor non-polarized by design, construction, and physics.
The second most common parts we are asked about are inductors. Inductors are somewhat similar to resistors except they are specifically designed for storing electrical energy in the form of magnetic energy. Inductors can be simplified to fancy coiled wires around ferromagnetic material (or other magnetic material). Inductors do not care which way they are placed just like resistors due to the physics, design, and construction. I believe the confusion, again, comes from practices in analysis and certain applications where polarity is necessary when it comes to direction of current. Even though inductors aren’t polarized, the particular direction the current flows will change the magnetic poles present in and around an inductor. For example, a coil present on a relay has particular polarity markings on circuit diagrams simply to indicate that is the intended direction of current to properly acutate the relay, it does not mean that the coil itself is polarized. Magnetic poles may be effected by electrical poles, but are completely separate concepts.
Capacitors are a little more complicated when it comes to polarity. The material that the di-electric material in the capacitor along with the overall design of the capacitor define if it is polarized. Here are some examples:
- Aluminum Electrolytic Capacitors : Almost always polarized unless the datasheet or parameters specifically call out if they are bi-polar. Same thing goes for aluminum polymer capacitors.
- Ceramic Capacitors : Non-polarized; haven't seen a case where they are polarized.
- Film Capacitors : These tend not to be polarized, however, if anyone is ever unsure check the datasheet. Manufacturers often call out if things are polarized as that is crucial to applications where polarization is necessary. If a datasheet doesn't really say (from what I've seen of datasheets) then it most likely isn't polarized.
- Tantalum Capacitors : Inherently polarized, reverse voltage can destroy these types.
- Mica Capacitors : Non-polarized in general.
- PTFE Capacitors : These technically have a film design, so these also tend to be non-polar.
Metal Contacts / Switches / Other General Passive Devices
Any other type of passive device that does not specifically claim polarity is present, then it most likely doesn’t have polarity. Switches and metal contacts don’t care about polarity at all since everything is mechanical and metal conducts electricity anyway it can. Polarity is a type of attribute that makes or breaks a component and/or design, so it would be a necessary inclusion in a datasheet. One example of a component asked about is a reed switch. These are basically just simple electromechanical switches with simple metal contacts moved by a magnetic field, so they don’t have polarity.
Active components are almost always polarized based on the fact that they require power to function. However, depending on the type of active component there may be certain aspects like inputs and outputs that don’t care about the direction of current. Active components are far more complex than passive components, so it is always recommended to read the datasheets for particular question you have in mind about the component.
Here are some examples of active components:
- Diodes : typically polarized, dependent on type (Zener's are special case where they can be bi-polar dependent on voltage).
- Transistors : for power, yes they require a specific positive or negative source; for conducting current it depends on the type of transistor and mode being utilized.
- AC DC Converters : Input is bi-polar; output is either uni-polar (all positive or all negative voltage and ground reference or bi-polar with both positive and negative voltage outputs with ground reference). Power sources are inherently polarized.
- Crystal Oscillators/Resonators : If the crystal/resonator are two pin versions, they typically are not polarized, they can be powered either way. Make sure to check on multi-pin versions though in the datasheets.